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I TA L I A N H A B I TAT S
Mountain streams
5
Italian habitats
Italian Ministry of the Environment and Territory Protection / Ministero dell’Ambiente e della Tutela del Territorio
Friuli Museum of Natural History / Museo Friulano di Storia Naturale · Comune di Udine
I TA L I A N H A B I TAT S
Scientific coordinators
Alessandro Minelli · Sandro Ruffo · Fabio Stoch
Editorial commitee
Aldo Cosentino · Alessandro La Posta · Carlo Morandini · Giuseppe Muscio
“Mountain streams · Life in running waters”
edited by Fabio Stoch
Texts
Marco Cantonati · Valeria Lencioni · Bruno Maiolini · Mauro Marchetti · Karin Ortler · Mario Panizza ·
Sergio Paradisi · Margherita Solari · Fabio Stoch
English translation
Elena Calandruccio · Gabriel Walton
Illustrations
Roberto Zanella
Graphic design
Furio Colman
Photographs
Archive Museo Friulano di Storia Naturale (Ettore Tomasi) 47/1, 47/3, 48, 50/1, 50/2, 50/3, 51/1, 51/2,
51/3, 54/1, 54/2, 55/1, 55/2 · Marco Cantonati 31/1, 31/2, 32/1, 32/2, 32/3, 34/1, 34/2, 35/1, 35/2, 39 ·
Massimo Capula 134 · Ulderica Da Pozzo 9, 10, 27, 28, 46, 58, 110, 139, 148, 151, 153 ·
Adalberto D’Andrea 6, 64, 132 · Massimo Domenichini 141 · Maria Manuela Giovannelli 60/2 ·
Luca Lapini 90, 92, 98, 99 · Valeria Lencioni 20, 100 · Bruno Maiolini 56, 60/1, 65/2, 68, 71, 74/1, 74/2,
79/1, 112, 145 · Mauro Marchetti 124 · Michele Mendi 95 · Andrea Mocchiutti 23 ·
Giuseppe Muscio 26, 42, 52, 119, 120, 130, 135, 136 · Karin Ortler 40, 45, 49, 53 ·
Sergio Paradisi 85, 87 · Natural Park of Foreste Casentinesi (Nevio Agostini) 7, 18, 29, 80, 122, 129, 147 ·
Roberto Parodi 96, 97 · Ivo Pecile 13, 66 · Margherita Solari 47/2, 59, 116, 150
Fabio Stoch 62, 102, 103/1, 103/2, 114, 115, 138 · Roberto Zucchini 65/1, 67, 70, 79/2, 91, 93, 127
Mountain streams
Life in running waters
©2003 Museo Friulano di Storia Naturale · Udine
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or
by any means, without the prior permission in writing of the publishers.
ISBN 88 88192 10 7
Cover photo: springs of Arzino in Carnia (Friuli, photo by Ulderica Da Pozzo)
M I N I S T E R O D E L L’ A M B I E N T E E D E L L A T U T E L A D E L T E R R I T O R I O
M U S E O F R I U L A N O D I S T O R I A N AT U R A L E · C O M U N E D I U D I N E
Italian habitats
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Sergio Paradisi
Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Mauro Marchetti · Mario Panizza
1
Caves and
karstic
phenomena
2
Springs and
spring
watercourses
3
Woodlands
of the Po
Plain
4
Sand dunes
and beaches
5
Mountain
streams
Flora and vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Marco Cantonati · Karin Ortler
Invertebrate fauna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Bruno Maiolini · Valeria Lencioni
Vertebrate fauna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Sergio Paradisi
6
The
Mediterranean
maquis
7
Sea cliffs and
rocky
coastlines
8
Brackish
coastal lakes
9
Mountain
peat-bogs
10
Realms of
snow and ice
Ecology of mountain streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Fabio Stoch
Conservation and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Mauro Marchetti · Mario Panizza · Sergio Paradisi · Fabio Stoch
Suggestions for teaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
Margherita Solari
Select bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
11
Pools,
ponds and
marshland
12
Arid
meadows
13
Rocky slopes
and screes
14
High-altitude
lakes
15
Beech
forests of the
Appennines
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
List of species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
Introduction
SERGIO PARADISI
Mountains undoubtedly take on fascinating aspects when the sight of
snowclad peaks rising into the sky,
high meadows carpeted with beautiful
spring flowers, and mysterious, aweinspiring forests are accompanied by
water as an additional feature in a
grandiose landscape.
Man has always been attracted to running water, whether as a turbulent rush
of power gushing from the tongues of
glaciers, as myriads of silvery streams
spreading down hilly slopes, as the
pleasure experienced upon seeing a
clear stream flowing across a meadow,
or as the deafening roar of waterfalls.
In the past, man exploited mountains
The Acquacheta in the northern Appennines
for some of his activities; nowadays he
views them as “picture postcard” landscapes, sufficiently far from large towns
to be considered uncontaminated and rustically serene.
In fact, this oleographic picture is far removed from reality. Steep slopes and
the resulting high speed of water flow, the low or very low temperatures, and
reduced trophism all mean that mountain streams are rigorous and selective
environments.
Their animal and plant communities are poorer than those found in lowland
rivers, and are made up of specialized components. However, as specialization also implies vulnerability, high-altitude streams are essentially vulnerable
environments, by their very nature. Here, meteoric events produce an almost
continual renewal in the landscape, preventing animal and plant populations
from reaching a settled maturation stage: as if the stream biocoenoses were
perpetual pioneers. In such a context, any unwise human intervention, albeit
on a small scale, may upset the delicately contrived balance which makes
Mountain streams have always attracted man
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forms of life possible. The traditional activities of mountain people (livestock
rearing, forestry) had generally limited consequences on the stream ecosystem,
as at high altitudes there are few polluting sources and villages or towns are
small. In recent years, the well-known phenomenon of abandonment of
mountain areas (particularly high mountain ones) has greatly reduced human
activities. At the same time, and as a consequence of this demographic
trend, various kinds of interventions were needed. They were neither small
nor uninfluential, and often deprived the stream and its landscape of the
heavenly aura we have attributed to them so far.
Dams, canalizations, various kinds of hydraulic arrangements, deviations and
embankments all have a strong environmental impact aimed at controlling
water flow, to be balanced against their value for the safety of human settlements and hydraulic requirements. Streams are often viewed - paradoxically,
since they are commonly perceived as absolutely natural - as nothing more
than sterile conveyors, preferential ways in which melted ice from glaciers or
rainwater choose to reach the valley.
The animal and plant communities which populate streams are not considered
when hydraulic operations are executed, and these often deeply modify and
sometimes jeopardize those communities forever.
The present volume deals with this issue, and also examines the geological,
botanical and faunal aspects of Italian high mountain streams. When speaking
of streams, we do not refer to a single, homogeneous habitat, but a complex
network of environments which often reveal very different geological and biological aspects.
Watercourses in the Alps and Appennines are mountain streams, as are those
flowing down the lava spurs of Mount Etna and the small, temporary rivers of
inland Sardinia, the geological history of which has created a completely different biogeographical world from that of the Italian peninsula. The icy, fastflowing watercourses which descend from Alpine glaciers are streams, like
rivers which on hot summer days are reduced to gravel beds and look more
like deserts than watercourses. This is why this volume does not cover all the
environmental themes that the “stream” ecosystem provides. The present
work is devoted to mountain streams in the strict sense of the word, defining
the geographical area of the Alps and northern Appennines and, further south,
the streams of the Abruzzi and Latium regions.
The volume covers all naturalistic aspects of streams, but also analyses the
dangers which threaten the survival of their frail populations. We hope that the
spread of knowledge may make us more aware of what we are about to lose
and stimulate our will to preserve it.
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An Alpine stream in winter
Hydrogeology
MAURO MARCHETTI · MARIO PANIZZA
Waters flowing on the Earth’s surface make up the so-called superficial
flow, a subject studied by branches of Earth Sciences such as hydrology,
hydraulics, and geomorphology. Parameters relating to fluid physics (the
field of hydraulics), river geometry and the development of the hydrographic
network (hydrology), the structural factors of the catchment basin (lithology,
degree of fracturing, etc.) and climatic conditions, which influence river
modelling processes (geomorphology) are all necessary to understand the
characteristics and evolution of watercourses.
Rivers in the Italian Alps and Appennines differ greatly from one another, as
they are considerably influenced by the very varying rocks which outcrop in
their respective basins, and by very different relief energy, generally higher in
Alpine basins. The Alps and Appennines also have different climates, as the
former are often located at higher altitudes, where the periglacial environment
dominates.
■ Geological setting
Italy is particularly rich in mountains and hills, which respectively represent
35.2% and 41.7% of the entire surface area of the country. As is well-known,
the mountains of the Alpine and Appennine chains belong to a very long
mountain system stretching from the Straits of Gibraltar to Indonesia.
The evolution of this mountain system is extremely complex: as regards the
circum-Mediterranean area, the various portions of the chain are a series of
tectonically deformed stretches, in which the processes of shortening and
overthrusting of the continental crust did not take place in a homogeneous
way. These phenomena date back to the Jurassic-Cretaceous boundary
(about 150 million years ago).
Subduction of the ocean floor ceased in the Cretaceous and the two edges of
the Eurasian and African continents collided. Overthrusting began to involve
increasingly longer parts of the continental crust. The most active phase in
the Alps occurred during the Upper Eocene-Oligocene (40-20 million years
ago). The orogenetic phase continued into the Neogene and was particularly
Ravines are typically created by streams flowing across hard rocks
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intense during the Miocene in the Tuscan Appennines, and in the Plio-Pleistocene in the external Appennines.
Mountain chains in southern Europe are formed of overturned north-running
faults: their structure is highly asymmetric, as they are composed of overlapping faults translated sometimes hundreds of kilometres. The Appennine
chain overthrusts towards the Adriatic.
The Alps are a well-defined geographical unit, about 1000 km long and 150200 km wide, characterized by numerous peaks exceeding elevations of 3500
m. The intense tectonic event in the Oligo-Miocene, which formed the Alps,
was followed by an erosive phase which reduced the chain to a series of low
reliefs in the late Upper Miocene. The present conformation is due to later uplift in the Plio-Pleistocene.
The Appennine chain is about 1000 km long and only a few of its peaks exceed 2000 m: the highest ones lie between the Abruzzi and Latium regions
(e.g., Gran Sasso, 2912 m). Mount Etna is the highest (3223 m).
The greatest differences between the Alps and the Appennines lie in the extent of their tectonization, i.e., their degree of uplift and the ductile or fragile
behaviour of their rock formations. These factors directly or indirectly interact with others, such as the erodibility of rocks and climatic conditions, determining an erosion rate which varies significantly according to area. The
most significant difference in the evolution of mountain streams is certainly
the different lithogical composition of the outcropping soils in the Alps and
Appennines.
Despite their complexity, hard, erosion-resistant rocks outcrop throughout the
Alps: intrusive and metamorphic ones in the west, calcareous in the centre
HELVETIC PREALPS
CERVINO
EXTERNAL DOMAIN
Geological sketch of a section of the Western
Alps, showing overlapping nappes in the
mountain chain.
The Alps rest on a pre-Triassic basement
composed of granite or metamorphic rocks which
are harder than the sedimentary cover. This hard
basement outcrops in the Western Alps.
The Southern Alps, which comprise almost all the
central and southern Alps, are calcareous, and
rest on a paleo-African basement overlaid by
M. ROSA
13
SESIA
SIMPLO - TICINES NAPPES
layers of sediments (and rare vulcanites) over 10
km thick.
Compression during the formation of the Alps
and the development of a series of sliding planes
(faults) in the north gave rise to the subsequent
thrusting, folding and shrinking southwards of
these layers.
Yellow: Helvetic domain; green: Penninic domain
(light blue: Piedmont area); orange: Austro-Alpine
domain.
A stream carving its way through mountains in the Aurina Valley (province of Trento)
14
and east, and volcanic or dolomitic in the Veneto-Trentino (province of Trento)
area. In the Appennines, lithologies are less coherent, being composed of
clay, marl and sandstone, with few crystalline or metamorphic rocks. In
some areas, volcanic rocks occur, in places made up of tuff and ash with low
resistance to erosion.
However, less erodible lithologies stand out in the Appennines where the
landscape generally has gentle slopes: among these are the Latium volcanic
APPENNINE EXTERNAL FRONT
I
P
L
ALPINE/SOUTH ALPINE LIMIT
FOREDEEPS
PO PLAIN
A
FORELANDS
ADRIATIC FOREDEEP
GARGANO FORELAND
BRADANIC FOREDEEP
I
UR
LIG
AN
BA
SI
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OP
EN
IN
G
UP
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LIG
./M
ID. M
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H
TYRR
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ENIA
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P
./
IOC
P. M
GU
.-P
LIO
IS
LE
T.
SALENTO
FORELAND
GELA - CATANIA FOREDEEP
IBLEI FORELAND
The Appennines, at the edge of the western
African promontory (Adria microplate) feature
foredeeps (Po Plain, Adriatic, Bradanic, GelaCatania), later filled with sediments, on the
edges of which are marginal continental
areas (forelands: Gargano, Salento and Iblei).
The Appennines are composed of faults
which originated in the Cretaceous, when the
ocean closed up, continental plates collided,
and the Liguria-Provence and Tyrrhenian
basins were formed.
Compression caused tectonic plates to form,
which became detached from the crust
beneath and overlapped. Orogenesis in the
Appennines gave rise to anti-clockwise
rotation of the Appennines and the SardinianCorsican unit, which caused the Liguria unit
to slide over the Tuscan one during the
Oligocene. The southern Alpine and northern
Appennine margins warped downwards and
came into contact under the Plio-Quaternary
marine and fluvial deposits of the Po Plain.
covers, sandstone in the Tuscan-Emilian and Romagna ridge, limestone in
the Umbrian-Marches sequence, the crystalline and metamorphic rocks of
the Apuan Alps, and the remains of the metamorphic basement with granitoid
intrusions (which formed during the Hercynian orogenesis, dating back to the
end of the Carboniferous, about 300 million years ago) of the CalabrianPeloritan arc.
This variety of rocks implies varying resistance to external agents and surface water flow, and thus also influences solid transport by watercourses.
The total solid transport of the Appennine rivers has a higher percentage of
suspended components than bottom sediments, in comparison with Alpine
ones.
Another difference lies in the general distribution of relief energy, also defined as relative altitude. The mean altitudes of the various catchment basins
are greater in the Alps, as shown by the highest peaks of the two chains.
High altitudes influence the microclimate of catchment basins and therefore
the type of water supply: the highest ones have nivo-glacial situations. Ice
and snow also imply considerable physical disintegration (cryoclastism), capable of forming great quantities of debris. This is carried to rivers by surface
water flow, the force of gravity and avalanches.
Alpine catchment basins are also usually larger than Appennine ones, and
this generally influences the supply, flow and length of watercourses.
The two mountain chains also underwent different glacier modelling processes. During the last glacial maximum, which ended about 15,000 years ago,
the Alps were buried under a single ice cap, from which only the highest and
sharpest peaks emerged, whereas the Appennines had small glaciers which
originated near the highest north-facing ridges. Glaciers have not only modelled the main valleys, but have also produced huge amounts of sediments,
most of which are still stored in catchment basins. This debris is easily carried
by streams in far larger quantities than those deriving from physical disintegration in present-day morpho-climatic conditions.
■ Hydrography
Catchment basins. Water flowing on relief surfaces is organized in hydrographic networks, and all watercourses, including occasional ones, drain a
certain catchment basin. This basin is the area in which - assuming nil infiltration and evapotranspiration, that is, imagining surface impermeable and
devoid of vegetal cover - any liquid or solid precipitation (rain, snow, hail, etc.)
is conveyed to the main channel subtending water flow in the whole area.
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3rd ORDER BASIN
5 th ORDER BASIN LIMIT
2
2 nd ORDER BASIN
2
2
2
2
3
3
1st ORDER BASIN
2
3
2
3
2
a
b
c
d
e
f
2
4
2
2
2
2
2
2 3
2
2
4
5
3
3
2
2
2
5
2
1st ORDER
2nd ORDER
3rd ORDER
2
2
2
4 th ORDER
5th ORDER
4th ORDER BASIN
0
2
4 km
BASIN LIMIT
Catchment basins and hydrographical networks
may be analysed from the morphometric point of
view by dividing them into hierarchically
organized streams.
First-order streams are fed by springs or
streams; two first-order streams unite to form a
second-order stream; two second-order streams
unite to form a third-order stream, and so on.
Second-order streams subtend basins of the
same order. These basins constitute sub-basins
of higher order basins, e.g., the catchment basin
of the stream Isorno, between Piedmont and the
Canton of Ticino (Switzerland) and its catchment
basin.
One catchment basin borders on others and is separated from them by a
boundary called a watershed. Surface and subterranean watersheds do not
always coincide, as infiltration may produce subterranean flows towards other
basins (loss) or from them (gain).
The pattern of the hydrographic network of a basin, composed of the main
watercourse and its tributaries, is called drainage pattern. It may take on
many different forms, mainly according to geological structure.
Evolution of a hydrographic network. The development of the hydrographic
network of the entire circum-Mediterranean area, including the Italian reliefs,
was greatly influenced by an important event which occurred during the
Messinian (Upper Miocene, 4-5 million years ago), when the Mediterranean
was isolated from the Atlantic Ocean. Until then, the two had communicated
with each other through two passages, one north of the Betic chain, in present-day southern Spain, and another south of the Rif chain, in present-day
Morocco. The approach of Africa to Europe closed both passages (first north,
then south) in a few tens of thousands of years.
The result of this closure was the almost total evaporation of the Mediterranean during a hot period, to such an extent that evaporites (especially gyp-
Dendritic drainage patterns (a) are typical of soils
containing homogeneous clay, such as the hills of
the northern Appennines.
Parallel networks (b) are influenced by highgradient slopes, like those of Alpine valleys.
Rectangular networks (c) are associated with
faults and fractures and are typically found
in the Trentino Prealps.
Radial networks characterize isolated reliefs, e.g.,
the Euganean Hills (centrifugal, d), or depressions
such as the volcanic lakes of Latium (centripetal, e).
Deranged networks are found in geologically
young or particularly eroded areas, such as the
Carso (f).
sum) were deposited in some areas. At the same time, in land emerging
above sea level, hydrographic networks receded, due to regressive erosion
starting from the lowest sea level. The extent of this deepening of the hydrographic network may be fully understood if we consider that the depression
base of the main pre-Alpine lakes (Garda, Maggiore, Como, Iseo) is more than
500 m below the present-day sea level. Even though these lake depressions
have been filled in by sediments over the last 5 million years, their bottoms
are still below sea level.
Other critical periods for the Alpine and Appenine hydrographic networks
were the great Pleistocene glaciations (1,800,000-10,000 years ago). In the
Alps, glacial deposits identify at least five peaks which correspond to the
great glaciations, called Donau, Günz, Mindel, Riss and Würm.
During cold phases, catchment basins underwent complex, non-homogeneous situations all along the Italian peninsula. The Alps, for example, were
covered by a single great ice cap extending to the Po Plain, from which only
the highest reliefs emerged. Precipitation was less abundant than now, due to
the establishment of anticyclonic conditions over the glacial mass.
The surrounding plains were steppe-like (dry and cold), and reliefs were characterized by intense glacial processes which contributed both to remodelling
18
An Appennine stream (Ponte della Brusia, Bocconi, Forlì)
of the main Alpine valleys and to overall deepening of the network. In these
periods, many base levels were reshaped; glacial excavation of the main valleys
and the settling in them of very thick layers of ice supported higher base levels
for side tributaries, producing what today are suspended valleys, where the
tributaries flow into the main valley over a “throw”, giving rise to sometimes
spectacular waterfalls.
During the glacial epochs, debris was produced in great quantities and the
network deepened; huge deposits of sediments, for instance, accumulated in
the valleys at outlets to the plain, as far as the Po itself. The sea, the level of
which was about 110 m lower than it is today, did not influence the mountain
portion of the network to any significant extent.
During the glacial phases in the Appennines, there were no large glaciers, and
not even tongues of ice reached the surrounding plains. The landscape was
dominated by periglacial (cryonival) conditions; only in the innermost areas of
the northern Appennines (Tuscan-Emilian ridge) and on the Abruzzi reliefs did
small or medium-sized glaciers appear.
Periglacial conditions implied intense physical disintegration, which was not
followed by corresponding deepening of the hydrographic network. The latter
could not remove all the sediments produced on its flanks and conveyed to
its watercourses. “Grèzes litées” are the characteristic deposits still found
on the flanks of the Umbrian-Marches regions. In the Po Plain, for instance,
during the last glacial maximum, gravel forming alluvial fans was deposited,
and now makes up the feed area for the Emilian plain.
After the last glaciation (Holocene), changes in the network were minimal and
influenced the type of vegetal cover, and particularly its density. This was
partly conditioned by the climate, with slight variations in temperature and humidity during the Holocene, but mostly by the rise in human populations and
their relative activities, starting from the Neolithic and even more in Roman
times. Several periods may be identified: severe network erosion following
deforestation (especially in the Neolithic, in Roman times and the modern
age), and minor erosion, in the event of stability, following mountain abandonment and deforestation, as in the early Middle Ages or the second half of
the 20th century. The latter period is one of the most difficult to interpret, as
abandonment of mountain areas, which causes reafforestation, accompanied
other human actions, with contrasting effects.
Abandonment itself, for instance, produces increased soil erosion in the short
term, as woodland and slope maintenance are not carried out; lack of control
and economic interests may give rise to the practice of deliberately setting
fire to vegetation, resulting in large-scale erosion in the rainy season. Artificial
operations (works to regulate water flow, artificial reafforestation, etc.) have
the opposite effect, i.e., they thwart erosive processes.
Watercourse supply. Supply to watercourses is one of the most important
parameters which determine the quantity and quality of available waters in
streams, as well as their regimen, or seasonal flow capacity. Watercourses
which drain small basins with homogeneous lithologies are characterized by
simple supply due to precipitation, melting of ice or snow, or the emergence
of aquifers. In larger basins, water supply is complex, and is influenced by the
various types of flow supplied by tributaries.
There may be pluvial supply when the water flowing in rivers comes from rainfall. Rivers with this kind of supply have regimes characterized by high or low
waters, according to periods of maximum rainfall or dry seasons respectively.
This seasonal trend is therefore influenced by the general climatic conditions
typical of the catchment basin.
In the northern Appennines, where feed is typically pluvial, high waters are
more common in autumn than in spring. Low waters are typical of winter and
especially summer, as a consequence of both high evapotranspiration and
reduced supply. In areas with particular climatic conditions, like Liguria or
southern Piedmont, the proximity of the Alpine and Appennine chains to the
coast interferes with air masses coming in from the Atlantic, which may cause
19
20
intense, late summer storms which are
responsible for catastrophic floods.
Similar situations are typical of the
Calabrian region and to a certain extent the whole western Appennine
chain. Another peculiar situation may
be found in the Friuli and Veneto Prealps, where cold air masses wedge in
from central Europe and sometimes
Siberia. These cause fronts to settle in
the area for long periods, especially in
autumn, when they give rise to abundant rainfall.
Glacial supply occurs when watercourses derive from the melting of ice.
In this case, high waters are recorded
in periods when melting is intense,
Meltwater is a form of supply for mountain
streams
i.e., in summer, whereas low waters
are typical of winter.
When water supply does not depend on the melting of ice but of snow, high
waters are anticipated to spring and decrease in warmer periods, when the
snow cover is reduced. Snow supply prevails in the high-altitude basins of
the Italian mountain regions; high waters peak in May-June and then decrease until the following spring.
Watercourses with snow supply may be found at high altitudes in the Western
Alps, and the water consists of melted ice and snow. Many tributaries of the
Dora Baltea, Dora Riparia, Sesia and Ticino are supplied in this way, as well
as some tributaries of the Adige along the Eastern Alps.
Springs. In mountain streams, springs may determine the regime of watercourses. When the quantity of water only amounts to a trickle, the result is a
small pool; when the flow may be gauged, it is called a “true spring”. Many
names are given to water originating from springs: true springs, “fountainheads”, “veins”, and in the case of anthropic intervention, “fountains”.
Streams carry water emerging from aquifers. In the case of permeable rocks,
aquifers originate in sediments in which the speed of water flow depends on
the slope and permeability of the terrains crossed. Springs originate when the
roof of the aquifer meets the topographic surface. A spring is thus an area,
small or large, where the subterranean aquifer meets the surface of the
Types of springs
Mauro Marchetti · Mario Panizza
Emergence springs ( a ) have various
locations and may temporarily
disappear as the flow of the aquifer
varies during the year. If they emerge
in a valley, they appear at lower
altitudes as the watertable sinks, and
reach high altitudes during maximum
level phases. If they are in a cavity,
they may disappear during low
watertable levels.
Overflowing springs ( b ) originate
where waters in subterranean
chambers touch an overflow threshold
or reach the surface.
Contact springs ( c ) are very common
and flow out when the aquifer flows in
permeable soil after touching an
impermeable formation. This contact
may be stratigraphic (transgressions,
etc.) or tectonic in origin (overthrusts,
faults, etc.), or due to recent forms
and deposits (landslides, moraines,
a
b
e
d
floods, etc. on less permeable
lithotypes underneath).
These springs have fixed locations
which cannot be altered by changes
in watertable level. When the latter
changes, they may vary in flow, or
gush out only when the watertable
level is high.
Karstic springs ( d ) in carbonatic
massifs are a typical example: here,
permeability is principally due to flow
in cracks and fractures rather than to
the permeability of the rock itself.
Barrage springs ( e ) result from
subterranean water that emerges
when impermeable obstacles hinder
its flow.
Fissure springs are caused by the
presence of faults, fractures and any
other type of preferential course, e.g.,
karstic conduits, which allow water to
flow out at a precise spot.
c
21
22
ground and emerges. Water may leave the aquifer because it filters through
sediments, porous rocks, or preferential conduits, generally discontinuous
surfaces (stratified surfaces, formations with different degrees of permeability,
cracks, faults, etc.).
There are many kinds of mountain springs, among which are emergence,
overflowing, contact, barrage, and fissure springs.
Springs may be isolated, grouped in restricted areas, or aligned along particular geological structures (for instance, between permeable and impermeable formations). These alignments may be particularly frequent, as flysch
in the Appennines or sandstone overlying the fine, impermeable rocks of the
high Emilian Appennines (e.g., at the foot of the “Pietra di Bismantova”).
Chemico-physical characteristics of water. Mountain stream waters are
characterized by a thermal regime which depends on their supply, and by
chemical composition which depends on the surface terrains they cross and, particularly for spring waters, on the composition of the rocks crossed
before reaching the surface. Water chemistry is determined by dissolved
ions. Their transport in solution does not depend on watercourse energy, i.e.,
water flow and speed, but rather on the composition of the rocks crossed and
their aggressivity (pH).
The most common dissolved ions are carbonate (CO3=), chlorine (Cl-) and sulphate (SO4=) ions, as well as sodium (Na+), potassium (K+), calcium (Ca++) and
magnesium (Mg++) cations, and dissolved silica (SiO2).
The predominance of some ions over others depends on the terrains crossed,
for example, waters flowing inside carbonatic massifs have basic pH and are
rich in calcium ions and carbonates in general. Waters that cross crystalline
massifs where quartz and feldspars abound (e.g., granite, gneiss and schist),
have acid pH and are rich in silica, like those of the innermost part of the
Alpine chain. Waters crossing a particular terrain, like Triassic gypsum in the
Umbrian-Marches chain, are rich in calcium and sulphates.
Sodium and chlorine waters are found in salty soil recently abandoned by the
sea (generally in coastal plains) or areas with mineral springs. Higher concentrations of magnesium ions are recorded in areas rich in this element, as in
the Dolomites.
Transport in solution is gauged through laboratory tests which determine the
contents of dissolved ions; this datum is expressed in different units of measurement. One of the most common is hardness, i.e., the content of calcium
and magnesium in water (total hardness).
Hardness is measured in hydrotimetric degrees, the most common of which
23
High-altitude karstic area (Julian Alps) without surface drainage network
are French degrees (1°F corresponds to 10 mg/l of Ca++ ion). According to this
unit of measurement, waters are classified as: very soft (0-7), soft (7-14),
moderately hard (14-22), quite hard (22-32), hard (32-54) and very hard (>54).
Total solid residues at 180°C are easy to find: a litre of water evaporates at
180°C. Waters derived from the melting of snow are particularly acid, due to
few dissolved cations and abundant carbonic acid.
In some areas of the Italian reliefs water chemistry may be modified by
anthropic causes, mainly related to the intense exploitation of Alpine areas
by tourists. The consequences of this great demographic pressure has led to
sometimes significant increases in pollutants: coliform organisms, heavy
metals, nitrates and phosphates.
Abnormal increases in hydrocarbon and chloride are recorded near main
roads, due to the scattering of chlorinated salts for de-icing. Livestock farming areas (cattle-breeding in the Alps and sheep-breeding in the Appennines)
lead to high values of dissolved nitrates and nitrites.
Hydraulic systems. Watercourses may be divided into two groups: rivers
and streams. The two groups partially overlap over a wide transition area.
A river is a perennial watercourse with low speed and slope (less than 0.5%
gradient). Streams or seasonal torrents are characterized by higher speeds and
gradients; the term torrent, in Latin torrens, derives from “torreo”, or bubbling.
Velocity and stream discharge
Mauro Marchetti · Mario Panizza
Velocity
Stream discharge
Water velocity is the best indicator
of stream energy. The faster waters
flow, the more intensely stream beds
are eroded; slow-moving waters give
rise to sedimentation processes.
Velocity is not constant or regular
along streams; in rectilinear
stretches, fast waters usually
concentrate in the middle of the
channel, just under the free surface;
in curved ones, they concentrate
near the concave bank.
Stream discharge (Q) is the volume of
water flowing in a stream in a specified
unit of time, and is expressed by the
formula:
Q=Av
in which A is cross-section area and v
is average stream velocity. Stream
discharge is measured in m3/sec or
sometimes in l/sec, by calculating the
hydrometric level of water.
Their trends in time are analysed with
diagrams called hydrograms.
1000
cm/sec
RECTILINEAR STRETCH
+
EROSION
100
10
CURVED STRETCH
TRANSPORT
+
NV
EX
NK
CO
BA
1
BA
This term is often also used in a figurative way to describe the intensity of
flow in particular circumstances, like “a torrent of words”.
The speeds of mountain streams vary and are affected by water flow and
type of supply. Low waters may be perfectly calm and their speed very low,
about 0.1 m/s, whereas high-altitude waters have a high speed, over 10 m/s.
Therefore, even if streams have minor total flows, they have a far greater
competence.
Watercourse competence is determined by the maximum size of the single
clastic rocks carried. This size is strictly controlled by the stream speed and its
depth. Instead, total solid transport is proportional to the water flow. Increased
volume of transported material (solid load) and corresponding decrease in
the particle size of transported material (competence) are observed as
springs flow from the upper parts of their catchment basins to the river
mouth.
The collection and sedimentation which occur along streams change their
longitudinal profile. Erosion along a portion of the stream lowers the gradient
of the watercourse downstream and raises it upstream, increasing the erosion rate upstream and lowering it downstream. Similarly, sedimentation
along a portion of the stream lowers its gradient upstream and raises it downstream. When every part of the longitudinal profile maintains the same gradient over a sufficiently long period (e.g., one year), the watercourse is said to
have reached balance.
Streams reach their characteristic balanced profile through erosion and sedimentation processes. When this occurs, the whole volume of debris which
reaches the stream bed from nearby slopes and upstream, is carried away
downstream. From the viewpoint of transported sediments, a balanced
stream therefore has a nil balance. A diagram with the ordinates showing elevations above sea level and the abscissa the distance from springs, will show
that the longitudinal profile of a balanced watercourse is theoretically hyperbolical, with gradients steadily falling from spring to river mouth.
The river mouth is a fixed point of reference, called base level, below which
watercourses cannot deepen their beds. The general base level is the sea,
although there may be local base levels due to the confluence of streams in
hierarchically more important watercourses, or their flow into natural or artificial lakes.
Huge clastic concentrations, larger than those of stream competence, are
visible along mountain streams. These large pebbles or boulders are not
carried by the stream water, but by gravity along its flanks or exhumation by
water of smaller clastic rocks, until the channel banks and bottom are almost
NK
SEDIMENTATION
VE
24
CO
A
NC
0.1
0.001
0.01
0.1
1
10 mm 100
Distribution of lines connecting points where
the current has equal velocity in rectilinear (up)
and curved stream beds (down).
Fast currents are shown in red: in a curved
strectch they are close to the concave bank
Hjulström’s diagram, showing that erosion,
transport and sedimentation all depend on
water velocity.
x: diameter of debris in mm;
y: current velocity in cm/sec
In the early 20th century, it was
already clear that erosion, transport
and sedimentation all depend on
water velocity, as shown in
Hjulström’s diagram of 1935.
This demonstrates that the finest
particles (diameter less than 0.05
mm) are not sedimented, even if the
water is almost still, but float in
suspension as far as the streams
reach the lakes or seas, which
represent base levels
In Italy, the National Hydrographical
Service provides many measuring
stations, and the data collected are
published.
Past floods recorded over long
periods and then statistically
processed, can predict peak
stream discharges in 100, 200 or
even 1000 years. Those with
particular return periods are used to
design special works along
watercourses.
25
26
completely made up of immovable
material. In such conditions, channels
cannot be eroded further, as the
coarse clastic rocks protect underlying deposits with finer particle sizes
from erosion.
The arrangement of large rocks in
mountain streams is very important
because it contributes to the formation of clastic aggregates supported
by larger blocks. These determine the
typical terraced profile which AngloSaxons call “step and pool”. The
spacing and relief of this type of discontinuity is important both to assess
the dispersion of energy in the stream,
due to the roughness of the river bed
Large amounts of solids make waters milky
(Savio stream, northern Appennines)
and the hydraulic throws which are
responsible for downstream deceleration at every step, and for the ecology of the stream itself, as shallow, fast
stretches alternate with calm, deep ones.
Solid transport in mountain streams is deeply affected by the rocks of the
catchment basin, the stability of flanks near the stream, active processes prevailing in those reliefs, and the type of feed of the stream itself. Prevailing
types of solid transport are thus impossible to define according to hydraulic
characteristics (e.g., flows and speeds). Streams crossing easily erodible fine
rocks, like those of the Emilian Appennines, have higher suspended transport
than their bottom sediments. In such areas, mainly during intense autumn
rains, the waters become cloudy following surface leaching of rocks in the
supply basin and slow gravitational processes along flanks.
In streams supplied by ice meltwater, suspended transport makes waters
milky-blue in colour. Streams which cross areas with outcropping competent
rocks - calcareous, dolomitic or crystalline - have more bottom sediments
than suspended components, especially if the material reaching the river bed
derives from cryoclastism and if transport down slopes is due mainly to collapse rather than to surface leaching. Generally, there is a clear difference in
types of transport during high and low water phases. Low waters are characterized by little bottom transport, whereas suspended transport continues,
particularly if clay or fine sediments are occur along the river bed.
27
Steps and pools
Flora and vegetation
MARCO CANTONATI · KARIN ORTLER
■ Algae and aquatic lichens
In normal conditions, i.e., when disastrous events like sudden, great floods
or chemical pollution have not occurred, the bed of a stream sparkles
with shades of colour ranging from
dark brown or black, to reddish, to all
nuances of green and blue-green.
Streams owe their bright colours to algae, aquatic lichens and mosses
(whereby German scholars use the
Vegetation along the banks of a stream
term Vegetationsfärbungen = colours
due to aquatic vegetation). The banks,
unless artificially straightened or reinforced with concrete, are never bare, but
richly covered with aquatic plants, bushes and trees which need increasingly
less water and become more and more intolerant of submersion with distance
from the stream. While aquatic and hygrophilous mosses are frequent in
spring areas, and higher aquatic plants prefer slow-moving waters with sandy
or muddy bottoms typical of large plain rivers, mountain streams with fastmoving waters are characterized by algae. These organisms are generally associated with beaches, or sometimes lakes, not with mountain streams. However, even if often overlooked, they are quick at colonizing any watercourse,
whether small or impetuous, in plains as in mountains.
The term “algae” is commonly used, but it often inappropriately identifies true
higher plants which have adapted to life in an aquatic environment, such as
the water buttercup. The term is inaccurate also from a strictly systematic
point of view, and is nowadays mostly used informally to indicate a highly varied group of organisms. “Algae” better defines aquatic plants of various sizes
and with heterogeneous organization, without specialized vascular tissues,
and not separated into the usual roots, stem and leaves (i.e., they are thallophytes, not higher plants).
Springs of Arzino in Carnia (Friuli): algae and lichens cover most of the emerging rocks
29
Algae range in size from few thousandths of a millimetre in the case of
cyanobacteria (blue-green algae) to tens of metres in giant brown algae
(Macrocystis), which grow on ocean shores. The cells of these plants present
the two main structural patterns of the animated world: prokaryotic, having
cells without a differentiated nucleus, typical of bacteria and cyanobacteria,
and eukaryotic, with a nucleus clearly distinguished from the cytoplasm and
enveloped in membranes, typical of all other organisms. The feature these algal types have in common is the way in which they procure energy for life,
through oxygenic photosynthesis, by means of a green photosynthetic pigment called “chlorophyll a”, typical of both cyanobacteria and plants. However, individual algal groups differ enormously for the other pigments they
contain (accessory and photoprotective), as well as for their ultrastructural,
physiological and biochemical characteristics and reproductive system, in
the same way as man is different from a sea-urchin or an insect.
CYANOBACTERIA
DIATOMS
GREEN ALGAE
MARCH
1 mm
30
APRIL
MAY - JUNE
Seasonal variations in algal populations (sizes of the individual algae are indicative and colours are only
evocative) on stream rocks and pebbles (microscopic aspect)
■ Seasonal variations of algae in streams
In temperate climates, seasonal trends affect the presence and development of
many species of algae in streams. The cyanoprokaryote Phormidium autumnale, as its name suggests, is usually found in autumn; the chrysophyte Hydrurus foetidus prefers the cold months of autumn and winter; the rhodophyte
Bangia atropurpurea develops in early spring. The variations in the abundances
of these macroalgae are macroscopically visible and can be quantified by measuring cover and thickness on the spot. As regards diatoms, this is only possible for exceptional blooms of some species occurring in specific seasons
(e.g., Diatoma spp., Gomphonema olivaceum var. calcareum, Melosira varians).
Many works devoted to streams indicate that variations of the most abundant
species in diatom associations, in percentual terms, are not very great.
Algal groups in mountain streams
Cyanobacteria. Cyanobacteria often
colonize the most inhospitable areas of
streams, where environmental conditions
are the most severe, e.g., areas where
water may flow suddenly, change and
periodically cease entirely, or rocks along
the banks that are only occasionally
splashed or sprayed by turbulent waters.
Cyanobacteria are also called
cyanoprokaryotes, as they do not have an
eukaryotic structure like algae, but are
prokaryotic, like bacteria. Unlike
photosynthetic bacteria which contain
bacteriochlorophyll, they contain
chlorophyll a, and their photosynthesis is
oxygenic like that of algae, mosses and
higher plants. Many of them have a
sufficiently high number of characteristics
to be taxonomically analysed with
methods traditionally applied to algae,
i.e., morphological observations by
optical, Scanning or Transmission
Electron Microscopyes.
So far, cyanobacteria have therefore been
studied by phycologists as “blue-green
algae” or “cyanophytes”.
Marco Cantonati
However, “los bacteriólogos
chauvinistas”, as the well-known Spanish
ecologist and phycologist Ramón
Margalef defined them, claim that
cyanobacteria should only be studied
using methods applied to bacteria.
This controversy is based on the
structural (prokaryotic) simplicity of these
organisms, which resembles that of
bacteria and enables them to colonize
inhospitable environments like the Arctic
tundra, rocky mountain slopes and hightemperature waters (for this very reason,
bacteria were among the first organisms
to appear on the Earth billions of years
ago). In these environments,
cyanobacteria are subject to intense solar
radiation, particularly by ultraviolet light,
which can seriously damage their DNA.
They react with a whole series of
adaptations, like actively moving away
from excessive light, generating efficient
mechanisms to repair molecular damage,
and producing photoprotective
substances. Nowadays, concern for the
reduced ozone in the upper atmosphere
Dark brown spots are colonies of Chamaesiphon geitleri, a cyanobacterium typically found in mountain
streams with carbonate substrates and clean, gushing waters. Right: cells of the cyanobacterium
enveloped by an irregular sheath, as seen under the microscope (1000 X)
31
32
requires deeper knowledge of
contrivances adopted by organisms to
protect themselves from ultraviolet
radiation, and these physiological and
biochemical aspects are therefore studied
very closely.
The main accessory pigments of
cyanobacteria are blue phycocyanin and
red phycoerythrin, which endow
mountain stream beds with bright
colours: red or reddish in some areas or
on rocks; small, dark brown or turquoise
spots glowing under stones, and dark
green spots on pebbles. Some species
protect themselves from dehydration with
strong sheaths enveloping their cells, like
Chamaesiphon polonicus, which is
generally found in areas of the stream
characterized by occasional or seasonal
low water or even none at all. Stream
cyanobacteria prefer epilithic substrates,
i.e., rocks and pebbles. They may also
grow on other substrates, such as moss,
aquatic plants, or other cyanobacteria
(like Chamaesiphon amethystinus, which
is found on the filaments of another
cyanoprokaryote, Tolypothrix distorta).
Most taxa (= taxonomic groups, like
genera and species) are widespread.
However, there are also very rare species,
such as the above-mentioned
Chamaesiphon amethystinus. In 1999,
Eugen Rott and co-workers identified
algae in Austrian waters (analysing 225
watercourses from various aspects), and
classified almost half the cyanobacteria
as “very rare” on the basis of their
frequency. Many taxa are good indicators
of water quality.
Diatoms. Diatoms make up the largest
group of algae as regards number of
species and individuals in a stream and,
generally, in aquatic environments. There
are few kinds of watercourses which
diatoms do not colonize, among which
Banks of a mountain stream covered by the efflorescence of the microalga Diatoma. Bottom right:
bands formed of algal cells (about 60 X); top right: the silicic frustules of these diatoms have a
rectangular section when seen from the side (400 X)
are “typical” glacial streams,
characterized by high discharge
(especially in the hottest hours of the day
in spring and in summer), and rich in
suspended sediments (glacial flow).
Otherwise, these algae are found in
almost any humid area, from cracks in the
desert floor to the oceans. They are
microscopic (a few thousandths of a
millimetre) unicellular organisms, but they
may aggregate in large groups which,
when they bloom, form golden to dark
brown patinas, velvety covers, or thin
filaments. This is the case of species
belonging to the genus Diatoma, whose
cells adhere to one another forming long
bands and, in particular conditions,
accumulating to cover the beds of slowmoving watercourses.
Every diatom cell is contained in two
valves made of amorphous silica,
structurally very similar to opal and glass.
These valves fit together like the box and
lid of a shoe box: bilateral and radial
symmetries are the most common types
among diatoms. According to this simple
criterion, diatoms are subdivided into two
large groups, Pennales and Centrales.
This subdivision has deep ecological and
morphological significance, as it is based
on the two large habitats colonized by
diatoms: benthic, i.e., shores and
watercourse bottoms, and planktonic, i.e.,
free waters typical of lakes and oceans.
Diatoms are so small that they are little
known to non-specialists, but they are
very important in aquatic environments.
They frequently dominate oceanic
plankton and constitute the basis of the
food chains on which large-scale sectors
of the fishing industry, and therefore the
nourishment of entire human populations,
depends. They make up about a quarter
of global primary production, exceeding
rainforests and savannahs which are by
far the most productive terrestrial biomes.
The elegant shapes of diatoms are still a
source of inspiration for artistic works, in
which valves are embedded in resin and
juxtaposed on slides. The valves owe
their beauty to the rich and complex
series of their morphological details
(spots, lines, bands, etc.) which are the
signatures of the various species.
Identification is carried out by arranging
the valves on slides after cleaning them
of organic substances and carbonates.
Identification of diatom species is
therefore based on the structural details
of the silicic “box” which contains their
cells. One of the most evident structural
characteristics of Pennales is the thin slit,
called raphe, which runs along the valve.
It has long been thought that this
structure enables certain diatoms to
move, but the exact mechanism is very
complex and is still being studied.
The cell seems to emit “small molecular
rods” from the raphe opening: these rods
adhere to the substratum at one end,
and hook themselves to the cell
membrane on the other. The cell moves
forward by contracting, the rods unhook,
and then the process is repeated.
The ability to move actively is particularly
important for species living on mud
(epipelic habitat) or other soft substrates,
as the micro-algae are enabled to move
to illuminated surfaces when they need
to (species of the genera Navicula and
Nitzschia are typically mobile and
epipelic).
Other species (like those of the genus
Cocconeis) have a raphe only in one of
the two valves, and anchor themselves
firmly to hard substrates (e.g., stones
33
34
and rocks, epilithic habitat) by means of
mucilage produced by the raphe itself. In
yet other cases, they adhere with strong
mucilaginous stalks which are secreted
by pores at one or both ends of the
valve. The variety of movement and
adherence mechanisms enables diatoms
to colonize all areas of mountain
streams: central zones with fast-moving
water, or small side bays where organic
material collects easily.
The many species and their different
ecological needs allow the colonization
of various kinds of streams, from waters
with very little mineralization to those rich
in carbonates, from clean waters to
others organically polluted or
contaminated with heavy metals.
Chrysophytes. Chrysophytes, or goldenbrown algae, are similar to diatoms as
regards their accessory pigments and
reserve substances, and include
planktonic species with silicic scales
which form typical silicified cysts.
However, only one species of this group
lives in mountain streams, Hydrurus
foetidus. Fringed filamentous structures,
several tens of centimetres long, which
characterize these chrysophytes and turn
the beds of mountain streams almost
black, particularly on carbonatic rocks,
are often seen in autumn or winter.
Great quantities of these algae may even
be smelled in areas with a moderate
increase in algal nutrients (e.g., near an
Alpine cattle barn). This smell
(unpleasant, recalling that of rotten fish)
becomes pungent if one of these
filamentous, mucilaginous structures is
squashed - hence its name “foetidus”.
Microscopic examination reveals that the
cells of this alga, which are greenish and
golden-brown ovoid structures a few
thousandths of a millimetre in size, are
actually immersed in a mucilaginous
matrix inside the filamentous structure,
technically called coenobium. The
species is usually found in cold waters,
from the mouths of glaciers to medium
altitudes. Flow and light may play an
important role in the development of this
alga, since it tends to reduce its presence
in summer even in mountain springs
where water temperature is almost
constant and low also during summer.
Chlorophytes. In mountain streams, the
brown shades of diatoms and
chrysophytes alternate with the bluegreen, turquoise or reddish cyanobacteria
and, in particular, when organic pollution
The filamentous structures are cenobia of Hydrudus foetidus, typical of fast-flowing stream stretches,
particularly in autumn and winter. Right: the cells of Hydrudus foetidus (green ovoid structures) under
the microscope; the elongated, arched shapes on the right are cells of the Diatoma Fragilaria arcus
occurs and in summer with patches of
greenish filaments or green crusty covers.
Tufts of green filaments frequently grow
on aquatic moss. These algae belong to
the large group of green algae or
chlorophytes, the only ones to contain the
same chlorophyllic pigments (chlorophyll
a and b) and the same reserve substance
(starch) as mosses, ferns and higher
plants. Areas of the stream subject to
flow variations reveal unbranched
filaments of green algae of the order
Ulotrichales. Those belonging to the order
Zygnemales are found in calm, clean
water. Microscopic examination easily
identifies the three most common genera
of this group according to the shape of
their chloroplasts: spiral in Spirogyra,
band-like in Mougeotia and star-shaped
in Zygnema. Representatives of the
Desmidiales are rare in streams, although
they may be found in small side bays
where sand and organic substances are
deposited. These are considered among
the most beautiful of all algae. Desmids
are frequently found in environments like
peat-bogs, and are characterized by cells
symmetrically arranged on both sides of a
narrow waist or isthmus, with a richly
decorated cellulose wall which makes
them look like tiny jewels. Springs and
streams on carbonatic rocks may
sometimes reveal the presence of
Oocardium stratum, a rare, clearly
declining species. Its cells are inserted
like plugs in tubular calcareous deposits.
Members of the Cladophorales are
frequently found at mid-low altitudes,
particularly on limestone rocks or in water
rich in nutrients, where they cover
surfaces with their dense, branching
filaments. Their most common genus is
Cladophora, which is characterized by
particular branching and multinucleate
cells, as microscopic examination reveals.
Green algae, however, do not necessarily
turn streams green. In spring and
summer, the banks of at least seasonally
fast-flowing and turbulent mountain
streams are coloured orange and red. The
colour is actually due to green algae of
the order Trentepohliales, located on the
evolutionary line which led to the higher
terrestrial plants. Their cells are filled with
a mixture called hematochrome,
containing carotenoid pigments.
Charophytes. In the past, charophytes
were classified as green algae, as they
share the same pigments (chlorophyll a
and b) and reserve substances (starch).
Today, charophytes or stoneworts are
A fast-flowing mountain stream. The felty red covers on rocks are due to the green alga Trentepohlia sp.
Right: enlarged photograph showing branched filaments, which owe their red colour to the presence of
carotenoid pigments
35
generally considered an independent
group, especially because of the peculiar
structure of their thallus and reproductive
structures. They generally live in still
water, but may be found on the sides of
streams, in secondary channels, or areas
where the current is slow and fine
sediments deposit. They definitely prefer
clean waters and carbonatic substrates.
Although non-specialists often confuse
them with aquatic higher plants, they are
macroalgae. They look like, long, small
plants, extending for several tens of
centimetres, with multi-cellular thalli
made up of elongated internodal cells
and shorter nodal cells bearing whorls of
branches, each of limited growth. The
plant is usually covered by carbonate
precipitates, which must be removed with
dilute hydrochloric acid prior to
examination. Many species typically smell
of garlic. The most common genera are
Chara and Nitella, which may form
submerged meadows.
Xanthophytes. Green thalli of peculiar
shape may be found in streams,
sometimes so densely packed that they
resemble that kind of stiff synthetic sponge
used by florists for flower compositions.
They are not green algae, but a species of
the genus Vaucheria, belonging to the
xanthophytes or yellow algae. This group
does not contain chlorophyll b and its
reserve substance is not starch but
chrysolaminarin. Xanthophytes may be
distinguished from green algae by using
reagents which colour starch, like iodine.
Equally important and widespread, the
genus Tribonema has very different
morphology. It includes several species
which form long, unbranched filaments in
mountain streams.
Rhodophytes. Red algae or rhodophytes
are very often found in seawater, and their
presence in fresh water is relatively rare
and limited to a few, albeit very interesting
species.
Hildenbrandia rivularis forms beautiful,
bright red circular spots, especially in
low-altitude streams. Lemanea fluviatilis
has thalli with nodose structures
separated by internodes. It is a
rheophilous species (adapted to the
current) with a wide distribution in
streams and which may even appear in
glacial streams at high altitudes.
The most peculiar species is Bangia
atropurpurea, a red filamentous alga,
almost identical forms of which inhabit
areas of varying altitudes, ocean, sea and
lake shores, rocks and other hard
substrates in various types of streams. In
the last few decades, detailed studies on
its morphology, reproductive cycle,
ecophysiology (adaptation to different
degrees of salinity in cultures in the
laboratory), karyology (analysis of
chromosome number, form and size) and
lately, biochemistry and molecular
genetics, have aimed at determining if
these sea and fresh water forms alike
belong to the same species, or if they are
to be viewed as separate entities.
Audouinella hermanni is frequently found
in mountain streams. It lives on rocks or
other aquatic plants, where it forms
purple-violet thalli a few millimetres long.
Phaeophytes. Very many species of
phaeophytes or brown algae live along
sea and ocean coastlines, including the
largest of all algae. Their presence in
fresh water is limited to a low number of
species (e.g., Heribaudiella fluviatilis),
mainly found at low altitudes.
■ Biogeography of algae in streams and endangered species
The importance of biogeography in explaining algal distribution is difficult to
interpret. The same species may be common to several mountain chains in
different continents (e.g., the Alps and the Himalayas), whereas others seem to
live only in a single mountain massif. Some species of diatoms, for instance,
although widely distributed, cannot be found in some areas: Fragilaria arcus
var. recta lives in Arctic and Antarctic areas and in eastern Asia, but not in
the Alps.
Generally speaking, as most groups of algae develop resistance forms which
can be efficiently carried by the wind, aquatic birds, etc., most species are
cosmopolitan and widely distributed. As regards diatoms, this is particularly
true of species living in polluted or moderately contaminated waters.
Methods for evaluating the quality of waters based on diatoms are therefore
easily applicable or adaptable to streams throughout the world. It is not surprising then, that there are no great differences between the algal flora of
Alpine and Appennine streams. In the Appennines, longitudinal distribution
areas of algal species, detectable along watercourses, are compressed and
located at lower altitudes with local variations.
Natural causes and phenomena of organic pollution which generally increase
EPIPHYTIC
EPILITHIC
ENDOLITHIC
36
sand
mud
EPIPSAMMIC
EPIPELIC
The main habitats of microalgae in streams, showing algal populations typically associated with
substrates, including those which may pierce through carbonatic rocks to reach the interior (endolithic
habitat)
37
downstream, give rise to the typical
longitudinal distribution of algal
species along watercourses. High
altitudes are characterized by epiliChamaesiphon geitleri
thic species which live in clean, cold,
fast-flowing waters (e.g., the cyanobacterium Chamaesiphon polonicus,
6
Hydrurus foetidus
4
diatoms like Diatoma mesodon and
2
0
Fragilaria arcus, the chrysophyte
6
Hydrurus foetidus and the rhodophyte
5
4
Lemanea fluviatilis).
3
Medium altitudes are marked by
2
1
species which require some nutritive
0
salts and can withstand variations in
10
8 °C
temperature (e.g., diatoms like Cym6
4
bella affinis and Cocconeis placen2
0
J F M A M J J A S O N D J F M
tula, the chlorophyte Cladophora glo1989
1990
merata and the xanthophyte Vaucheria
Seasonal variations in the surface cover of
geminata). Species which can tolerate
some algal species, corresponding to variations
in discharge (m /s) and temperature (°C)
high quantities of nutritive salts and
higher temperatures (e.g., the cyanobacteria Oscillatoria spp., diatoms Navicula spp., Nitzschia palea and Surirella
ovata, and desmids Closterium leibleinii and Staurastrum punctulatum) are
found at low altitudes.
In view of the often wide distribution and intimate relationship between environmental conditions and algal species, attempts at drawing up “Red Lists”
(i.e., of endangered species) for algae and particularly diatoms, have led to
the identification of some of the endangered aquatic environments in which
these algae live. For diatoms, these are mainly nutrient-poor (oligotrophic)
freshwaters.
According to Horst Lange-Bertalot, who drew up the Red List of diatoms in
Germany, protection of these environments is particularly difficult, as they
undergo not only direct impacts (e.g., organic pollution), but also receive
supplies of atmospheric pollutants (some of which may be nutrients) with
precipitation. As regards the distribution and rarity of algal species, much
still remains to be done, especially in Italy, since the algal flora of large areas
has not yet been explored or is still scarcely known.
The presumed rarity and distribution of certain taxa may therefore be due to
lack of available information.
14
12
10
8
6
4
2
0
8
6
4
2
0
Phormidium autumnale
m3/sec
38
3
■ Aquatic lichens
Not all the patinas and colourful crusts
found in mountain streams are due to
algae and cyanobacteria: many are
composed of aquatic lichens.
These organisms have not yet been
studied thoroughly, and available information is scarce. Aquatic lichens,
like all lichens, derive from the association (symbiosis) of an alga (chlorophyte or cyanophyte) with a fungus
A lichen of the genus Verrucaria
(ascomycete). The alga provides glucids (sugar) through photosynthesis,
and the fungus offers a protected environment and supplies sufficient water
and salts from outside. Covers with variously well-defined edges may often
be seen in streams.
The colours of their thalli range from grey, black and dark brown to greenish,
and they bear black ovoid structures. These are called perithecia and contain the spores of the fungus. The perithecia may be very obvious and look
like verrucas or warts - hence, the common name of the widespread genus
Verrucaria. Its different species colonize siliceous and carbonatic substrates
and fast-moving areas of streams, as well as marginal ones barely sprayed
by water.
Light conditions are very important in the distribution of aquatic lichens.
Among the species which can tolerate reduced light are Verrucaria hydrela
and Porina chlorotica. Toleration to drought varies according to species.
Some (e.g., Verrucaria funckii) cannot withstand even short periods of subaerial exposure. Others are sensitive to prolonged submersion (e.g., Dermatocarpon luridum, which colonizes areas that are submerged only during
floods).
The result is vertical zonation of species with respect to low and high water,
which becomes particularly evident on rocks with large homogeneous surfaces which are partly bathed by water. This kind of vertical distribution is also
typical of aquatic and hygrophilous mosses and will be discussed later. Some
lichen form typical associations with species of moss and hepaticas in small
streams at medium altitudes, especially on siliceous substrates. Analysis of
these associations provides useful information about the quality and integrity
of such mountain streams.
39
40
■ Bryophytes and higher plants
Mountain streams have fast, turbulent waters, undergo intense erosion and
are cold - all unfavourable to the development of bryophytes and vascular
plants. Although fast-moving waters do limit the growth of mosses and
aquatic plants, many species are widespread. Current speed and substratum
lithology (the latter influences water chemistry: acid, around neutral or alkaline
waters) are the main factors in determining which plant communities can
adapt to life in streams.
Vegetation on banks and beds is more highly developed. As water flow
undergoes seasonal changes, there may be different types of plant associations. These parts of stream beds which frequently undergo small variations in water level are colonized by pioneer plants, capable of growing and
fructifying rapidly during periods of low water, before later floods submerge
the area again.
The edges of this vegetation, in areas subjected to seasonal regular (but less
frequent) variations in water level, is characterized by bushes which can tolerate flooding, such as willows. In areas which are only flooded occasionally,
colonies of bushes and trees make up the riverbank woodland.
In order to survive in these particular environmental conditions, subjected to
considerable changes over the seasons, higher plants have had to adapt
themselves to both flooding during high water, and drought during periods of
little or no water. The following paragraphs explain what mechanisms they
exploit.
The distinctive and perhaps most fascinating feature of these environments
is their remarkable dynamism. During high waters, the stream shifts its bed,
carries considerable quantities of large-sized material (up to one-third of
the water volume!), reshapes the course of its channels by submerging old
obstacles and creating new ones, and erodes its banks and the foot of
mountain slopes while depositing coarse material in plain areas.
This continual, extreme dynamism contrasts with the need for stability of
man’s villages and his works. People living near watercourses, whether in
the Alps or in the Appennines, have often had to face the consequences of
hydrogeological disorders such as landslides, instability, solid transport
and erosion. The vegetation growing on banks outside the area flooded
during high water (alluvial plain), is not especially linked with the watercourse (except for some species which reveal the humidity of the undergrowth in these areas).
A typical mountain stream bed
In mountain streams, various kinds of vegetation colonize areas which are frequently flooded
41
43
42
Comparison between natural and semi-artificial stream environments
Bank vegetation
Disorders caused by streams, such as erosion at the foot of slopes, may be
remedied by planting suitable grasses, shrubs and trees on the banks.
Unlike rthe zonal vegetation which, if undisturbed, depends on local climatic
conditions and is therefore restricted, the vegetation which is closely linked
with watercourses depends to a lesser extent on climate and is therefore very
uniform in its essential structure throughout central Europe.
Mountain streams in general must be considered as habitats deserving rigorous protection. Streams in natural conditions, like those described in this
chapter, the flow rates of which vary greatly and are colonized by great quantities of bank bushes and trees, are becoming more and more rare. Human
intervention has had a strong impact: besides causing pollution, man has
redesigned the course of streams, constructed artificial banks, or lowered
their beds by excavating sand and gravel from them.
Many specialized plants can no longer survive in such revolutionized environments. In natural conditions, the bank vegetation of a mountain stream carries out essential functions: it offers food and shelter to the local fauna, contributes to the removal of nutritive salts (nitrates and phosphates) which flow
into the stream from the catchment basin, and reinforces banks.
The valleys of watercourses are also important channels for the diffusion of
plants belonging to different vegetational and floristic areas.
44
Many factors contribute to this process: the fact that moving water does
not only carry seeds and vegetative
propagules, but also whole plants or
fragments of them; the reduced competitive pressure of alluvial deposits
and temporarily flooded areas; the formation of new soil during high water;
an efficient supply of nutrients and
water which enable sometimes very
demanding plants to develop; and the
ability of animals living near streams
to transport seeds (essential for the
colonization of upstream areas).
Aquatic flora. There are no vascular
plants adapted exclusively to life in
Bryophytes on the calcareous banks of a
fast-running waters.
mountain stream. Aquatic bryophytes, such as
Platyhypnidium riparoides ( 1 ) live underwater.
Some species, typical of still waters,
Just above the level of mean flow are
Cratoneuron filicinum ( 2 ) and Dichodontium
may also be found in streams, but
pellucidum ( 3 ), followed by Brachythecium
only in areas where the water is calm.
rivulare ( 4 ) and Didymodon spadiceus ( 5 ).
Hygrohypnum luridium ( 6 ), Fissidens
Their distribution is irregular and
cristatus ( 7 ) and Ctenidium molluscum ( 8 )
are found at maximum flow level. Thallose
depends on environmental condiliverwort (Conocephalum conicum) ( 9 ) grows
tions, which change with any marked
in areas which are never splashed by water
increase in flow.
The reproduction of vascular aquatic
plants is mainly vegetative, since their seeds cannot germinate in running water. In this case, colonization occurs through fragmentation, i.e., plant parts
which are able to generate new individuals are carried by the current.
Instead, the reproduction of bryophytes (mosses and liverworts) depends on
water. Mosses do not reproduce by means of flowers and seeds, but from
male and female gametes born on the protonema. This is a thin, green, primordial filament which forms only when spores germinate. Male gametes
need water - even just a drop of rain or dew - to reach and fecundate female
gametes, from which a new bryophyte will develop.
Unlike vascular plants, bryophytes cannot regulate their transpiration
processes actively, and therefore live in more or less humid environments.
They avoid areas with fast-moving waters and prefer calm waters (springs),
where they carpet large areas, both submerged and emerging.
Bryophytes are systematically placed
between algae and vascular plants
(pterydophytes and higher plants) and
include liverworts and mosses. Liverworts - the most primitive group - may
have either a thallose form with a
lobed structure, or a foliated form with
stalks and leaves without ribbing, as in
Scapania undulata.
The delicate morphology of aquatic
liverworts makes them vulnerable to
fast-moving water and the material it
carries.
Unlike liverworts, mosses have a more
complex structure, with clearly differentiated leaves and stalks, more similar to that of vascular plants. HowLiverwort Scapania undulata
ever, aquatic mosses living in running
water also have to be considerably
flexible and traction-resistant, as an efficient anchoring system may be fundamental for their survival.
Some species (e.g., Fontinalis antipyretica) carpet pebbles thickly by adhering
to them with their primitive rhizoids. The leaves of aquatic mosses are often
dense and unilaterally arranged. Stalks and branches are elongated and the
tips of leaves are small, with reinforced ribbing. Mosses are very sensitive to
drought, but few bryophytes grow directly in water: most of them prefer only
periodically wet or submerged areas.
Bryophytes have also adapted to the typically low temperatures of mountain
streams. Their photosynthesis requires free carbon dioxide, which is dissolved
in higher quantities in cold waters. Recent studies show that the photosynthetic rate of mosses adapted to life in cold streams decreases with increasing
water temperature.
The type of substratum is essential in determining the presence of many
species of bryophytes in streams. Thus, the genus Cratoneuron is only found
on calcareous substrates and Scapania undulata on siliceous ones.
Bryophytes also feature indifferent species, like Brachythecium rivulare.
Communities of aquatic mosses are quite homogeneous in Italy and central
Europe. The Italian Alps and Appennines host communities of Platyhypnidium
riparioides (which adheres to rocks, forming sometimes quite large tufts),
45
46
Mosses may cover most of the rocks on river banks
with its partner species Fontinalis antipyretica and Brachythecium rivulare.
The extent of cover may vary considerably, according to the environmental
characteristics of specific areas (especially current speed).
Pioneer vegetation of stream beds. When the banks and beds of mountain
streams are frequently submerged even in periods with low water level (ansd so
enriched with nutritive substances), we find communities of annual grasses,
such as several species of polygons (Polygonum spp.).
In natural conditions and in periods of low water, grasses colonize the beds of
streams. This pioneer vegetation is organized mosaic-wise, as it depends on
the distribution of gravel and sand deposited during periods of high water,
according to current speed.
Orophilous species like alpine gypsophila (Gypsophila repens), alpine toadflax (Linaria alpina) and alpine willowherb (Epilobium dodonaei) may also be
found in streams. These plants - called in German Alpenschwemmlinge - are
typical of Alpine and sub-Alpine gravelly debris. The seeds of these plants
are carried by currents to alluvial areas, find the proper conditions for their
development. There they have to germinate, flower and fructify quickly.
Rapid, abundant production of seeds guarantees efficient recolonization
after a possible increase in water
level. If there is a violent storm, for
instance, the level of the stream may
rise and submerge plants, which are
then carried away by the current, but
their seeds may start a new life-cycle.
In this environment, herbs do not only
have to tolerate periods of submersion, but also drought. This is why
their roots follow the level of the
aquifer, or why they reduce their
transpiration both through the xeromorphic structure of their leaves and
their own shape, which is similar to
that of dwarf shrubs.
This is the case of mountain avens
(Dryas octopetala), which forms low
carpets with creeping woody stalks. Its
horny leaves look like those of oak
trees: they are oblong-elliptical, dentellated, and up to 3 cm long, with
white down on their underside. This
grass produces single, cup-shaped,
creamy-white flowers with upturned
corollas up to 4 cm wide, with yellow
stamens. Its feathery silver fruit is
equally beautiful and lasts all summer.
Pioneer vegetation alternates with areas
of thicker vegetation, made up of
stretches of steppe-like meadows with
grasses like Calamagrostis pseudophragmites, typical of Calamagrostietum pseudophragmitis, an association generally found in south-eastern
Europe.
Its erect, leafy culm may reach a
height of 1.5 m, and its underground
offshoots enable it to colonize new
sandy areas very quickly, even after
47
Alpine toadflax (Linaria alpina)
Alpine willowherb (Epilobium dodonaei)
Mountain avens (Dryas octopetala)
48
being submerged. It may surface
again after being covered by gravel
and sand.
The alluvial plains of streams are
therefore a meeting point for species
coming from other, different environments, which give rise to very peculiar
vegetation.
Areas which undergo seasonal
variations in water level. Areas
which are less frequently submerged
by seasonal variations in water level
are colonized by thickets. Seeds of
willows and other shrubs may develop
into plants of a certain height before
the next flood.
Purple willow (Salix purpurea)
Many species of willows have adapted to environmental conditions of
streams and, while submerged, withstand the power of the current by means
of efficient anchoring systems - networks of roots, lanceolate leaves, and
considerable flexibility of branches and trunks.
Frequent floods generally do not only imply accumulation of debris, but also
ability to regenerate: these willows quickly start growing again by developing
strong new shoots from their collet and adventitious roots.
High waters may give rise to anaerobic conditions, during which plants
adopt another contrivance aimed at avoiding damage caused to roots by
insufficient oxygen. The possible accumulation of ethanol, a toxic product of
anaerobic respiration, is avoided in many ways. Numerous lenticels in the
lower part of the trunk facilitate the diffusion of both oxygen to the roots and
of ethanol towards the surface of the trunk. Many species of willows may
produce non-toxic metabolites like pyruvic and glycolic acids.
Streams in the hills and mountains of the Alps and northern Appennines are
flanked by pure and mixed willow groves, whose dominant species are purple
willow (Salix purpurea) and riparian willow (Salix elaeagnos).
Wet and cool Appennine ravines typically contain Appennine willow (Salix
apennina). Purple willow and Alpine tamarisk (Myricaria germanica) are found
in sandy and silty soils which are frequently flooded.
Purple willow (Salix purpurea) looks like an expanded shrub, with easily
recognizable thin, flexible, purpleshaded branches. Its oblong, dark to
blue-green leaves, up to 8 cm long,
have serrated apexes. Its delicate
catkins appear in early and mid-spring
before the leaves; the males have
purple, then yellow anthers.
Alpine tamarisk (Myricaria germanica)
is very elegant, with light, glaucous
leafy fronds, pale pink flowers, and
seeds characterized by tufts of stipitate hair. It is sensitive to drought and
in the gravelly beds of high mountain
streams, where the level of the water
table varies considerably and is particularly dry in summer, it is replaced
by riparian willow (Salix elaeagnos).
Alpine tamarisk (Myricaria germanica)
These are thick, erect shrubs with delicate, velvety grey shoots. Their linear,
whole leaves are up to 20 cm long, grey when young, then dark green and
downy on their underside. Their edges are parallel and revolute with acute
apexes. In spring, delicate, green catkins 3-6 cm long flower with the leaves.
The males have yellow anthers.
According to changing climatic, geomorphological and pedological
conditions, these species are associated with more thermophilic shrubs,
such as almond leaved willow (Salix triandra), continental forms like dark
leaved willow (Salix myrsinifolia), or typically mountainous ones like violet
willow (Salix daphnoides, the Salicetum elaeagno-daphnoidis association,
one of the most typical vegetal communities of Alpine valleys and therefore
among the first to be described). Plants must also withstand drought during
low water and their roots follow phreatic water.
In particular, sea buckthorn (Hippophae rhamnoides) reduces transpiration
through the xeromorphic structure of its leaves. This shrub has thorny shoots
bearing linear grey to green leaves up to 6 cm long, with silvery-bronze
scales on both sides. In spring, it produces tiny yellow-green flowers on
racemes up to 2 cm long. In female plants, flowers are followed by round,
bright orange fruit 8 mm across, particularly rich in vitamin C, which last until the following spring. Hippophae rhamnoides colonizes silicic terraces
above high water level, and is particularly lush in south-facing valleys.
49
50
Willows
Willows, of which there are about 300
species, usually prefer open areas in
northern temperate regions, although
they may be found all over the world,
except in Australia. High-altitude, boreal
regions, are colonized by strong and
cold-resistant shrubby types, which
grow tree-high in milder climates. A
large number of species live on fluvial,
alluvial and open soils. As willows grow
rapidly and develop strong new shoots
from its collets, many species are used
to consolidate stream and river banks or
highly eroded mountain flanks.
Willows growing in dry environments
have ovate leaves, which are lanceolate
in species colonizing stream banks.
The shape of their leaves and the
considerable flexibility of their branches
and trunks allow willows to bend with
the current during floods, so that they
avoid being broken or uprooted.
In the past, the great flexibility of the
twigs of almond leaved willow (Salix
triandra) and purple willow (Salix
purpurea) lent themselves to many
forms of wickerwork.
In late winter, the small flowers of willow
appear before or at the same time as
the leaves. Flowers do not have petals
and are borne in female and male
catkins on different trees. Male catkins
are usually more conspicuous than
female ones, and their pollen is carried
by the wind. Willows also have
important melliferous (honey-producing)
species which produce nectar in early
spring, when other flowers are rare.
Small seeds develop in female capsules
and are carried by the wind. They are
permeable, and germinate immediately
when the climate and season are
favourable. This enables willow to
colonize broad areas with surface
Karin Ortler
watertables, but it has fatal
consequences when seeds fall in shady
areas or insufficiently damp soils,
because the small plants die
immediately. In the past, the cultivation
of willows was important, especially in
the Po Plain, where various species
were grown to produce fuel for bakeries,
and were also used for basketwork,
carpentry, tools, etc.. Wickerwork was
not restricted to the production of
baskets, but included other products
such as fish traps, supports for vines
and other woody plants which grow on
espaliers, and strings to tie and fix the
thatched roofs of country cottages.
It is also worth remembering the
pharmaceutical use of the glycoside
salicin as an efficient febrifuge (antifever agent). This substance is
extracted from the bark of almost all
willow species, from which salicylic acid
(now a component of aspirin) was
obtained in the past.
Riparian willow (Salix elaeagnos)
Leaves of riparian willow (Salix elaeagnos)
Morphological adaptations to currents of leaves of various types of willows. From left to right: white
willow (Salix alba), crack willow (Salix fragilis), laurel or bay willow (Salix pentandra),almond leaved
willow (Salix triandra discolor and Salix triandra triandra), common osier (Salix viminalis), European
violet willow (Salix daphnoides), riparian willow (Salix elaeagnos), purple willow (Salix purpurea)
Purple willow (Salix purpurea)
Purple willow (Salix purpurea)
White willow (Salix alba)
White willow (Salix alba)
51
52
Areas flooded only during high water. Along mountain streams, bushes are
followed by grey alder (Alnus incana association: Alnetum incanae, especially
in the valleys of high-altitude mountains and dominating areas with very little
humus), is replaced by black alder (Alnus glutinosa) in areas where winters
are milder. These trees may grow where willow groves and Hippophae rhamnoides have previously prepared the soil by gradually collecting sand and
gravel. Alders have adapted both to occasional high water and insufficiently
aerated wet soil. Lenticels in the lower part of the trunk provide air to superficial, submerged roots. Roots have tubercules which contain bacteria, living in
symbiosis with the plant. These absorb nitrogen from the air, compensating
for the lack of nitrogen in the soil.
The combination of certain conditions determines which species colonize particular areas after floods. Alder seeds germinate in the summer, when water
levels decrease and enable the growth of alder. The seeds of white willow (Salix alba association, Salicetum albae, the vegetal community of alluvial plains
which flanks the main rivers of the Po Plain and central Italy) and crack willow
(Salix fragilis) flower in June, when mountain stream waters are high and areas
for colonization are not yet available.
The germination period of these seeds is limited to a few days, and they cannot be carried by water. They find more favourable conditions for their development along the lower stretches of watercourses, on sandy and silty soils,
where they form thick riverbank
forests. Low winter temperatures also
play a fundamental selective role.
Pioneer plants and willows which
usually live near streams and prefer
sunny areas may no longer live in the
shady underbrush of alder woods. The
soils of these habitats, although still at
an early evolutionary stage, are
enriched in nutritive substances at
every flood.
Thus, associations of megaphorbies
(term derived from the French
megaphorbiées) are usually found at
the edge of alder woods and glades,
e.g., bishop’s weed (Aegopodium
podagraria) and figwort (Scrophularia
White willow (Salix alba)
nodosa).
Alder
In Italy, there are four spontaneous
species of alder: grey alder (Alnus
incana), found in alluvial soils of
central-northern Italy; black alder
(Alnus glutinosa, see photo), which
colonizes river and stream banks and
brackish woodland; green alder (Alnus
viridis), typical of the Alps, usually
covered with snow, where avalanches
are frequent; and Italian alder (Alnus
cordata), which lives in mountain
woodland in southern Italian regions.
Alder is suited to very damp soils and
perhaps this is why many botanists
believe that the Latin word Alnus
derives from the Celtic “al lan”,
meaning “living near banks”. Since
black alder wood hardens if
submerged for lengthy periods, it was
commonly used for piles and other
hydraulic works.
Alder wood turns deep orange when
freshly cut, and was thus endowed
with symbolic meanings, e.g., the tree
of life after death.
Black alder may be higher than 30 m,
round or pyramidal in shape and very
strong, with easily recognizable
leaves. The oval, round leaves are
irregularly serrated and without a
proper apex, which is usually gently
lobed. The upperside is a dark, glossy
green; the underside is a lighter, matt
green with rusty tufts of hairs where
the main veins branch.
Grey alder, which grows to 10-15 m,
is recognizable by its dark grey bark.
Its oval leaves are up to 10 cm long
and 5 cm wide, doubly serrated along
the margins, with sharp apexes. The
upperside is dark green and the
underside grey and downy.
Female and male flowers are born on
separate catkins on the same tree,
Karin Ortler
and form in late winter or early spring,
before the leaves open. Male catkins
are about 10 cm long and hang in
groups from the tips of branches.
Their pollen is carried by the wind.
Clusters of female catkins, oval and
about 1-2 cm long, produce small,
woody, dark brown fruit about 2 cm
long. These woody nutlets stay on the
tree throughout the year. Their seeds
have a special structure full of air,
which enables them to be easily
carried by running water. Winds carry
them for about 30-50 m, and they
may germinate for 12 months.
As alder is a pioneering plant, it
grows rapidly during its juvenile
stages, but seldom lives for longer
than 100-120 years.
Alder wood, which hardens in water,
is used for hydraulic constructions. It
is sensitive to exposure to air and is
seldom used as fuel or as carpentry
material. Its bark is rich in tannin and
was once used for tanning hides.
53
54
These hygrophilous and nitrophilous
plants grow rapidly to over 1 m.
Among the flora of the underbrush is
ostrich fern (Matteuccia struthiopteris),
up to 1.5 m high, and butterbur (Petasites spp.), whose dense stalks in
spring bear round, purple and white,
male and female flowers on different
plants. Base heart- or kidney-shaped
leaves grow after flowering. Their diameter is about 60 cm and they form
dense herbaceous communities.
Ostrich fern (Matteuccia struthiopteris)
Areas which are not subject to floods
are colonized by alder and also by
trees typically found in riverbank forests of hilly areas, like common ash
(Fraxinus excelsior), white poplar (Populus alba) and black poplar (Populus
nigra). Common ash has pinnate leaves about 30 cm long, with 9-13 small,
oblong, ovate leaves, about 10 cm long and 3 cm wide. Its flowers are delicate, violet, without petals, and bloom in spring, before the leaves, from almost black buds. Its winged, hanging fruit is greenish, then light brown, and
ripens in thick bunches.
Poplars belong to the willow family.
Their flowers, like those of willows,
have hanging male and female catkins
about 5 cm long (in black poplar) or 10
cm long (in white poplar). However,
their seeds are larger: the small, green
capsules open to release seeds covered with a white down similar to raw
cotton, which are easily carried by the
wind. The leaves of black poplar are triangular or ovate, about 10 cm long and
wide, with glossy uppersides and
Black poplar (Populus nigra)
opaque undersides. The leaves of white
poplar have 2-5 lobes, and are about 10
cm long and 7 cm wide. When ripe, their surface is glossy, and a dense, white
down covers their underside. Another type of riverbank forest grows along
streams and is characterized by Scotch pine (Pinus sylvestris). These conifers,
together with willows, colonize calcareous, alluvial streams, which are submerged for only 1-2 days during exceptional years. Although Scotch pine has
great ecological value (it grows in peat-bogs, on dry soils of rocky slopes or on
alluvial debris), it cannot withstand floods lasting more than a few days.
Butterbur association (Petasites spp.)
Black poplar (Populus nigra)
55
Invertebrate fauna
BRUNO MAIOLINI · VALERIA LENCIONI
Benthic invertebrates are defined as small animals which live close to the substratum for at least a period of their life. Those of larger size (over 1 mm long),
are called “macroinvertebrates”, of which insects constitute the most important part. In mountain streams, orders of dipterans, plectopterans,
ephemeropterans and trichopterans prevail. Invertebrates in mountain
streams have to face two life-threatening obstacles: current speed and cold
temperature, which they overcome by contriving extraordinary adaptations.
In order to contrast the current, these animals may have flat bodies (like
heptageniid ephemeropterans), strong suckers (blepharicerid dipterans), and
crown-shaped hooks (simuliid dipterans). The current is not only obstacle, but
also produces a constant flow of food downstream, and is a good mean for
dispersion.
It is far more difficult for invertebrates to adapt to the cold. Many scientists
consider temperature the main factor determining the ecology and evolution of
aquatic and terrestrial invertebrates (egg-laying and hatching, growth rates,
mating, reproductive strategies, activity models, types of food). Invertebrates
living at high altitudes and latitudes have adopted various strategies to overcome the long, cold winters. Snow and ice, drought, wind, cold and little food
are their most feared enemies. Aquatic invertebrates may exploit the current
to move downstream and avoid freezing or drying up, or they may shelter in
underground waters. But they frequently spend winters in the same place:
the capacity of these organisms, particularly insects, to survive in these harsh
environments is due to a series of physiological, biochemical, morphological,
behavioural and ecological adaptations. Among these is the production of
melanin, their small size and hairiness, capacity for feeding and mating on the
ground instead of in flight (they therefore have small or totally absent wings),
the building of cocoons, quiescence, diapause, and resistance to cold.
Quiescence is a direct, temporary reaction to unfavourable conditions, which
is interrupted as soon as conditions become favourable again. Diapause is a
direct reaction to unfavourable conditions which cannot be interrupted until
after a certain period of time. For aquatic insects, diapause regards eggs and
mature larvae, seldom pupae, and almost never adults. Day length (photoperiod)
Stonefly larva
57
59
58
Cold challenges mountain stream invertebrates
Currents: obstacles for invertebrates, but also efficient means of transport
is one of the factors which governs the transition between different physiological states. In Arctic environments, prolonged diapause has been recorded,
i.e., organisms may even protract their inactivity for even many years, until environmental conditions are favourable (the life-cycle of some dipterans of the
chironomid family may last 6-7 years).
Organisms resistant to cold, i.e., able to survive to temperatures below 0°C for
long periods (even many months at -40/-50°C), without damage, apply two
strategies: hibernation and supercooling. Hibernation freezes their
haemolymph and all their cell fluids except the cell matrix. The temperature at
which crystals form and the duration of hibernation are controlled by chemical
substances, such as polyhydric alcohol, sugars, amino-acids, and complex
proteins of high molecular weight called THPs (Thermal Hystereris Proteins),
which lower the freezing-point of body fluids by raising their density and, linking with water molecules, prevent the formation of ice crystals.
Spring “awakening” is usually a quick process, whereby the same substances
which enabled hibernation now provide amino-acids and carbon at thaw,
when there is still little food in the environment. Hibernating insects die only
when exposed to very low temperatures (-30/-50°C) for many months, when
the temperature drops further (-60/-80°C) even for short periods, or when
freezing and thawing cycles alternate.
Super-cooling, the most typical strategy of aquatic insects, enables them to
survive when temperatures drop below 0°C (maximum –20°C) for long periods,
during which their body fluids remain liquid. The freezing point of these insects
shifts from –5/-10°C (as in most insects) to –20/-25°C, as they may eliminate or
reduce substances which favour the formation of ice, and their body fluids take
in anti-freeze substances similar to those used by hibernating insects. Glycerol
is frequently accumulated as an anti-freeze, and may constitute up to 10-14%
of the weight of the animal.
60
Taxonomy of invertebrates
Triclads. Triclads, or planarians, are
characteristically found in cool,
mountain streams rich in oxygen,
especially if small and of spring origin.
These small animals are easily identified
by their extremely long and flat bodies
which remain tenaciously attached to the
substratum, even in particularly fastflowing waters.
They prey on small invertebrates such as
insect larvae, crustaceans, gastropods
and annelids, but may also feed on
carrion.
In the upper course of mountain
streams, the most common species is
Crenobia alpina, well adapted to fast
water speeds and low temperatures.
Further down, we find Dugesia
gonocephala and Polycelis felina.
Although fewer than 20 species are
known in Italy, mostly typical of slowflowing or standing waters, their
presence in streams may be relevant
Planaria
Bruno Maiolini · Valeria Lencioni
with regard to population density and
influence on macrobenthic communities,
as they are specialized predators.
Molluscs. There are few molluscs in
mountain streams, as most freshwater
species prefer still or slow-flowing
waters, due to their scarce mobility and
found in areas of submerged vegetation.
They crawl on the bottom or swim freely
in water among aquatic plants. They are
detrivorous, herbivorous, or even
carnivorous. Tubificids and lumbriculids
are generally found in fine sediment,
feeding on bacteria associated with the
sediment itself. Tubificids may abound in
sediments rich in organic debris and are
generally dominant at depths exceeding
one metre. Enchytraeids, which may also
colonize terrestrial environments, are
present where waters are rich in
oxygen, particles are coarse
and currents fast. Among the
oligochaetes which
populate running
waters, only naidids,
lumbriculids and tubificids are
strictly aquatic families, of
which the first two are
generally found in glacial and
non-glacial reaches above the tree-line.
Ancylus fluviatilis
difficulty in finding the right food.
A few species constitute an exception,
one of which is the pulmonate Ancylus
fluviatilis. Middle-shallow reaches feature
species of the genus Pisidium, small
freshwater mussels which live in sandy
or pebbly substrates.
Oligochaetes. Oligochaetes colonize
various freshwater environments and are
found in several stream microhabitats:
sandy or muddy bottoms, hard, pebbly
or gravelly substrates, areas with
deposits of organic debris or submerged
vegetation, etc..
Small oligochaetes (some naidids, for
example), may also colonize interstitial
environments. Naidids are frequently
Hirudineans (leeches). Hirudineans,
better known as leeches, are mostly
freshwater species found in running and
standing waters. They adhere to the
substratum and move by means of
suckers, thus requiring environments
with hard substrates like rocks, stones
or pebbles. The torrential reaches of
rivers are therefore optimal for species
which prefer running water.
Their presence is limited upstream by
temperature (they cannot reproduce at
temperatures below 10-11°C) and
downstream by increased sandy or
muddy substratum.
Few species are found in intermediate
stretches, like Erpobdella testacea,
E. octoculata and Dina lineata, all
preying on other benthic invertebrates
and resistant to moderate organic
pollution. Other species (Dina krasensis,
Trocheta bykowskii) may live in streams
at low altitudes, even in large numbers.
Mites. Mites (see drawing) are arachnids
a few millimetres long; they abound in
soil fauna, particularly in stream beds,
where they provide an excellent example
of adaptive radiation, with tens of
thousands of known species. Only a few
families have adapted to life in
freshwater (water mites). A group of
families, known as
hydrachnids, count
thousands of species in
freshwaters, and even
halacarids, a sea taxon, may be
found in freshwaters. Mountain
springs and interstitial
environments are among the
freshwater environments
colonized by the most interesting water
mites. In springs, water mites constitute
an important part of the so-called crenal
fauna, typical of these habitats, which
requires perennial, clean springs. In the
interstitial environment, i.e., in gravelly
and sandy stream beds, species have
devised adaptations for life in
subterranean water: they have no eyes
and are colourless.
The biological cycle of freshwater mites
is extremely complex, being exceptional
right from the beginning: eggs do not
originate larvae; rather, pre-larvae
develop inside eggs. The latter turn into
larvae which leave the eggs and become
parasitic on aquatic insect larvae
(especially chironomids), the internal
liquids of which (haemolymph) they
suck. Insect larvae are also exploited as
“means of transport” which carry larvae
61
62
to new habitats (phoresis).
From this point onwards, the cycle
becomes even more complex and
develops in three successive steps
(protonymph, quiescent; deuteronymph,
similar to an adult but with three pairs of
legs, free and predatory; tritonymph,
quiescent again).
Tritonymphs finally develop into adults
with four pairs of legs, like all arachnids,
bodies divided into two parts, front
(gnatosome, bearing mouth parts) and
rear (idiosome, bearing legs). The body
surface is covered with bristles and
glands, which probably emit an
unpleasant smell and protect water
mites from predators.
All water mites prey on other aquatic
invertebrates and are very sensitive to
water quality. Italian springs and streams
are colonized by many genera and
species, the most common of which are
Lebertia, Hydrovolzia, Partnunia,
Sperchon and Thyas.
The phoretic or transport stage of their
life-cycle explains why endemic species
of mites are so rare; most aquatic
species are widely distributed and may
be found randomly almost everywhere in
the Alps and Appennines.
Alpine streams; they are absolutely the
most common organisms of our planet,
and have colonized all habitats
containing water (seas and oceans,
lakes, ponds, marshes, rivers and
streams, even tiny pools in tree-trunks,
mosses and wet soil, caves, interstitial
waters, and even the small lakes which
form in favourable conditions in
Himalayan glaciers at 5000 m a.s.l.).
These crustaceans have unmistakable
features: they have only one eye
(Cyclops is one of the most common
genera) and their bodies have two long
antennules and two short antennae, with
five pairs of legs (the term copepod
derives from the Greek meaning “oarfeet”) ending in a pair of caudal rami
bearing long bristles.
The female bears one or two egg-sacs;
the eggs hatch to become larvae
(nauplia), which are very different from
the adults and, after a number of moults
(generally five) they turn into sub-adults
(copepodids), which do not reproduce.
After five further juvenile stages,
copepods become adults after the
sixth moult.
Sexes are easy to determine because
most male copepods in Italian streams
Crustaceans. In mountain streams,
most crustaceans are less than 1 mm
long, and belong to the so-called
“meiofauna”. These organisms live on
the bottom of slow-flowing
watercourses, in close contact with the
substratum (epibenthic) or inside it,
between gravel and sand granules
(interstitial fauna), even at considerable
depths.
Copepods are undoubtedly the most
numerous crustaceans in springs and
Harpacticoid copepods (Bryocamptus
tatrensis) mating
have swollen, prehensile antennules
which they use to seize females during
mating.
Moss and interstitial species which are
frequently found in high-altitude streams
and springs belong to the harpacticoids
(particularly to the genera Attheyella,
Bryocamptus, Maraenobiotus,
Hypocamptus, Moraria and
Parastenocaris).
Some species are very rare and only
found in restricted areas of the Alps and
Appennines; others are typically boreoalpine, distributed in areas including
northern Europe and the Alps (seldom in
higher mountains on the Appennines) traces which reveal their much wider
distribution during glaciations.
Cyclopoid copepods prefer downstream
reaches, and many members of the
genera Acanthocyclops, Diacyclops and
Speocyclops are found in interstitial
environments.
Besides copepods, spring and interstitial
habitats typically contain other small
crustaceans. In karstic springs,
ostracods may be locally found in large
numbers. Their body is enveloped in a
bivalve carapace (ostracod derives from
the Greek word meaning shell) which
protects the soft parts of the body and
its appendices. They are round or beanshaped.
Ostracods also feature blind and
depigmented interstitial
species. Among the most
common genera in the
Dolomites (large quantities
of calcium are
indispensable for the
development of their
carapace) are Potamocypris,
Pseudocandona, Cypria (see drawing),
and the rarer Cavernocypris and
Cryptocandona.
In interstitial habitats, other taxonomic
groups of crustaceans are only
occasionally found, such as
cladocerans, widely distributed in
surface environments, and exceptionally,
the blind depigmented members of the
order Bathynellacea which, although
limited to subterranean waters, have
recently been found in the Alps at high
altitudes.
Larger crustaceans are more common in
some streams, particularly amphipods,
which are found in large numbers
downstream. Their biomass makes them
one of the most important food sources
for fish. They are generally associated
with middle-shallow stretches and slowflowing waters. At high altitudes, they
are present only in a few relict sites.
Along the Alpine chain, Gammarus
fossarum, G. balcanicus and
Echinogammarus stammeri are common
species. In the Appennines, they are
replaced by Gammarus aff. italicus,
Echinogammarus veneris and E. tibaldii.
Other crustaceans found in springs and
interstitial habitats are completely blind
and depigmented, and belong to the
genus Niphargus, with about 100
species in Italy. N. strouhali often lives in
mountain streams and prefers
elevations exceeding 2000 m
a.s.l.. This genus has recently
been found in interstitial
environments at high
altitudes, e.g., in glacial
streams at the foot of the
Bernina massif (Switzerland).
Other crustaceans colonize low-altitude
streams, and are only exceptionally
found in mountain stretches.
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64
Freshwater crayfish (Austropotamobius pallipes
fulcisianus)
Among these, freshwater crayfish
(Austropotamobius pallipes fulcisianus)
are widely distributed. They belong to
the decapod order and prefer clean,
well-oxygenated stretches at low
elevations, but in particular conditions
(Trentino), they may be found at 1500 m
a.s.l. Crayfish are common in northern
Italy and colonize the Appennines down
to the Basilicata region, decreasing in
number southwards. In the past, they
were much more numerous and widely
distributed, but were harvested
indiscriminately for long periods, since
they are delicious to eat. In recent years,
their numbers have fallen still further,
partly owing to their sensitivity to
worsened water quality. Today, many
regional laws forbid them to be taken,
and they are further protected by the
Habitat Directive of the European
Community.
Ephemeropterans (mayflies).
Ephemeropterans owe their name to the
short span of their adult life (in Greek,
ephemeros = lasting one day, and pteron
= wing). The masticatory apparatus of
adults does not often function and they
can only live as long as their store of
energy, accumulated during their aquatic
larval stage, lasts. They are found in
lakes, marshes and ponds, but also
running waters, preferably at middle-low
altitudes. Many species tolerate
moderate organic pollution, and
considerable numbers may therefore be
found in quite poor-quality
watercourses. In Italy, 94 out of 300
European species are found.
Mayflies are considered a primitive order,
similar to odonates, with a hemimetabolous development.
The ecological role of mayflies in
mountain streams is one of considerable
importance and, if conditions are
favourable, they may prevail over the
macrobenthic community, both in
number of individuals and total biomass.
Most nymphs are herbivores and are an
important source of food for other
aquatic invertebrates and fish.
After emergence, they become the prey
of many birds, bats and fish which catch
them during the delicate stage of
metamorphosis, often on the water
surface. Fishermen are aware of this,
and adult mayflies are the most imitated
in the refined art of artificial bait
construction.
Mayflies occour in various fluvial
microhabitats and resort to peculiar
morpho-physiological adaptations.
Four genera belong to the heptageniids
(Epeorus, Rhithrogena, Ecdyonurus and
Heptagenia) and, as nymphs, share
dorso-ventral flattening. Their
hydrodynamic aspect, with flattened
head, abdomen and legs, is enhanced
by the shape and position of abdominal
gills, used as efficient suckers to
increase their adherence to the
substratum. Thus, nymphs can tolerate
strong currents by remaining in the thin
Mayfly nymph of the genus Baetis
layer close to the substratum where
water flow is slow. The most common
species found in high-altitude stretches
belong to this family: Epeorus alpicola,
E. sylvicola, Rhithrogena loyolaea,
R. alpestris and Ecdyonorus helveticus.
The only Italian species of the
oligoneuriid family - Oligoneuriella
rhenana - has characteristics similar to
those of the heptageniids, and lives in
fast-flowing Alpine and Appennine
streams.
The nymphs of these two families are
generally flat-bodied, and are also called
lithophilous, as they live on the surface
of stones and submerged rocks.
A second group (hyponeophiles) of
families includes water nymphs, whose
tapering, hydrodynamic body has three
strong, fringed caudal filaments, used as
efficient rudders or oars.
Baetids and siphlonurids belong to this
group, the latter represented in Italy by
only one species. They are typically
found in lakes and slow-moving waters.
The many species of the large family of
baetids usually live in the middle-low
stretches of mountain streams. Some,
like Baetis alpinus, which is found in
many Italian regions, may colonize
upstream reaches. B. rhodani is also
common in streams and has great
ecological value, being found in both
polluted and unpolluted downstream
stretches.
A third group of nymphs has adapted to
crawling on the bottom (herpophiles) and
includes the caenid, leptophlebiid and
ephemerellid families. These mayflies are
generally associated with slow-moving
waters rich in organic sediments, but
may locally abound at middle-low
altitudes.
There are also burrowing nymphs
(oryctophiles), belonging to the
ephemerid and polymitarcid families,
with modified mandibular processes and
forefeet which enable them to dig
tunnels in river and stream bottoms.
A few live in stream stretches, as they
prefer slow-flowing waters.
The only exception is Ephemera danica,
locally abundant, inhabiting gravel and
sand streams.
Subimago of a mayfly of the genus Baetis
65
This order is divided into two groups,
zygopterans or damselflies, and
anisopterans or dragonflies. In the
former group, only one genus lives
exclusively in running waters,
Calopteryx, with three Italian species
(C. virgo, C. splendens and C.
haemorrhoidalis) found at middle-low
altitudes. The latter group includes two
families with species adapted to running
water, cordulegastrids and gomphids.
The former has only one Italian genus
(Cordulegaster), which includes a few
very large species, whose larval stage
occurs in slower areas of streams.
Gomphid nymphs are flatter and
stumpier, and live in the mud and gravel
of mountain stream beds.
66
of sound signals, which are produced by
drumming the substratum with their
abdomen. This phenomenon has been
thoroughly studied, and the recording
and analysis of sounds reveal that these
are true languages.
The rhythm of beats and the frequency
of sequence repetition differ between
species and sexes. In some cases, even
“dialects” have been recorded in
populations of the same species but
geographically apart.
Stoneflies of the northern hemisphere
(Palaearctic and Neartic) are divided into
two large groups: Systellognatha and
Euholognatha. The buccal apparatus of
adults of the former group does not
Cordulegaster bidentatus
Odonates (dragonflies and
damselflies). Dragonflies and
damselflies are the largest insects with
larval stages (up to 6 cm long). The
adults are excellent fliers and may be
seen far from the point of their
emergence from the water.
They then colonize many aquatic
environments including standing or
slow-flowing waters.
A few species live in stretches or pools
at the edge of streams, rich in aquatic
vegetation.
Nymphs, like adults, prey on other
invertebrates and vertebrates, especially
tadpoles and fry which they catch with
their typical buccal apparatus, called
mask. This looks like a pair of pincers,
which are kept folded when at rest and
snap forward to grasp the prey.
Therefore, their method of hunting is by
stalking, during which motionless
nymphs mimic aquatic vegetation.
Plecopterans (stoneflies).
Plecopterans, or stoneflies, derive their
name from the Greek pleco = fold, and
pteros = wing, due to the typical position
in which adults keep their wings at rest,
folding them scissorswise to cover their
abdomen. This is probably the most
typical order of benthic fauna in
mountain streams, since many species
are particularly suited to cold, welloxygenated, clean and fast waters.
This, and downstream pollution of rivers,
confines stoneflies to streams at
altitudes over 600-700 m.
Their distribution is not only limited by
their strict ecological requirements
(stenoecious nature), but also by the
poor flying skills of the adults.
Over 3000 species are known in the
world, 400 of which belong to
European fauna. In Italy, 144 species are
known, divided into 21 genera grouped
in 7 families.
One peculiarity of stoneflies is their use
Stonefly nymph of the genus Perla
function, making it impossible for them
to feed. Nymphs are generally
omnivorous and good predators in the
last stages of their development.
Euholognatha include species which
feed as adults, meaning that they may
lead a subaerial life for some weeks.
Both adults and nymphs are
phytophagous or detritivorous and feed
on algae, mosses and vegetal debris.
Their development is hemi-metabolic
and their larval stage lasts for about one
year, although biennial cycles are known,
e.g., in Dinocras cephalotes.
A large number of moults is necessary to
achieve the adult stage, ranging
between 10 and 20, with a maximum of
33 in Dinocras cephalotes, the largest
species, the winged females of which
are over 30 mm long. When mature,
nymphs leave the water and have their
last moult, to become adult insects.
This generally occurs in spring, but there
are many known cases of winter
emergence from the water, with flying
adults which mate on snowy shores.
There is usually only one generation a
year (univoltine species), but in
favourable conditions, a second
autumnal generation may follow the first
(bivoltine species), e.g., Nemurella
pictetii.
Nymphs generally look like mayflies, but
have only two caudal cerci and lack leaflike abdominal gills.
Nymphs of stoneflies, although typical of
mountain streams, are not particularly
suited to life in water, and colonize
microhabitats characterized by slow
currents, like leaf debris, the lower parts
of stones and rocks, and the quiet
waters of pools.
Stoneflies are widely distributed in all
mountain streams and, in favourable
conditions, constitute more than 50% of
the macrobenthic population. After
dipterans, they are the group most
frequently found in high-altitude
streams, even with large species, like
Dictyogenus fontium, populations of
which have been found in the Alps at
elevations over 2800 m a.s.l.
Other smaller species living upstream
(rheophilous-orophilous species) are
Protonemura ausonia, P. caprai,
P. elisabethae, P. brevistyla, Nemoura
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Adult stonefly of the genus Chloroperla
mortoni, N. obtuse, Leuctra rosinae,
L. festai, L. teriolensis, Isoperla rivulorum,
Perlodes intricatus, Siphonoperla
montana and Chloroperla susemicheli.
At middle altitudes, there are medium
and large species like Perla grandis,
Dinocras cephalotes, D. ferreri,
Dictyogenus ventralis, Perlodes intricatus
and Isoperla rivulorum.
The genus Tyrrhenoleuctra, with only one
Italian species - T. zavattarii - deserves
special mention. It is found in Sardinian
and Corsican streams.
Heteropterans (bugs). Bugs are
commonly found in aquatic
environments, in both larval and adult
stages. Their name derives from their
two “different wings” (hetero = different;
pteros = wing), i.e., partially sclerotized
front wings and totally membranaceous
rear wings. Nymphs are very similar to
winged adults, and turn into adults by
means of gradual transformations in
shape and size called paurometabolous
metamorphosis. Aquatic bugs prefer
quiet waters, although many species are
found in running waters, near shores, in
pools, in calm river sides, and generally
where water current is slow.
Some bugs called gerrids live on the
water surface, and others, nepids, live
underwater. The former walk or skate on
water surface exploiting the superficial
tension, the latter walk on the bottom or
swim in the vegetation. Most of them
prey on other insects, such as mites and
spiders. Larger species suck tadpoles,
fry and the eggs of amphibians and fish.
Smaller species are omnivorous and also
feed on benthic meiofauna, microscopic
algae and organic debris.
Among the most common genera in
streams are Gerris, Aquarius and Velia.
Aquarius najas lives exclusively in
streams, in calm stretches and pools.
Coleopterans (beetles). Beetles
represent the largest animal order, with
at least 350,000 species, 10,000 of
which belong to Italian fauna. Some
families are found both in standing and
running waters. They are the only
holometabolous insects which live in
aquatic environments as both larvae and
adults. Adult water beetles can always
fly and seek better environmental
conditions. There are two categories:
swimmers, such as diving beetles
(Dytiscidae), crawling water beetles
(Haliplidae) and water scavenger beetles
(Hydrophilidae), and walkers,
represented by long-toed water beetles
(Dryopidae), riffle beetles (Elmidae),
minute moss beetles (Hydraenidae) and
another family of crawling water beetles
(Helophoridae). The former have fringes
of swimming hairs on their legs, and the
latter have strong claws to cling to the
substratum. Aquatic adults have tracheal
respiratory systems like those of
terrestrial species, and have contrived
adaptations to store air for their quite
long immersions. Diving beetles, water
scavengers, crawling water beetles, and
minute moss beetles sometimes emerge
to store air. They adopt different
positions in order to store oxygen.
Diving and crawling water beetles, for
example, stand on their heads, obliquely,
until the rear part of their body touches
the air-water interface. They then
capture an air bubble, which they tuck in
the chamber under their wing surface,
i.e., in the cavity between abdomen and
elytrum. Exhaled gases are released as
bubbles. Instead, water scavengers,
minute moss beetles (see drawing) tilt
their bodies upwards and convey
air to the chamber under their
wing surface through their
antennae. Minute moss
beetles are aquatic only as
adults, whereas marsh
beetles (Helodidae) are
aquatic only as larvae.
Larvae and adult beetles eat
various kinds of food. Among
adult water beetles, diving
beetles are predators, water
scavengers are omnivorous,
and the other families are
generally herbivorous.
Larvae of diving and water
scavenger beetles are predators,
larvae of crawling water beetles
are herbivorous suckers and the other
families are herbivorous/detritivorous.
Aquatic beetles prefer river bank
environments characterized by slow,
shallow waters, rich in vegetation and
organic matter. This is why they are not
found in upstream stretches, where fastflowing water, stream bed instability and
severe erosion make colonization
difficult. They may live at higher altitudes
in perifluvial wet areas, the ecological
role of which is that of “refuge areas”
where environmental conditions are not
so extreme.
Dipterans. The order of dipterans
includes the highest number of species
in the class of insects. With the sole
exception of oceans, dipterans colonize
almost all environments during the
different stages of their life-cycle. They
include more than half of all aquatic
insects and, in the larval stage, live in a
large number of environments - streams,
lakes, ponds and marshes. Their
development is complete, with
terrestrial or aquatic larval stages
and subaerial adult stages. Some
species are semi-aquatic
and live in wet soil, rotting
hollows in plants or animal
carcasses. Larvae of dipterans
feature all nutrition models,
including plant feeders, detritus
feeders and predators.
The name of these animals,
“with two wings”, derives from
the fact that adults have only
one pair of front wings, since
the rear ones are transformed into
equalizers (appendices with a
club shape) which are used as
flight stabilizers.
According to the shape of the antennae
in adults, dipterans are divided into two
sub-orders, nematocerans and
69
70
Larvae of non-biting midges (Chironomidae)
brachycerans. The former have long
moniliform antennae made up of many
joints, a slender body like that of
mosquitoes and long, thin legs.
Brachycerans are characterized by short
antennae and a stocky aspect, like flies.
Dipterans include a large variety of forms
and adaptations, and may colonize
highly polluted or environments which
are “inhospitable” for most insects, like
wastewater, sulphurous and thermal
waters and cold glacial streams.
This is due to the ability of larvae to
tolerate low concentrations of dissolved
oxygen, owing to oxygen storage
induced by respiratory pigments similar
to haemoglobin (e.g., Chironomus of the
group thummi), or anaerobic respiration
(i.e., without oxygen).
Larvae are sub-cylindrical, thin and
worm-shaped, or fleshy and stocky
(dorso-ventral flattened shapes are rare),
and are characterized by jointless legs.
They may have pseudopods (false legs)
in various positions, with or without
hooks, spiracles, tubes or respiratory
filaments. During the pupal stage,
individuals may be free or enclosed in
rigid envelopes, in the water or on the
soil. Adults generally live for about one
month, during which mating (in flight, on
vegetation, the ground or even in water),
and egg-laying occur. Dipterans include
various haematophagous adult species
which attack humans and other warmblooded vertebrates (ceratopogonids,
culicids and simuliids are known for their
irritating bites), and they may also carry
diseases.
Although the order features many
different forms, single families are
characterized by uniformity of shape.
Nematocerans have completely
sclerotized and evaginated heads
(cephalic capsules), which are therefore
clearly visible (eucephalous larvae), or
small, only partially sclerotized heads,
set deep in the thorax (hemicephalous
larvae). Larvae of brachyceran species
are, with rare exceptions, hemicephalous
or acephalous, i.e., their heads are very
small, with few sclerotized parts, and
partially or completely set inside the
thorax.
Body segmentation is usually evident,
but thoracic and abdominal areas are
not clearly distinguished.
The larvae of many dipterans,
particularly those with aquatic juvenile
stages, are little known, and it is often
necessary to breed them to their adult
stage to identify the species.
The most numerous families of
nematocerans in mountain streams are
net-winged midges (Blephariceridae),
biting midges (Ceratopogonidae),
non-biting midges (Chironomidae), dixid
midges (Dixidae), moth flies
(Psychodidae), blackflies (Simuliidae),
solitary midges (Thaumaleidae) and
craneflies (Tipulidae and Limonidae).
Among brachycerans are houseflies
(Anthomiidae or Muscidae), snipe flies
(Athericidae), long-legged flies
(Dolichopodidae), dance flies
(Empididae), and soldier flies
(Stratiomyidae). Non-biting midges and
black flies are the largest groups in
high-altitude streams, and the only ones
present, apart from a few exceptions, in
glacial streams. Proceeding
downstream, we find dance flies and
crane flies and, still further down,
solitary midges, net-winged midges and
snipe flies.
Chironomids (non-biting midges).
The family of chironomids is the largest
in freshwater environments, both for
number of species and individuals.
About 15,000 species are known in the
world, 400 of which are found in Italy.
They are divided into 8 subfamilies, 5 of
which live in Italy: Tanypodinae,
Diamesinae, Prodiamesinae,
Orthocladiinae and Chironominae.
Adults live in an aerial environment but,
as larvae, they colonize various
freshwater environments, such as lakes
and mountain streams, rivers, ponds and
pools, clean waters rich in oxygen as
well as very polluted ones, both poor
(oligotrophic) and rich in nutrients
(eutrophic). Some species colonize sea
water, particularly areas between low
and high tide, and others live on land.
Thus, non-biting midges are a definitely
cosmopolitan group of insects.
They include many species of high
ecological amplitude (euryecious), but
also stenoecious ones, i.e., they are
unable to live in conditions different from
those to which they have become
adapted. Therefore, they are good
biological indicators as they reveal even
71
Larvae of midges (Diamesa, Chironomidae)
minimal variations in the environmental
conditions in which they live.
For example, the presence of
Diamesinae larvae in watercourses, and
particularly species of the genus
Diamesa, such as D. steinboecki,
indicates “glacial” conditions, as they
are typical of these environments.
In mountain streams, non-biting midges
are certainly the largest group of
dipterans, both in terms of number of
individuals and species.
At high altitudes and in extreme
environments, like the upstream
stretches of mountain streams,
chironomids are usually the only
invertebrates. A clear longitudinal
succession is not only evident in
species, but also in subfamilies.
In upstream stretches, where waters are
cool and well-oxygenated, Diamesinae
and Orthocladiinae abound. These two
subfamilies include several rheophilous
and cold stenothermal species, i.e.,
adapted to life in the fast-flowing, cloudy
and always cold waters of glacial
streams. In particular, larvae of Diamesa
spp. have contrived several adaptations
72
to fast currents and unstable substrates,
like a) long and strong claws with which
to cling to the bottom; b) elongated rear
pseudopods to cling to larger stones and
increase the stability of larvae; c)
construction of sand “tubes” cemented
with saliva to protect themselves from
the current. Besides these strategies,
many of them colonize small depressions
in pebble surfaces, to avoid being
crushed by stones rolling in the stream
bed. These species feed on organic
matter originally carried by the wind
(spores, pollen, vegetal fragments, dead
insects), which is trapped in ice and
released in stream water during the thaw.
The length of chironomid larvae ranges
from 2-30 mm and their colour from grey
to dark yellow, violet, orange, red and
green. Some species of the
Chironominae subfamily
(e.g., Chironomus of the thummi group),
are typically red: these are the ordinary
“bloodworms” on which aquarium fish
often feed. In nature, they live in waters
characterized by high organic pollution.
Their blood-red colour is due to the
presence in their haemolymph of a
respiratory pigment very similar to
human haemoglobin, which enables
them to survive even in poorly
oxygenated environments. All larvae are
worm-shaped, i.e., have elongated,
segmented bodies, often covered with
bristles and without real legs.
They only have two pairs of hooked
“pseudolegs”, front and rear, with which
they cling to the substratum.
The lives and even the shapes of
chironomid larvae are considerably
determined by their feeding habits.
Some larvae are herbivorous and scratch
the algal and bacterial cover of the
substrates they colonize; others are
detrivorous and collect particles of
organic debris from the bottom; and yet
others are carnivorous and prey on small
animals. These generally belong to the
Tanypodinae subfamily. Predatory larvae
have contrived various adaptations to
this kind of life: a barely sclerotized chin,
in order to swallow prey whole,
scythe-shaped jaws like pincers, a large,
sclerotized tongue which rolls
backwards to push the prey down the
pharynx, retractile antennae on
hydrodynamic heads, and greatly
elongated pseudopods to move quickly
and jerkily.
In other subfamilies, sclerotized chins
and oblique-moving jaws scratch algae
and debris off submerged surfaces.
Non-biting midges living on the
sandy-muddy bottoms of lakes generally
produce tubes, in which the larvae
create currents of water by waving their
bodies to and fro. This movement
conveys food particles (algae, bacteria,
etc.) to a specially constructed net,
which is produced with their own saliva
and lies on the bottom.
Periodically, the larvae swallow this net
and replace it with a new one.
Some species burrow thin tunnels in the
submerged leaves of aquatic plants.
Others are symbionts or parasites of
other invertebrates.
Pupae and larvae have similar sizes and
colours, but the front part of their body
is swollen with the cases of adult
antennae, legs and wings. Pupae may
swim freely or, for those species living in
tubes, remain partially enclosed in the
larval tube itself. The length of adults
ranges from less than 1 to about 14 mm
and they have very long antennae,
feathery in males. Their thin body looks
like that of mosquitoes, but chironomids
have a humped thorax which partially
shields their head, and sucking mouth
parts instead of biting ones.
This is why chironomids are known as
“non-biting midges”.
Their long legs shake in a typical way
during flight, so that they appear to be
“gesticulating” (their name comes from
the Greek chironomeo = to gesticulate).
Females lay eggs on the water surface
or near the shore, gathering them in
gelatinous masses which may be free or
attached to the substratum.
The new-born larvae moult 4 or 5 times
before metamorphosis. Emergence may
occur in spring, summer and/or autumn.
Adult life is short and there is only time
to mate and lay eggs.
As regards altitudinal distribution of
chironomids in streams, higher
stretches, where water temperature in
summer never exceeds 3-4°C, are
dominated by the subfamily of
Diamesinae with the genus Diamesa,
represented by species like Diamesa
steinboecki, D. latitarsis,
D. goetghebueri, D. bertrami and
D. zernyi. Another Diamesinae found in
upstream stretches, and sometimes the
most frequent (in non-glacial streams) is
Pseudokiefferiella parva. In the same
stretches, Pseudodiamesa branickii may
also be found, particularly between
mosses and algae. Together with
Sketch representing the life-cycle of non-biting midges (chironomid dipterans): eggs ( 1 ); larva ( 2 );
pupa ( 3 ); adult or imago (male 4 and female 4 )
73
74
Diamesinae, there may also be
Orthocladiinae, such as Orthocladius
rivicola gr., O. frigidus, Eukiefferiella
minor/fittkaui, E. claripennis gr. and
Tvetenia calvescens/bavarica.
At medium altitudes, Orthocladiinae
becomes the largest subfamily, for
number of individuals, genera and
species (Chaetocladius piger gr.,
Heleniella ornaticollis, Corynoneura spp.,
Thienemanniella partita, etc.).
Diamesinae and Orthocladiinae are
replaced downstream by
Chironominae, particularly Micropsectra
atrofasciata gr.
Simuliids (blackflies). Together with nonbiting midges, blackflies are the first
colonizers of high-altitude streams,
where they may assemble in large
numbers if conditions are favourable.
Their larval stage is spent in running
water, particularly in fast-flowing
stretches.
Adult specimen of black fly
75
Larvae of black flies (simuliid dipterans)
The special technique adopted by blackfly larvae to move upstream
They are a very old group, as shown by
a fossil pupa which dates back 160
million years, well into the Jurassic
period. Today, 1500 species are known,
400 of which live in Europe and 70 in
Italy.
The life-style of blackflies is particularly
interesting at both larval and adult
stages. Larvae are typically pearshaped, with a swollen abdomen.
They feed on fine organic particles and
bacteria, which are filtered by special
organs called cephalic or mandibular
fans. These derive from the
transformation of labial parts which
evolved into fans, formed of a series of
rami, generally between 30 and 50, upon
which many hairs grow, creating an
efficient capturing system. The larvae
have silk glands which produce a
viscous liquid with which they adhere to
the bottom, by means also of hooks at
the end of their abdomen and on their
single thoracic pseudopod. The two
series of hooks alternate with new
adhering systems, enabling them to
move on smooth surfaces upstream, in
search of micro-habitats where the
current is faster and more water can be
filtered. The silk glands are also used
when moving downstream. In this case,
the larvae stick silt to the substratum
and quickly spin a “safety rope”, holding
on to it until they reach the desired
position. This original system enables
the larvae of black flies to occupy even
smooth surfaces of rocks exposed to the
current, where few invertebrates can
resist. Thus, they may avoid competition
for space and the danger of being
preyed upon. In stream beds with
macrophytic vegetation, these larvae
adhere to various plant parts, and a
single emerging grass stem may host up
to 200-300 of them. The larvae have 6-9
instars until they build a triangular
envelope in which they begin their
metamorphosis. In the Prosimuliinae
subfamily, the envelope is very small and
its shape is not well-defined.
Adults look like small, humped flies, with
a “flattened” nose (from which their
name derives), and the females of many
species are haematophagous (bloodsuckers). A “blood meal” is needed to
take on organic iron to ripen eggs.
Very restrictive conditions govern which
host’s blood is sucked and thus some
species attack birds (subgenera
Eusimulium, Nevermannia), horses
(subgenus Wilhemia) or bovines
(Prosimulium latimucro, P. rufipes,
Simulium ornatum, S. intermedium,
S. variegatum, S. paramorsitans).
If cattle are severely attacked, an intense
anaphylactic reaction may even kill the
animals, and in Alpine areas there have
been cases of murrain since the early
1970s. As attacks occur during the day
and never in closed areas, serious
damage may be avoided by quickly
providing shelter. Humans may also be
bitten, generally with no complications
except for those who suffer from allergic
reactions.
The development of larval groups of
black flies is helped by slightly increased
organic pollution, and consequent larger
numbers of bacteria and organic
particles in the water. High levels of
pollution limit the presence of these flies,
because smooth surfaces are covered
with bacterial and periphytic covers.
The altitudinal distribution of species is
76
relatively predictable, as many species
are closely linked to their environment
(stenoecious).
Upstream areas are characterized by the
Prosimuliinae subfamily and some
species (Prosimulium latimucro and
P. rufipes) may colonize the lower
stretches of Alpine glacial streams, where
the mean temperature is about 0°C.
In Appennine areas live P. albense and
P. calabrum, the former extending as far
as Sicily, the latter proper to Calabrian
streams. At lower altitudes,
besides Prosimuliinae, there is
also the large Simuliinae
subfamily which belongs to
the subgenus Nevermannia,
found in both Alps and
Appennines, such as Simulium
vernum. Typical Appennine
species are S. fucense and
S. marsicanum.
In medium stream stretches,
there are many species of the
subgenera Simulium,
Eusimulium, Obuchovia and
Tetisimulium, of which some
(Simulium ornatum and S. variegatum)
are tolerant or highly tolerant
(S. intermedium) of organic pollution.
Limoniids (crane flies). Limoniids are a
dipteran family with many species
(about 320 in Italy), most of which are
found in northern regions; larvae are
usually terrestrial although a few live in
streams. The larvae of some species
typically live on muddy banks, others
live in silk tubes covered with debris
between emerging plants, and may be
herbivorous, detrivorous or carnivorous.
The most common genera in
high-altitude streams are Dicranota,
Rhypholophus and Tricyphona.
Thaumaleids (solitary midges). In Italy,
there are 14 species belonging to 4
genera, of which Thaumalea is the most
common in the Alps. The larvae are very
similar to those of chironomids and may
easily be confused. They differ for their
cephalic capsule, which is slightly
flattened, setose and covered with small
dorsal protuberances. They have only
two pseudopods, front abdominal and
rear, and a large sclerotized dorsal
plaque on each segment of their body.
Thaumaleid larvae are frequently
found in mountain streams on rocks
either splashed or covered by a
thin layer of water (hygropetric
larvae).
Blepharicerids (net-winged
flies). Among dipterans, these
are the best suited to life in
fast-flowing waters, due to the
peculiar and unmistakable
“architecture” of their larvae
(see drawing). The dorsal part
of their body is made up of a
series of round arches which can
withstand the pressure of the current.
Their ventral part has strong suckers
which enable them to adhere to the
substratum even under powerful
waterfalls. Blepharicerids have the most
functional and complete suckers of all
insects: they have a real air-pump,
made up of powerful muscles.
This produces a vacuum, enabling them
to adhere to the substratum perfectly.
Each adhesive disc has a front slit to
detach the sucker and to allow
movement. They have segmented
abdomens and small, conic
pseudopods, branchial tufts and lateral
appendices.
The larvae feed by scratching the
bacterial or periphytic covers of the
substratum, usually large, smooth
surfaces washed by fast-flowing water.
Pupae, like larvae, live attached to the
substratum in the same environments,
and their size is similar to that of mature
larvae (5-12 mm). The pupae adhere to
the substratum with 3 or 4 suckers
located ventrally along both sides of the
abdomen.
Net-winged midges are considered
indicators of good environmental quality,
as they are very sensitive to several
forms of pollution. This is not only due
to their physiological intolerance to
chemico-physical variations in water, but
also to a series of indirect effects, above
all reduced adherence to the substratum
caused by the overgrown bacterial
covers, filamentous algae and mosses
which coat the surfaces of eutrophic
environments.
The poor mobility of the larvae and their
habit of colonizing the upper part of
stones makes blepharicerids sensitive
to sudden variations in water flow typically streams which are subject
to hydroelectric regimes.
The presence of these animals
indicates good trophic and
hydraulic conditions.
In Italy, there are 5 genera and
13 species, all living in
freshwater.
Among these, Liponeura
cinerascens with its two
subspecies minor and
cinerascens,
lives at high
altitudes, in
the Alps
and in the ridge between the
Appennines and the Maritime Alps.
Athericids (snipe flies). This is a small
family of brachycerans, with aquatic
larvae and adults which sometimes suck
the blood of other arthropods or
mammals (see drawing). They are
frequently found in streams with low or
moderate quantities of organic matter.
The larvae live in calm areas of the
stream, burrowing in sand or gravel,
under the bark of submerged branches,
or on mosses. The females of many
species lay eggs in a peculiar way:
many of them gather on the branches of
a tree suspended over water, lay
masses of eggs, and then die, remaining
suspended from the branch. Later, the
larvae plunge directly into the water
below.
Three species are usually found in
streams: Atherix ibis, Ibisia marginata
and Atrichops crassipes.
Empidids (dance flies). This is a large
family, with 270 species known in Italy.
It is widely distributed and lives in
almost all environments, including high
altitudes (over 2000 m). The larvae may
be terrestrial, aquatic or semi-aquatic.
In streams, they live among mosses
and stones, or on fine, wet sediments
on the bank. Their cephalic capsule
is small and their mandibular
hooks are particularly welldeveloped. This is due to the
feeding habits of the larvae,
which prey on simuliids and
chironomids voraciously.
Other dipteran families.
These
families
include
species which
live in peculiar environments, like drain
residues, waste, thermal and sea
77
78
waters. Among these are moth
flies (Psichodidae), biting
midges (Ceratopogonidae),
houseflies (Muscidae) and soldier
flies (Stratiomyidae). These families
generally live in areas of streams
rich in organic debris, submerged
vegetation or fine sediments (sand
and silt).
Trichopterans (caddisflies).
Trichopterans derive their name
from the Greek words trichos (hair)
and pteros (wing). Flying adults
look like butterflies with hairy,
light-coloured wings. When at
rest, the four wings fold roof-like
to cover the animal. More than
300 species are known in Italy,
divided into 20 families. Adults are
crepuscular or nocturnal, and often fly in
large swarms in autumn evenings near
watercourses or lights. The life of the
larvae is particularly interesting, aquatic
in both running and standing waters
(except for the genus Enoicyla, which
lives on land). The uniqueness of
trichopteran larvae lies in their ability to
construct variously shaped cases for
different purposes.
As regards larval shelters, trichopterans
may be divided into three groups: with
mobile, fixed, or free cases.
Most species belong to the first group.
They construct cases around their
bodies to protect themselves and to
increase their resistance to the current.
Their constructing technique consists in
the production of a silk-like secretion to
which inorganic (sand, pebbles; see
drawing) or organic materials
(mollusc shells, pieces of leaves, stems
or twigs) stick.
The case is conic, with inorganic
material in the sericostomatid,
odontocerid, and beraeid families
and with organic or mixed
material in the brachycentrid,
lepidostomatid, leptocerid and
limnephilid families.
Hydroptilids (microcaddis)
make peculiar cases.
These tiny species build
seed-like cases made of silk
and grains of sand.
Goerids insert small stones
next to cylindrical cases, used
as stabilizers to prevent
overturning; the cases of
glossomatids are like camel humps,
with an opening in each apex.
The building technique of
trichopterans larvae has even been
adopted by jewellers who breed larvae
in aquariums containing grains, sticks,
gold leaf and small precious stones of
various kinds, to obtain very original
and unique “biological” jewels.
The larvae of other families
(hydropsychids, polycentropodids,
philopotamids) build silk cases not to
protect themselves, but to collect food.
These are real “cobwebs” with a conic
shape, open to the current, which filter,
or rather capture dead or living organic
matter carried by water.
These larvae have pygopods with long
bristles which form small “brooms”,
used to sweep the nets and collect the
catch to eat it. Nets are built under
stones, rocks, or where water cannot
damage them.
Trichopterans also include species
(rhyacophilids) the larvae of which do
not construct cases, but live free and
prey on benthic macroinvertebrates.
79
Larval case of caddisfly (limnephilid trichopteran)
Adult caddisfly
Their development is holometabolous
and when metamorphosis approaches,
the larvae, which live in mobile cases,
climb rocks or similar projections and fix
themselves near the water surface.
The front opening closes, and the larva,
now a pupa, begins the extraordinary
process which will transform it into a
winged adult.
Long, orderly strings of pupal houses are
frequently seen on submerged rocks
along the water level.
Non-case-building trichopteran larvae
pupate inside a specially built silk case,
fixed under rocks.
The poor mobility of most trichopteran
larvae makes them vulnerable to sudden
changes in water flow, and they are often
miserably stranded after storms, when
water retreats.
This phenomenon is more frequent in
streams subjected to hydroelectric
peaking. Variations in flow are
considerable and sometimes due to
natural causes, as in streams supplied
with meltwater from glaciers. Therefore,
poor mobility, preference for mediumslow-flowing waters, and their prevailing
trophic role as shredders-collectors of
coarse organic matter (mostly dead
leaves) limit the presence of
trichopterans to stretches under trees.
However, this does not prevent them
from colonizing favourable environments
at elevations exceeding 2500 m a.s.l.
Higher stretches are colonized by
orophilous and stenothermal species of
cold waters, like Rhyacophila tristis,
Philopotamus montanus, Plectronemia
conspersa and Drusus discolor.
In downstream stretches live other
species of high ecological value, such
as Rhyacophila torrentium
Hydropsyche instabilis, Potamophylax
cingulatus and Sericostoma
pedemontaum.
Vertebrate fauna
SERGIO PARADISI
■ Fishes
Salmonids of Italian streams. The species which popular belief
immediately associates with mountain streams is trout. However, the
distribution of this fish is not limited only to this kind of water, but includes
pedemontane streams, large pre-Alpine lakes of glacial origin and plain
streams. These environments are all characterized by cool or cold water,
with tempe-ratures seldom reaching 20°C, usually well below that figure,
and therefore, generally well-oxygenated. It is the oxygen requirement of
trout and its very active metabolism which closely link this fish to highaltitude mountain streams, where water turbulence increases the percentage
of oxygen to saturation levels. Hydrobiologists classify this stretch as “upper
and medium salmonid area”, or simply “trout zone”.
As regards fish, trout is therefore the guide-species of these waters, the best
suited to these ecological conditions, and sometimes the only one present.
But trout is an easy definition. Which trout lives in Italian freshwaters? The
answer to this question is all but obvious, and this issue is the basis of
research and discussion for many scientists who still have not come to an
agreement. According to Tortonese, whose two volumes in the series “Fauna
d’Italia”, devoted to bony fish appeared in the early 1970s, all Italian trout
belong to the polymorphic and polytypic species Salmo trutta, characterized
by variable size, skin and behaviour, with local varieties and ecotypes. The
nominal variety Salmo trutta trutta, to which stream or brown (“fario”) trout
belongs, is found in all regions (including Sicily and Sardinia). There are also
two endemic subspecies: “marble” trout (Salmo trutta marmoratus), found in
the river Po and its Alpine tributaries, and the Lake Garda trout (Salmo trutta
carpio), typical of the largest Italian lake. The “lacustrine” and “Sardinian” or
“macrostygma” trout do not have systematic value.
Twenty years after Tortonese’s work, Gandolfi and Zerunian proposed a different systematic interpretation which, however, they did not consider definitive. In
their opinion, Lake Garda trout is one species (Salmo carpio) and Lake of Posta
Fibreno trout (Salmo fibreni) is another. They proposed calling Salmo trutta a
Spectacled salamander (Salamandrina terdigitata)
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