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I TA L I A N H A B I TAT S
Volcanic lakes
17
Italian habitats
Italian Ministry of the Environment and Territorial Protection / Ministero dell’Ambiente e della Tutela del
Territorio e del Mare
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 committee
Aldo Cosentino · Alessandro La Posta · Carlo Morandini · Giuseppe Muscio
“Volcanic lakes · Fire, water and life”
edited by Fabio Stoch
Texts
Silvia Arisci · Marcello Bazzanti · Arnaldo Angelo De Benedetti · Renato Funiciello · Mauro Iberite ·
Laura Lepore · Fiorenza Gabriella Margaritora · Luciana Mastrantuono · Giuseppe Morabito ·
Michela Rogora · Marco Seminara · Fabio Stoch · Daria Vagaggini
In collaboration with
Raffaella Berera · Vezio Cottarelli
English translation
Elena Calandruccio · Gabriel Walton
Illustrations
Roberto Zanella
Volcanic lakes
Fire, water and life
Graphic design
Furio Colman
Photographs
Nicola Angeli 47, 48 · Archive Museo Friulano di Storia Naturale 58, 60, 61, 62, 63, 64/3, 106 ·
Andrea Balestri 64/5 · Raffaella Berera e Vezio Cottarelli 90 · Compagnia Generale Ripreseaeree 10, 116 ·
Vitantonio Dell’Orto 98, 107, 109, 111, 112, 114, 115, 134, 135, 137 · Giuseppe Di Lieto 113 ·
Dario Ersetti 59 · Paolo Fabbro 6, 7, 15/1, 15/2, 18, 21, 38, 40, 139 · Renato Funiciello 11, 22, 132, 145 ·
Mauro Iberite 55, 64/4 · Giuseppe Ippolito 86 · Luca Lapini 108, 136 · Giuseppe Morabito 45, 51, 52, 53 ·
Giuseppe Muscio 13, 26, 41, 46 · Naturmedia 99, 102 · Roberto Nistri 64/2, 103, 105, 130, 138 ·
Fabio Stoch 9, 16, 17, 20, 23, 27, 28, 29, 33, 34, 37, 42, 43, 44, 57, 64/1, 64/3a, 65, 66, 67, 68, 69, 70,
71, 72, 73, 74, 75, 80, 84, 85, 87, 89, 92, 93, 95, 96, 97, 104, 110,121, 122, 123, 125, 126, 129, 131,
142, 143, 144 · Damiano Vagaggini 24, 25, 35, 54, 78, 79, 88, 100, 101, 117, 118, 119, 120, 124, 128
©2007 Museo Friulano di Storia Naturale, Udine, Italy
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system or trasmitted in any form or
by any means, without the prior permission of the publishers.
ISBN 88 88192 34 4
ISSN 1724-6539
Cover photo: Lakes Nemi and Albano, Latium (photo Compagnia Generale Ripreseaeree)
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 E D E L M A R E
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
Contents
Italian habitats
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Fabio Stoch · Daria Vagaggini
Geological aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Arnaldo Angelo De Benedetti · Renato Funiciello
1
Caves and
karstic
phenomena
2
Springs and
spring
watercourses
3
Woodlands
of the Po
Plain
4
Sand dunes
and beaches
5
Mountain
streams
6
The
Mediterranean
maquis
Hydrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Silvia Arisci · Laura Lepore · Michela Rogora
Phytoplankton. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Giuseppe Morabito
Macrophytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Mauro Iberite
7
Sea cliffs and
rocky
coastlines
8
Brackish
coastal lakes
9
Mountain
peat-bogs
10
Realms of
snow and ice
11
Pools, ponds
and
marshland
12
Arid
meadows
Zooplankton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Fiorenza Gabriella Margaritora · Daria Vagaggini
Zoobenthos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Marcello Bazzanti · Luciana Mastrantuono
Vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Marco Seminara
13
Rocky slopes
and screes
14
High-altitude
lakes
15
16
Beech forests The pelagic
of the
domain
Apennines
17
Volcanic
lakes
18
Mountain
conifer forests
Conservation and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
Fabio Stoch · Daria Vagaggini
Suggestions for teaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Marco Seminara
Select bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
19
Seagrass
meadows
20
21
Subterranean Rivers and
waters
riverine
woodlands
22
23
Marine bioLagoons,
constructions estuaries
and deltas
24
Italian
habitats
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
List of species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Introduction
FABIO STOCH · DARIA VAGAGGINI
Even the most inattentive passenger
in an aircraft flying over the Italian
peninsula cannot fail to notice round,
sky-blue bodies of water dotting the
landscape below. Although these
lakes, large and small, apparently calm
and surrounded by steep shores, are
spotted at a glance, only those who
know them really well are aware of
their true nature and complexity.
The protagonists of this pubblication
are volcanic lakes, unique Italian
environments which formed in ancient
times, at the dawn of civilisation. They
have therefore witnessed man’s social
and cultural development in a strikingly
beautiful natural context. Findings of
Lake Nemi (Latium)
Neolithic human settlements along the
shores or below the surface of volcanic lakes show how closely associated
mankind and lakes have been since prehistoric times. Later, the Etruscans and
Romans were the first populations to use these environments for water supplies.
An example is Trajan’s aqueduct, later called dell’Acqua Paola which, even
today, after a thousand years, conveys the water of Lake Bracciano directly into
the Janiculum Fountain in Rome. Volcanic lakes were not only used to supply
water, but also aroused curiosity and fear, as well as poetical inspiration. For
instance, in the 6th book of the Aeneid, Virgil tells us that the Cumaean sibyl took
Aeneas to the gates of the Underworld near the Lake Averno (facilis descensus
Averno: noctes atque dies patet atri ianua Ditis - the descent into the Averno is
easy: the gate to dark Hades is open night and day). Researchers are tracing the
invisible line that connects the past to the present, aiming at improving
knowledge of the mechanisms that regulate these delicate ecosystems and, as
lovers of nature, at discovering and learning to respect volcanic lakes.
Lake Albano (Latium): the crater-shaped rim of the lake
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8
Early research dates back to the late 19th century, when enthusiastic scientists
with simple instruments walked long hours to perform their geological surveys,
or sailed across these lakes to draw bathymetric maps and to collect samples
for chemico-physical and biological analyses. Although modern scientists have
better instruments and more knowledge, they share the same eagerness to learn
and understand these habitats.
Prolonged research has highlighted several characteristics that make volcanic
lakes extremely valuable environments - first, their origin due to volcanic action in
Quaternary times, which was very powerful in Italy. It created the craters and
calderas (wide, shallow craters due to collapse of the central part of a volcano)
that today contain these lakes. The particular morphology of these bodies of
water, which are generally very deep for their total area, is due to their particular
formation. The apparent tranquillity of their waters actually conceals deep
changes in the Earth’s crust caused by still unremitting volcanic action - there are
still phenomena of secondary volcanism, such as the release of gases - the same
that terrified people in ancient times. In addition to their peculiar geological
characteristics, volcanic lakes host several animal and plant communities which
either live in their waters or use them for their nutrient content. The plant
successions circling their shores and submerged aquatic macrophytes are the
“lungs” of these ecosystems, together with various kinds of micro-algae, the
fundamental link in the food-chain. These lakes are also inhabited by free-floating
animals and those closely associated with lake bottoms. These links in a foodchain dominated by fish and aquatic birds all live and reproduce in these
environments, invaluable troves of biodiversity. Precisely because of the
importance of their fauna, all these lakes are included in protected areas, Sites of
EU Interest (Habitats Directive) or in Special Conservation Areas (Birds Directive).
We may be led by these words to think that the relationship between man and
nature is compatible with the welfare of both. Unfortunately, this is not so.
Several threats to their integrity and equilibrium loom over volcanic lakes as
broken links of the invisible line connecting past and present. The nonsustainability of some human activities - such as intensive agriculture, excessive
animal grazing and tourism near these standing waters, the introduction of alien
species into them and changes in shore profiles - are only some of the factors
which in recent years have caused gradual deterioration of the quality of the
waters, and have jeopardised their flora and fauna.
This new volume of the Habitat Series has two objectives. One is to describe
volcanic lakes from several viewpoints, from their origins to their chemicophysical characteristics and biological communities. The other is to emphasise
their value for the conservation of nature by focusing on the main problems
related to their management. We hope this volume may contribute towards
protecting these precious environments; which have supported man for
millennia and which now risk irreversible deterioration.
The area around Naples in a map of 1817-19, before the lakes in the Astroni crater were reclaimed
Lush vegetation along the banks of Lake Martignano (Latium)
9
Geological aspects
ARNALDO ANGELO DE BENEDETTI · RENATO FUNICIELLO
Over the past two million years, Italy is
the European area that has most been
affected by Quaternary volcanism.
This phenomenon is associated with
the faulting of the Tyrrhenian edge of
the Apennines, along Tuscany and
Campania, an event which took place
over the last 10 million years.
The peri-Tyrrhenian volcanic arc
(Pleistocene-Holocene epochs) runs
for 420 km NW-SE from Val d’Era in
the north (Orciatico and Montecatini,
Val di Cecina), to Vesuvius in the south,
with only a few points inland (San
Venanzo, Cupaello and Pollino) or near
the Apennine faults (Monte Vulture).
This volcanic arc developed along the
Outcropping eruptive rock forms the island of
Martana in the middle of Lake Bolsena
western margin of the Tyrrhenian backarc basin.
A thinning of the Earth’s crust is visible in the geophysical structure of the
lithosphere, together with extensive, progressive thermic flow within the
Tyrrhenian basin which, in its southern part, gives rise to one of the most
active geodynamic elements in the Italian area. Thinning allowed the
subcrustal intrusion of a type of syenitic rock at Larderello (Tuscany) to the
area around Rome (Sabatino District), together with mantle upwelling in the
southern Tyrrhenian basin. Distensive tectonics began in the Middle-Upper
Miocene through the formation of normal faults running NW-SE and
subsidence to the NE, and perpendicular transcurrent NE-SW-running faults
that dismembered the Apennine chain. The result was the formation of
sedimentary basins filled with sand and clay to depths sometimes reaching
1000 m, and also deposits from environments transitional to shallower sea
basins before external volcanism started.
Aerial view of the Sabatino Volcanic District. Left: eastern shore of Lake Bracciano; right: Lake
Martignano and (top) crater that once hosted Lake Stracciacappa.
11
12
deep basins in
African - Adriatic domain
newly formed
oceanic crust
Miocene
mountain chain
slip vectors of orogenetic transport
strike - slip faults
inactive front of Europa-verging mountain chain
active thrust front
active normal faults
Simplified sketch showing the formation of the Tyrrhenian Sea, and front migration of the Apennine chain
Volcanoes are closely associated with
this faulting network that produced
tecto-volcanic structures like the
calderas of the Alban Hills and the
Phlegraean Fields or large dyke
networks like the Somma-Vesuvius
structure.
Integrated geological and geophysical
data show that, under the volcanic arc
and post-orogenic clastic deposits,
the Mesozoic-Cenozoic (the Mesozoic
epoch dates back 248-65 million
years ago; the Cenozoic followed)
carbonatic
succession
thinned,
overthrust and laterally extended
along a décollement surface on the
crystalline basement at a depth of
Volcanic deposits in the Vulture mountains
(Basilicata)
about 7-8 km. According to spacetime distribution and petrographic
criteria, the Apennine chain is divided into three petrographic areas called
magmatic provinces: Tuscan, Roman, and the recently identified Lucan. In
each of these provinces, igneous rocks with differing petro-chemical affinities
outpoured inside the main magmatic associations.
From the morphological viewpoint, volcanism gave rise to: caldera edifices like
those of Bolsena-Latera, Bracciano-Sacrofano, Vulcano Laziale (Alban Hills)
and the Phlegraean Fields; stratovolcanoes like Vico, Faete, Roccamonfina,
Ventotene, Ischia, Procida, Monte Somma-Vesuvius and Vulture; domed
shield volcanoes like Monte Amiata, the centres of Tolfa, Ceriti-Manziate,
Monti Cimini and the Ponza islands; or monogenic volcanoes like those of the
Umbrian District, Monti Ernici, and Ponza.
Many volcanic structures such as cones, craters, calderas, lava flows and lava
fields are still well-preserved. Calderas and craters are the best places for rain
and hydrothermal water to collect. This is due to the elevated rims of both
structures and to the not very permeable nature of volcanic products, which
hinders water drainage. The formation of a lake is therefore the result of a
series of factors, the most essential of which is the substrate and its
relationship with the surface watertable, as craters and calderas are like large
natural wells fed by these waters. According to their origin, volcanic lakes are
divided into primary and secondary.
13
Primary lakes are standing waters
contained in volcanic edifices like
calderas, tecto-volcanic depressions
and craters, and are described in this
volume. Secondary lakes are all basins
whose origin, at least partially, was due
to volcanism, but they formed in rocks
which are not of volcanic origin (like
sinkhole lakes, particular types of
basins described on p. 16).
Lastly, there are pseudo-volcanic lakes
forming from the outflows of hot
springs and mineral waters. In Italy,
there are only a few lakes of this type,
examples being Bagno dell’Acqua on
the island of Pantelleria, and the lake at
Arquà, in the Euganean Hills.
14
TN
AO
VE
TS
MI
TO
BO
GE
FI
AN
1
2
PG
AQ
3
5
4
7
6
ROMA
CB
8 9 10
11
NA
12
17
18
13 14 16
15
BA
15
Lake Bagno dell’Acqua (Pantelleria, Sicily region)
19
PZ
■ Tecto-volcanic depressions and
calderas
CA
22
20
PA
RC
21
Volcanic edifices
1
2
3
4
5
6
7
8
9
10
11
ORCIATICO-MONTECATINI
AMIATA-RADICOFANI
LATERA-BOLSENA
CIMINI
VICO
TOLFA-MANZIANA-CERITE
BRACCIANO-SACROFANO
COLLI ALBANI
MEDIA VALLE LATINA
S.VENANZO-CUPAELLO
ROCCAMONFINA
Age (years)
onset
end
4,100,000
1,000,000
720,000
1,310,000
410,000
2,500,000
600,000
600,000
680,000
420,000
600,000
200,000
<130,000
900,000
90,000
<40,000
6,000
<80,000
55,000
Volcanic edifices
12
13
14
15
16
17
18
19
20
21
22
PONZA GROUP
MONTE GUARDIA
VENTOTENE
ISCHIA
PROCIDA
PHLEGRAEAN FIELDS
VESUVIUS
VULTURE
IEOLIAN ISLANDS
ETNA
USTICA
Age (years)
onset
end
2,500,000
1,230,000
>1,000,000
150,000
<50,000
60,000
400,000
600,000
1,300,000
500,000
750,000
Ages of onset of activity, and time-span involved, of the main Italian volcanic edifices
1,070,000
480,000
1306 D.C.
1538 D.C.
1944 D.C.
110,000
PRESENTE
PRESENTE
130,000
Some Italian volcanoes have calderas, or
tecto-volcanic depressions, definitions
that in the past were distinct, but which
today are considered synonymous.
Lake Arquà (Euganean Hills, Veneto)
They are vast depressions inside the
original crater caused by the violent
emanation of large quantities of gases, partial emptying of the magma chamber,
and consequent collapse of the central part of the volcano above the vent due to
modified mechanical conditions underground. Large Italian volcanic lakes are
Bolsena, Vico and Bracciano.
Lake Bolsena. Once known by its Latin name of Lacus Volsiniensis, this is the
largest Italian volcanic lake (114 km2, 151 m deep). Located in upper Latium,
near Viterbo, it has two islands, Bisentina (0.17 km2) and Martana (0.10 km2). Its
origin was due to the outflow of large quantities of magma during the early
stages of the history of the Vulsino Volcanic District (576-500,000 years ago)
which, according to the most reliable models, caused the top of the partially
emptied magma chamber to collapse and the above area in turn to collapse
16
Sinkhole lakes in volcanic areas
Sinkhole lakes are associated with
volcanic activity. They are created by
the collapse of a generally circular
area, between a few and one hundred
metres in diameter, caused by the
formation of shallow cavities
underground. Thanks to their
particular shape, sinkholes often fill
with water, giving rise to ponds or
small lakes.
The original cavities giving rise to
sinkholes may be produced by poorly
coherent sediments migrating to the
bottom of palaeo-karstic cavities,
which gradually fill with water.
Others may form as a result of
chemical dissolution - closely
connected with volcanic activity since the aggressive substances
responsible for this phenomenon are
rich in carbon dioxide and hydrogen
sulphide.
Renato Funiciello
Solution greatly affects limestone,
which is widespread in Italy, giving
rise to karstic cavities.
Among the many bodies of water that
have formed in this way, there are
those near Rome (Lago di Puzzo, or
Lago di Leprignano, Lago Nuovo and
Lago di Giulianello); in the Piana di S.
Vittorino (Rieti), the best-known of
which is the Lago di Paterno; at the
foot of the Lepini mountains (Laghetti
del Vescovo); and near Caserta (Lago
di Vairano, Lago di Corree).
Lastly, in the Acque Albule area (Bagni
di Tivoli, near Rome), a series of
dolinas have become small lakes,
some of which are still active.
Examples are the Lago di S. Giovanni,
Laghetto Regina and Laghetto delle
Colonnelle: these sulphur springs
are called Albulae because of their
milky colour.
Laghetti del Vescovo, at the foot of the Lepini mountains (Latium)
inwards. The distribution of alternating
volcanic and lacustrine deposits shows
that, over time, the lake underwent
deep modifications associated with the
complex evolution of the Vulsino
District. The first great eruptions
released about 50 cu.km of magma - a
far greater amount than the catastrophic
eruption of Vesuvius in 79 AD which
destroyed Pompeii (about 1 cu.km). This
phenomenon occurred after large
quantities of lava scoriae, deposited in
Lake Bolsena (Latium)
the northern circum-caldera area (360352,000 years ago), were followed by
ignimbritic scoriae alternating with lava and lake materials. Recent underwater
archaeological findings of Bronze Age settlements show that the lake level
remained the same (294 m a.s.l.) for a long time. In the Iron Age, it rose rapidly for
reasons still unknown, up to the highest level allowed by the natural point of
flooding (inlet) at 306 m a.s.l.. Later, to reclaim land for agriculture, the Etruscans
lowered its level to 303 m by cutting a rock spur that dammed the flow of the river
Marta, in an area that is still called “Sasso Tagliato” (Cut Rock). The current level
of the lake (303.5 m) is maintained by a masonry inlet and sluice-gates
constructed in the Middle Ages in the port of Marta, at the outlet.
Lake Vico. Lake Vico, or Lacus Ciminus, is the highest Italian volcanic lake (510
m). According to legend, it was produced by the club Hercules thrust into the
ground to challenge the local population. Nobody was able to remove it. When
Hercules pulled it out, water flowed and filled the valley, thus creating the lake.
Lake Vico is actually a partly filled-in caldera which underwent several
collapses due to the settling of types of ignimbrite like Tufo Rosso a Scorie Nere
(red tuff with black scoriae). When the eruptions of the whole Vico area ended
(80,000 years ago), underground water and rain gradually filled the caldera. The
lake is surrounded by the Monti Cimini, Monte Fogliano (965 m) and Monte
Venere (851 m). Lake sediments reveal that, for several millennia, the lake was
larger, and its water reached the rim of the caldera, Monte Venere then being a
peninsula in the north-eastern part of the lake. It is still not known whether the
Etruscans or the Romans modified its structure by lowering the water level by
20 m through an artificial underground canal. What we do know is that the canal
was built before the Cassia Cimina, during Roman Imperial times. In the 1500s,
17
18
the Farnese family re-opened the canal (today’s Rio Vicano), lowered the level
of the lake by another 3 m, and regulated it with a weir that is still in place. The
lowering of the water level provided farmland and favoured the settlement of
farmers and shepherds. Today, the lake covers 12 km2 and is 49.5 m deep.
Lake Bracciano. This lake is contained inside a caldera in the Sabatino
Volcanic District (Rome), in central-northern Latium. With its area of 57 km2, it
is the second largest in Latium and the eighth largest in Italy. This lake, which
the Romans called Lacus Sabatinus, is round, with a maximum depth of 165 m
and a volume of over 5 billion m3. Its shores host the villages of Bracciano,
Anguillara and Trevignano, placed at regular intervals along the perimeter.
About 800,000 years ago, a series of volcanic eruptions gave rise to the
Sabatino volcanic system and, when they ceased, the craters filled with water
and formed several lakes, the largest of which is Lake Bracciano.
This vast depression formed as a result of the combined action of local faults
and a large magma chamber. The latter, which was probably a few km below
the surface and fed all the craters of the district, gradually emptied, and its top
collapsed along distension faults which, in turn, produced massive lava flows.
In successive stages, the area which today is a lake, became a large caldera,
forming over a period between 400,000 and less than 150,000 years ago. Its
only natural outlet is the stream Arrone, now dammed, and only used when in
Lake Bracciano (Latium)
flood. Otherwise, the water flows into the river Tiber. In Roman times, the lake
provided drinking water. Trajan’s ancient aqueduct was restored by Pope Paul
V in the early 1600s, and is therefore called “Acqua Paola”. It carries a type of
water known for its low mineral content, which is taken to the fountain
decorating the square on the top of the Janiculum hill in Rome.
■ Crater lakes
Volcanic lakes are ideally shaped for collecting water. Their shape and
impermeable bottom deposits, which prevent water from seeping in depth,
give rise to crater lakes. Generally, crater lakes form in maars, volcanic
craters produced by phreatic and phreato-magmatic explosions associated
with interactions between the watertable and upwelling magma. The word
maar (in German sea) originated in the Eifel area in northern Germany, where
there are many such lakes. When magma comes into contact with
underground water, energy is produced by the sudden change in the state of
the water - from liquid to vapour - which causes it to increase in volume about
100 times. Magma rapidly wells upwards and is fragmented into minute
elements the size of ash particles. The limestone basins of the Apennine
chain provide the water supply that interacts with magma causing this type of
eruption. Interactions may take place at various depths, and even surface
Two types of volcanic lake: a caldera lake inside a crater, and a dammed lake, at the foot of a volcanic edifice
19
waters may contribute in marshes and
shallow lakes. Lake water itself may
favour an explosion. However, this
hypothesis contradicts the evidence
whereby most volcanic systems of
this type are produced by a single
explosion, and their craters are
therefore called monogenic (having a
single origin). Instead, there are maars
Lake Nemi (Latium)
where multiple eruptions occur over
time (polygenic), and therefore the
water of crater lakes plays an active role. This is why the level of crater lakes
and the chemico-physical characteristics of their waters are carefully
monitored. Morphologically speaking, maars are typically funnel-shaped, due
to the destruction by explosions of pre-existing rock formations composed of
deposits deriving from previous volcanic activity.
Maars are volcanic formations that must be kept under control in the
assessment of volcanic risk, because of their nature and genesis. The
particular shape of maar craters makes them vulnerable to collapses and
landslides. In addition, the phreato-magmatic explosions that produce them
give rise to pipes which, although filled with various types of deposits, link the
deep sub-volcanic areas to the surface. These conduits can be filled with
gases which accumulate and flow through. The most dangerous gas is carbon
dioxide, and its accumulation under the surface of maar lakes may produce
sudden gas releases, like the one which recently occurred in Lake Nyos in
Cameroon (1984), as well as lake water overturn and flooding. Maar lakes
must therefore be continually checked for seismic activity, gas flow and flank
stability. In Italy, lakes belonging to this category are the lakes of Mezzano in
the Vulsini chain, Martignano and Monterosi in the Sabatini chain, Albano and
Nemi in the Alban Hills, Monticchio in the Vulture, and Averno in the
Phlegraean Fields.
20
Lake Mezzano (Latium)
Lake Mezzano. Known by the Romans as Lacus Statoniensis, this lake lies in the
Caldera of Latera which, in turn, is contained within other calderas. The crater
hosting Lake Mezzano formed at the margin of the Caldera del Vepe, the most
recent of the monogenic Latera calderas. The crater is a maar, which originated
with the last explosion occurring inside the caldera about 160,000 years ago. The
materials released were mainly fine ash and falling blocks of lapilli which
accumulated around the crater, producing its margins. Although the inner margin
21
22
Reclaimed volcanic lakes
If volcanic areas filled by lakes are
reclaimed, they provide fertile land and
lake silt rich in minerals. Many volcanic
lake basins have therefore been
exploited in this way by man. In the
Vulsini mountains, the areas of
Lagaccione, Latera and Montefiascone
once hosted lakes. In the 17th and 18th
centuries, in the Sabatini mountains,
the marshy lakes Stracciacappa (in the
past known as Lago di Straccio) and
Baccano were reclaimed. In 1828, the
Presidenza delle Acque e delle Strade
(the authority in charge of roads and
water supplies) opened a tunnel
between lakes Martignano, Bracciano,
Stracciacappa and Baccano. In his
“Viaggio nel Lazio: la Tuscia e l’agro
pontino” (Travels in Latium: the Tuscia
area and the Pontine agricultural land)
(1815-1818), the Italian mineralogist
and geologist Giovanni Battista
Brocchi wrote: “The inn at Baccano is
located in a basin completely
surrounded by reliefs, which was once
Arnaldo Angelo De Benedetti
a lake and perhaps, in ancient times, a
crater. For as long as man can
remember, it has been partially filled
with water, which was released by
creating an outlet that drained the
water […] which previously used to
stagnate in that location.”
Other fossil lakes are those of Cese,
Morto, Riano and Polline. In the Alban
Hills, there once were lakes Ariccia,
Castiglione, Gabii, Laghetto a Pavona,
Prata Porci, S. Giuliano, Valle Marciana
and Regillo. The last one was drained
in the 17th century, and the plain that
now covers the area is called Pantano
secco, or dry marsh. In the Phlegraean
Fields (Campania), although the Lake
Agnano was reclaimed in 1870,
seventy-five springs with temperatures
up to 75°C have survived. North-west
of the lake, near Astroni, historical
maps show that there were once lakes
called Grande, Cofaniello Piccolo, or
Lago di Mezzo, and Cofaniello Grande,
which are now very small.
is very steep, almost vertical, the outer
margin slopes gently, never exceeding
an angle of 10-15°. The lake (area = 0.5
km2, maximum depth = 31 m) has an
outlet (Fosso Olpeta) that crosses the
caldera and flows into the river Fiora.
Lake Martignano. Located east of
Lake Bracciano, at 207 m a.s.l., this
lake was known by the ancient name of
Lacus Alsietinus. Although it has an
area of 2.4 km2, it is very deep in
proportion (60 m). The Alsietinus
aqueduct, which was built in 2 BC to
exploit the water of the lake that lacked
natural outlets, supplied Augustus’s
Naumachia (an artificial body of water
Lake Martignano (Latium)
surrounded by rows of seats, where
ancient Roman spectacles resembling naval battles were performed) - at the
foot of the Janiculum hill, together with Caesar’s Gardens, private farms, and
the Janiculum fountain. The crater hosting the lake formed after at least three
phreato-magmatic eruptions (there are three overlapping eruptive units,
separated by palaeo-soils or erosive surfaces showing quiescent volcanic
activity), and is considered, at present, the last active centre of the Sabatino
Volcanic District. Underwater explorations in the lake have revealed that the
area has been inhabited by man since Neolithic times. A wooden structure and
evidence of fires 32 m below the current level of the lake show where its margin
was, and large fossil oak trunks indicate that the lake underwent great changes
in level over time.
Lake Monterosi. Known in ancient times as Lacus Janulae, this lake is found in
the Sabatino Volcanic District at 276 m a.s.l. It has an area of 0.3 km2 and a
diameter of 600 m, but is only 7 m deep. It is also historically known as the
place where, in 1155, Frederick I, called Barbarossa (Redbeard), refused to hold
the stirrup of Pope Hadrian IV’s horse, thus causing a diplomatic incident. The
lake formed after a single phreato-magmatic eruption of medium intensity.
The area originally occupied by Lake Ariccia (Latium)
Lake Albano. This lake (Lacus Albanus) is in the Alban Hills (293 m a.s.l.,
covering an area of 6 km2), 15 km from Rome. It is the deepest Italian crater
23
24
lake (-165 m). It is a polygenic maar and was therefore created by more than
one explosion. Stratigraphic analysis of the volcanic centre of Albano,
prompted by seismic risk for the city of Rome, has revealed that at least
seven explosive eruptions occurred at intervals ranging between 70,000 and
29,000 years ago. The exact date of the last explosion is still unknown. Lake
Albano is the most important and recent volcanic centre of the Alban Hills
and morphologically belongs to the “lithosome of the Via dei Laghi”, i.e., the
volcanic edifice created during the last phreato-magmatic phase of the
volcano. In ancient times, the inhabitants of this area continually moved to
various altitudes inside the rim of the crater as the level of the water in the
lake changed. In 394 BC, the Romans built an artificial outlet about 1,200 m
long, that both provided the area with water by means of wells, and drained
it at a height of about 70 m from the lowest point of the crater rim. Recent
analysis has shown that, during the Middle Bronze Age (1700-1350 BC), the
lake frequently flooded the area near Rome, destroying human settlements.
The artificial outlet built by the Romans is therefore considered the first
modern hydraulic work in the world especially constructed to reduce
volcanic risk.
From the geological viewpoint, the volcanic strata inside the crater are made
up of a bottom layer of volcanic material before the explosion of the maar,
which have been completely removed within the crater area. Overlying this
bottom layer are the stratified deposits of the seven eruptions, alternating
with palaeo-soil that formed during quiescent periods. At present, the lake
level is a few metres below the flood line of the draining outlet. The chemicophysical characteristics of the lake are constantly monitored due to the
hazard posed by the great flux of carbon dioxide (CO2) from deep inside the
area. Possible accumulation of CO2 at the bottom of the lake may cause the
lake water to rise suddenly and jeopardise the towns nearby.
Lake Nemi. The second largest lake of the Alban Hills, this lake has an area of
1.6 km2 and a depth of 37 m. Called Lacus Nemorensis by the Romans, it is
316 m a.s.l. and was produced by explosive eruptions that occurred about
150,000 years ago. The lake basin is composed of deposits of the multiple
phreato-magmatic eruptions that produced the crater itself. The crater rim,
which lies on top of deposits of previous volcanic activity, is made up of 2
overlapping ancient craters. Oval in shape, it is oriented along the meridian line.
Ancient waste dumps have been found along its banks, marking the presence
of Neanderthal man, as well as tombs going back to the Iron Age.
In the 1930s, the lake level was lowered by 22 m, in order to raise two
Roman galleys of the 1st century, which were later restored and displayed
nearby. Unfortunately, in 1945, the retreating German army burned them
down, and today their remains and two scale reconstructions are preserved
Lake Albano (Latium)
Lake Nemi (Latium)
25
26
in the Lake Museum. The shallow water of Lake Nemi, compared with that of
the nearby Lake Albano, may be due both to its long inactivity and slow
accumulation of lake deposits, and to the relative weakness of its eruptions,
the deposits of which lacked sufficient energy to be propelled to a distance
and therefore accumulated in the crater reducing its volume. The lake is
drained by a tunnel (1,653 m long, for a total difference of 12.5 m) probably
built by the Ariccia population. It was restored in 1927-28, when the lake was
partially emptied.
the eastern portion of the basin shows that the eruption was not powerful
enough to send its products beyond the 500 m separating the crater rim from
the bottom of the lake.
Lakes Monticchio. The two small Monticchio lakes (600 m a.s.l.) also known
as the “Vulture twins”, lie in the double central crater of the Vulture volcanic
edifice, along its western flanks, near the towns of Rionero and Melfi
(Basilicata). The Lago Grande (Large Lake) (area = 0.4 km2; maximum depth 38
m) and the Lago Piccolo (Small Lake) (area, 0.1 km2, maximum depth 35 m) are
separated from each other by a strip of land about 216 m across, and formed
110,000 years ago. The entire volcanic edifice of the Vulture was created over a
period ranging from 600,000 and 130,000 years ago, and is still affected by
seismic activity and considerable fluxes of carbon dioxide. The deposits
associated with the formation of the two lakes are composed of melilite
pyroclasts the particle size of lapilli, in dune-like layers (Case Agostinelli, at
most 4 m thick) outcropping to the west. The absence of the same deposits in
Lake Averno. This lake is found in the Phlegraean Fields volcanic region,
between Monte Nuovo and Monte Grillo. It is elliptical in shape and has a
maximum depth of 35 m. The lake basin is an extinct volcano that was
created about 4,000 years ago. In 38-36 BC, the dense forest surrounding
the area was dramatically modified by Marcus Agrippa, the Roman
statesman, who cut the trees down and converted the lake into a naval port
(Portus Iulius). In 1538, the eruption of the Monte Nuovo changed the shape
of the lake completely.
The name Averno derives from the Greek Aornon, meaning “area without
birds”, giving rise to the legend that no bird could fly across the lake and live
because of its poisonous sulphurous vapours. The lake was also mentioned
by Homer and Virgil, who represented it as the entrance to hell. It was
thought to be home to the Giants and of the population of the Cimmerians,
who were associated with the so-called “Catacomb” culture and were
believed to live in caves and flee the sunshine. The Greeks later identified
this area as that described by Homer in the Odyssey. The lake was thought
to be bottomless.
Lago Piccolo di Monticchio (Basilicata) and the abbey of San Michele
Lake Averno (Campania)
27
Hydrochemistry
SILVIA ARISCI · LAURA LEPORE · MICHELA ROGORA
From the hydrological viewpoint, lakes
are standing waters which gather in
basins within the Earth’s crust, and
which are not fed by the sea. The
sloping funnel-shaped area surrounding
a lake collects rain and is called its
catchment basin. The watershed is the
highest level in the catchment basin, so
that any water falling beyond it does not
flow into the lake. Catchment basins
greatly affect aquatic ecosystems, as
their area determines the volume of
collected water, their mineralogical
composition influences the basic
chemistry of lakes, and their plant cover
influences the division of flowing and
evapo-transpiring water. Drained water
Lake Bracciano (Latium)
is charged not only with organic and
inorganic matter, but also with pollutants produced by the main uses to which
they are put (agricultural, industrial, urban) in the area.
In addition to water flowing into the lake from the catchment basin, there are
underground springs from the watertable and, in volcanic lakes, seepage of
groundwater from the volcanic edifice, which has a very particular chemistry. In
this case, the lake may be the emerging portion of a larger catchment basin.
The geographical position of lakes affects their physical behaviour: those in
northern Europe freeze in winter and cannot exchange oxygen with air; those in
tropical and temperate areas have warm surface layers that float on deeper
layers, hindering water mixing and oxygenation of the bottom. Alpine lakes are
at their highest level in late spring or summer, when snow melts; lakes in central
Italy are at fullest in winter, when rain is more abundant. The Italian average
precipitation is 100 cm, and evaporation is similar. This means that the rain and
snow falling on Italian standing waters replenish, on average, only loss due to
Lake Vico (Latium)
29
30
The properties of water
Silvia Arisci · Laura Lepore · Michela Rogora
Water is the essence of life on Earth and
the basic chemical component of any
living organism. It also regulates the
metabolism of lakes, with its particular
properties like density, high thermic
capacity and behaviour in its different
states - liquid, solid and gaseous.
The unique properties of water are due
to its molecular structure. The formula of
water consists of two atoms of hydrogen
and one of oxygen. The three atoms are
bonded, with the oxygen atom between
the two hydrogen atoms. However, the
three atoms do not lie in a straight line the two hydrogen atoms bend towards
each other. This three-dimensional
structure is therefore unsymmetrical, the
oxygen atom having a partially negative
charge (δ-) and the two hydrogen atoms
partially positive charges (δ+). Opposite
the hydrogen atoms are two electronic
clouds of negative electrification, which
attract the hydrogen nucleus of an
adjacent water molecule to form what is
called a hydrogen bond. When water
freezes and becomes solid (ice), its
molecules form an ordered structural
lattice: one water molecule collects its
nearest four neighbours and arranges
them about itself in a tetrahedral
configuration. The peculiarity of ice is
that the intermolecular distance is higher
than in the liquid state, and therefore
density in a solid state is lower than in a
liquid state. Maximum density is
obtained at 3.98°C and pressure of one
atmosphere. Below this temperature,
density decreases again, until freezing
point (0°C) is reached. The most
important consequences of this
phenomenon are that, in winter, lake
surfaces develop floating ice sheets,
rather than freezing solid from top to
bottom, and that the temperature of the
water below remains almost constant,
due to the low thermal conductivity of
ice itself, which prevents heat from
being released into the atmosphere.
Water density varies with temperature,
pressure and salinity. It increases with
pressure (1 atmosphere every 10 m in
depth) and with the increased specific
weight of dissolved substances.
Hydrogen bonds between molecules
also determine the high specific heat
of water, i.e., the quantity of heat one
gram of water needs to increase its
temperature by 1°C. The high thermal
capacity of water explains why
thermic variations near large lakes are
unlikely, and why extensive bodies of
water can release great quantities of
heat accumulating in warm periods
into the atmosphere. Strong
intermolecular interactions influence
the viscosity of water, i.e., the great
resistance water opposes to motion
within it (775 times that of air).
δ+
O
LAKE
BASIN
VOLUME
DEPTH
AREA
EMIS. FLOW
REC. RATE
km2
km3
m
km2
m3/sec
years
Garda
2350
50.35
346
370
59.5
Iseo
1842
7.60
251
62
59.4
4
Como
4572
22.50
410
146
158.0
4
Maggiore
6559
37.50
370
212
297.0
4
10
0.46
165
6
-
47
-
15
Albano
Nemi
11
0.03
34
Trasimeno
376
0.59
6
124
0.9
21
Bolsena
273
9.20
151
114
2.4
120
41
0.26
12
0.5
17
147
5.05
165
57
1.2
137
0.004
38
-
110
Vico
Bracciano
Monticchio Grande
4
49.5
1.6
27
0.4
Characteristics of main subalpine and some volcanic lakes in central Italy
Compared with subalpine lakes, volcanic lakes have far longer recharge rates,
due to the weak hydrological conditions caused by their origin. Lakes Bolsena,
Vico and Bracciano, for instance, lie at the top of their volcanic edifices, and
their catchment basins are therefore extremely small, especially when
compared with their volume.
O
H
δ−
evaporation. Excess water, i.e., that drained by outlets, corresponds to the
supply provided by the basin, so that lakes fed by extensive basins, like Alpine
ones, have large outlets, and lakes supplied by small basins, like volcanic ones,
have small outlets. The theoretical recharge rate is the ratio between the
volume of water in the lake and the quantity of water drained by an outlet in one
year. The recharge rate is thus an index of the capacity of the lake to drain
through its outlet some of the pollutants flowing into the basin.
The table below compares the main morphometric and hydrological
characteristics of the largest Italian lakes (Garda, Iseo, Como, Maggiore) with
those of some volcanic lakes.
O
H
■ Temperature and oxygenation of lakes
O
O
O
A water molecule and its tetrahedral configuration in solid state (ice)
From the thermic viewpoint, the volcanic lakes of central Italy are temperate. In
winter they are homoiothermic (i.e., they have a relatively uniform temperature
from surface to bottom), and therefore all the water has the same density.
In spring, the sun heats only their upper layers of water, as shown by the fact
that infrared light, thermically the most efficient, can only penetrate the topmost
31
water layers. Wind can give rise to currents that distribute heat at deeper levels.
As summer sets in, the temperature rises and wind-generated currents can
overturn water to the bottom of the lake only at night and if the lake is not very
deep. When density differences are too great for the wind to maintain
homoiothermic conditions, temperature stratifies vertically between May and
October. In this period, the water is divided into three layers:
● A warm upper layer (epilimnion)
● A cold, deep layer (hypolimnion)
● A thin layer separating the epilimnion from the hypolimnion (thermocline, or
metalimnion), in which water undergoes sharp temperature changes.
As temperature decreases in autumn, the surface water cools and mixes with
deeper water, giving rise to winter mixing, a situation that maintains
homoiothermic conditions in the lake between January and March, and also
provides a certain chemical homogeneity. This phenomenon is particularly
important for the oxygenation of lake waters, especially deep layers where
dissolved oxygen is gradually consumed by biological decay, giving rise to
initial depletion (hypoxia) to total lack of oxygen (anoxia). These conditions
make life in the lake impossible for most organisms. Dissolved oxygen is
essential for life in lakes. Its main source is the passage of oxygen through the
air-water interface, and also by biological production (photosynthesis).
Oxygen can return to the atmosphere
through diffusion, be consumed
through respiration by aquatic
organisms, or be oxidised by chemical
processes. The balance between these
exchanges produces the spatial
distribution and time variations of
oxygen in a body of water.
In some lakes, deep water never
circulates with the other layers. This
condition, called meromixis, is common
to several volcanic lakes.
In meromictic lakes, isolation of the
deepest layers, oxygen depletion, and
processes of anaerobic decay produce
compounds like hydrogen sulphide
(H2S) (which produces a typical,
Lake Bracciano (Latium)
unpleasant odour), ammonia (NH4+)
and methane (CH4). Bottom sediments turn black or grey, and organisms living
at the bottom of lakes are negatively affected.
■ Other gases and ion compounds
Temperature (C°)
0
Depth (m)
32
5
10
15
20
25
0
5
10
15
20
25
30
February
April
June
August
October
December
35
40
Yearly temperature distribution in water column of Lago Grande of Monticchio (Basilicata)
30
From the chemical viewpoint, lakes are open areas interacting with the
atmosphere at the surface, the underlying bedrock, and water supply
underground and the surrounding area.
Gases in the atmosphere, especially oxygen and carbon dioxide, dissolve in
water according to Henry’s law, which states that the amount of gas absorbed
by a liquid is proportional to the partial pressure of the gas upon the liquid, by
means of a constant value (k) called Henry’s constant. The concentration of
dissolved gases decreases as temperature increases, and is influenced by
chemical and biological processes occurring in liquid.
Carbon dioxide (CO2) is supplied to lakes through exchange with the
atmosphere at the surface, in rain, which collects CO2 as it falls, and is released
by animal respiration. Carbon dioxide is very soluble in water and forms
carbonic acid, which in turn dissociates, raising the concentration of hydrogen
ions (producing bicarbonate and carbonate ions). These chemical reactions,
each of which is controlled by a balancing constant, are called carbonate and
bicarbonate balance, and determine the pH value of water at a given
33
temperature (generally between 7 and 9). The carbon dioxide system therefore
serves as a buffer, because any change in pH will cause a shift in the system
that will offset that change.
Water can dissolve not only gases, but also organic and inorganic matter which
is either polar or subject to polarisation. Salinity varies greatly and depends on
interactions with the atmosphere, drainage from the surrounding area, and
exchange with sediments inside the lake. The composition of natural water is
controlled by the bedrock and soil of the catchment basin, precipitation,
crystallisation and evaporation, and biological processes inside the body of
water (production-respiration). Volcanic lakes have far higher salinity than other
Italian subalpine lakes. Their total salinity ranges between 7000 and 11000
µeq/l - much greater than, for instance, that of Lake Maggiore (3000 µeq/l)
Salinity is computed from the cations (positively charged ions) of alkaline and
alkaline-earth metals like calcium (Ca++), magnesium (Mg++), sodium (Na+) and
potassium (K+), and anions (negatively charged ions) like carbonate (CO3--),
bicarbonate (HCO3-), sulphate (SO4--) and chloride (Cl-).
According to their salinity, natural waters are divided into two groups: soft water,
with low salinity when it erodes igneous rocks (poorly soluble), and hard water,
containing large amounts of alkaline and earth metals deriving from the
underlying calcareous bedrock (soluble). Natural water, the ion spectrum of
which is given by the bedrock of its catchment basin, is typically rich in calcium
34
Lake Bracciano (Latium)
Lake Albano (Latium)
35
36
and bicarbonate, and the relationships between ions are: Ca++>Mg++=Na+>K+
for cations and HCO3-> SO4-->Cl- for anions.
In central Italian volcanic lakes, this order is only partially maintained. Supply
from the sea is very important for these lakes, which have higher concentrations
of sodium and chlorine than standing waters far from the coast. In volcanic
lakes, in addition to marine aereosol, the main source of solutes is bedrock and
soil runoff from the catchment basin, which supply bicarbonate, calcium,
magnesium, sulphate and sodium. Among anions, bicarbonate dominates,
followed by chloride compounds, which in volcanic lakes are more frequent than
sulphate compounds. These lakes generally have a good buffer system, which
produces minimal variations in pH values at various depths. Cations like
calcium, magnesium, sodium and potassium have similar concentrations, and
sodium is slightly higher in lakes Bolsena and Bracciano.
Lastly, nitrates are barely present in volcanic lakes (<10 µeq/l), although they
are found in high concentrations in subalpine lakes (50-60 µeq/l). Nitrates
mainly derive from precipitation which, in northern Italy and densely populated
areas, is particularly rich in nitrogen oxides. Volcanic lakes are rich in organic
nitrogen produced by a complex cycle of metabolic assimilation of atmospheric
nitrogen by heterotrophic bacteria. They transform nitrogen into ammonia,
which either returns to the atmosphere or enters the plant-herbivore-carnivore
food-chain. Decomposition then triggers the cycle again.
µeq/l 0
1000
2000
3000
4000
MAGGIORE
5000
■ Algal nutrients: phosphate, nitrogen and silica
In addition to the inorganic constituents mentioned above, other inorganic
compounds are particularly important in lake waters. They are usually
referred to as nutrients, because they are exploited in various ways by algae
and micro-organisms.
Silica (SiO2) is essential to diatoms, a group of algae that use this compound
to produce their frustules (siliceous shells), thus regulating its dissolved
concentration. Dissolved silica in lakes undergoes great seasonal variations,
i.e., it accumulates in winter and decreases dramatically in spring, when
diatoms bloom.
Phosphorus, in its highly oxidised forms like orthophosphate (PO4---), plays an
active role in biological cycles as a nutrient for lake organisms. Phosphorus, like
silica, undergoes seasonal and spatial variations inside a lake: in the upper
portion, where photosynthesis occurs, it decreases when algal development
peaks; in summer, it increases in deep water, due to decay of biological material
sinking down from the upper layers.
Nitrogen, in the form of nitrate (NO3-), nitrite (NO2-), ammonia (NH4-) and
organic nitrogen, is an essential nutrient for several organisms. In lakes, the
dynamics of nitrogen and phosphorus are associated with biological
processes, and therefore with consumption by algae and bacteria. The
6000
Ca++
Mg++
ALBANO
Na+
BRACCIANO
NEMI
K+
HCO3NO3-
VICO
BOLSENA
Average ion concentrations in some volcanic lakes and in Lake Maggiore
SO4-Cl-
Lake Mezzano (Latium)
37
passage between the various nitrogen compounds is regulated by microorganisms. In summer, when temperature and light are sufficient for intense
algal growth, the production of nitrogen is controlled by the availability of
nutrient compounds. In particular, as it is present in lower quantities than those
required by algae, it thus limits their growth.
According to nutrient contents, lakes are divided into trophic levels, from
oligotrophic (few nutrients and low algal productivity), mesotrophic (an
intermediate stage), to eutrophic (very productive lakes rich in nutrients).
Trophic conditions are closely associated with the chemico-physical
characteristics of lake waters and the biocoenoses populating them.
Excessive nutrient enrichment (especially by phosphorus) due to human
disposal, is called eutrophication. This process diminishes water quality and
affects the use of the lake as a source of drinking water, for irrigation, bathing,
and other uses.
In the 1970s, results of monitoring analyses on Latium volcanic lakes by the
CNR (Consiglio Nazionale delle Ricerche - Italian Research Council) had
revealed low concentrations of phosphorus and low productivity of the lakes,
most of which were classified as oligotrophic. Recent increases in phosphorus
caused by human activities, has since positioned the lakes at the highest
trophic level. The quality of their water has therefore deteriorated (see chapter
on conservation and management).
38
µg/l 0
100
200
300
400
ALBANO
BRACCIANO
BOLSENA
NEMI
3000
MONTICCHIO
GRANDE
MONTICCHIO
PICCOLO
Lake Bracciano (Latium)
surface
bottom
Average concentrations of total phosphorus (in µg/l) at the surface and bottom of some volcanic lakes
39
■ Traces of heavy metals
40
41
Another typical characteristic that
distinguishes volcanic lakes from
other bodies of water is their heavy
metal content. The heavy metal
contents of large subalpine lakes (like
Lake Maggiore) are far lower, almost
undetectable.
The water of volcanic lakes is generally
rich in dissolved metals, especially
boron and strontium, both of which are
indicators of geothermic activity.
Other commonly found metals, like
iron, manganese and zinc, which
come from the underlying bedrock,
are far more frequent in lakes Bolsena,
Lago Grande of Monticchio (Basilicata)
Albano and Monticchio than in the
Maggiore. The reason for this characteristic lies in the lithology of the basins,
generally composed of incoherent lithoid tuff and effusive rocks. These
porous, permeable rocks facilitate runoff by rain and enrich lake water with
metals.
BOLSENA
MONTICCHIO
GRANDE
130 m
20 m
9
9
6
0
15
182
14
5
Boron
390
395
86
94
53
64
4
-
Barium
35
38
13
22
84
289
10
10
Copper
-
4
-
-
-
0.4
0.5
0.2
Iron
-
25
-
75
46
5900
8
4
Manganese
-
21
-
105
99
3572
1
5
Zinc
-
20
-
-
2
2
1
1
42
41
4
4
3
3
1
-
420
450
753
893
479
630
208
227
Lithium
Strontium
160 m
0m
35 m
MAGGIORE
0m
Alluminium
Lake Nemi (Latium)
ALBANO
0m
360 m
Average concentrations (in µg/l) of main cations and anions in some volcanic lakes and in Lake Maggiore
Phytoplankton
GIUSEPPE MORABITO
At first sight, volcanic lakes appear to be
special environments, as they have the
characteristic shape of a crater with high
rims, so that they actually look like large
bowls.However, examination of samples
of their waters and of the microscopic
algae (phytoplankton) contained in
them will not surprise limnologists and
naturalists greatly. Over the seasons,
volcanic lakes are not subject to the
extreme physical and chemical
conditions that would enable only some
selected algal species to survive.
On the contrary, these habitats are
usually inhabited by the same species
that normally colonise completely
different kinds of lakes. However, some
Lake Mezzano (Latium)
of these species find the ideal living
conditions that make them dominant precisely in volcanic lakes, due to the
optimal combination of physical and chemical parameters that is usually found
in such lakes. This section analyses the factors that lead to the development of
planktonic algae in lakes, highlighting the main characteristics of volcanic lakes
associated with several variables, and aims at providing a general overview of
the habitats in which these algae live. Readers will then understand why some
types of phytoplankton can be dominant in volcanic lakes.
■ Factors regulating the growth of phytoplankton
There are many variables controlling the growth of phytoplankton in lake water,
and they interact in often baffling ways. Yet, like all plant species, also
phytoplankton depend on certain light and temperature conditions, as well as
sufficient nutrients.
Ceratium hirundinella
43
45
44
Lake Vico (Latium)
Dinoflagellates of the genus Peridinium
Temperature and light. Analyses by phytoplankton experts reveal that, in
lake environments, there are two essential conditions for the life of single
algal species - temperature and sunlight. Just as on land, also in aquatic
ecosystems temperature and light must fall within the values compatible
with the life of plant species, and the range of these values may differ
between species. In addition, sunlight penetrating the water column
produces thermic energy and gives rise to layers of water with differing
thermic characteristics.
Seasonal temperature variations in lakes follow a regular cycle, alternating
periods of total water overturn and periods of water stratification.
Differences in temperature produce differences in water density, and these
small variations drastically affect the life of algae which, having a slightly
higher density than water, must contrast their natural tendency to sink
towards the bottom of the lake.
The survival of phytoplankton in lakes therefore relies on mechanisms that
enable them to slow sedimentation down. For instance, algae can resort to
adaptations such as changing shape to increase their cell area, forming
colonies, or regulating their position inside the water column by means of
propelling devices (flagella or mobile bristles), gas-filled vacuoles, or the
synthesis of compounds with low specific weight.
As regards sunlight, phytoplankton behave in various ways: some algae
thrive in broad sunlight on the surface, whereas others prefer to live a few
metres below. Both the quantity and quality of sunlight are important, and
depend on the nature of particles in suspension. These particles are like
coloured filters that select some wavelengths of the light spectrum, endowing
the lake with precise optical features and shades of colour, generally ranging
between blue and green, according to the quantity of growing phytoplankton.
Lakes can be brown or yellowish in areas with great quantities of complex
plant organic matter.
Phytoplanktonic biocoenoses adapt to the qualitative variations of the light
spectrum by selecting species with photosynthetic pigments (molecules that
capture light energy to carry out photosynthesis), which can better exploit the
available light in the environment.
Early limnologic studies describe Italian volcanic lakes as very transparent,
places where light - in qualitative and quantitative terms - is not a limiting
factor for the growth of phytoplankton. The excellent optical quality of
water is an original characteristic of these lakes, and it is probably
associated with their morphology. Lakes are often found at the top of
volcanic edifices, where water from rivers is scarce compared with the area
of the body of water.
46
Large quantities of water carrying suspended solids are therefore unlikely to
flow into the lake and alter the optical properties of its waters. Lake
transparency enables light to penetrate deep down into the water column.
For instance, in the late 1960s, a limnologic study by the Istituto Italiano di
Idrobiologia of Pallanza (today Istituto per lo Studio degli Ecosistemi of the
CNR) showed that lakes Bolsena, Bracciano and Vico had euphotic areas (of,
relating to, or constituting the upper layers of a body of water into which
sufficient light penetrates to permit the growth of green plants) down to 30 m.
Some algae find these conditions in volcanic lakes particularly favourable for
their development.
In volcanic lakes, this species replacement is also affected by the morphology
of the basin, which influences the processes and quantity of algal nutrients
available. Hence, when water overturn occurs in shallow volcanic lakes like
Monterosi and the Lago Grande of Monticchio, even the mineralised inorganic
nutrients sedimented at the bottom of the lake can return to the surface, to be
consumed by plankters. By contrast, deep volcanic lakes are unlikely to
undergo complete water overturns, and inorganic nutrients deposited at the
bottom remain inaccessible to phytoplankton.
Nutrients. The availability of nutrients, especially limiting ones, is perhaps the
third factor that most affects algal growth. Since the early 1970s, research
focusing on eutrophication has indicated phosphorus as the main limiting
nutrient of phytoplankton in lakes.
Most volcanic lakes have evolved from original oligotrophic conditions to
mesotrophic or even eutrophic conditions. These modifications have also
deeply altered the original phytoplankton communities, which have lost their
oligotrophic-loving species which were replaced by others better adapted to
the new environmental conditions.
The general introduction of the previous section provided a limnologic analysis
of volcanic lakes, and described the conditions algae encounter in colonising
them. Summarising briefly, an alga in a typical volcanic lake is in an
environment where temperature undergoes profound seasonal variations. In
summer, this produces layers of water all greatly differing in temperature from
one to another. These lakes are also transparent, so that sunlight can penetrate
to considerable depths, their water overturn is very slow, and supply from the
catchment basin is scarce. Nutrient availability varies according to the trophic
state and morphology of the basin, just as in other lakes.
Lago Grande of Monticchio (Basilicata)
Cyclotella comensis
■ Structures of phytoplanktonic associations
47
The specific components of phytoplanktonic associations undergo seasonal
modifications caused by changes in environmental conditions.
In late winter, when the water column is mixing and light becomes more
intense, diatoms start developing. These are typical pioneer algae, which
grow rapidly in the water still free of other algal groups thanks to their great
capacity for nutrient assimilation. Diatoms have a siliceous cell wall called
frustule which contains their cells - making them very heavy - and therefore
causing them to sink rapidly to the bottom, far from the upper layers
illuminated by the sun. This is why diatom growth is closely associated with
the period of maximum lake water turbulence, because water mixing
prevents them from sinking.
In Italian volcanic lakes the dominant diatom groups are the Centrales (discshaped) - as emerges from analyses conducted on lakes in Latium. According
to the scientific literature, in the years prior to trophic deterioration, the
dominating genus was Cyclotella, with species C. comensis, C. kuetzingiana
and C. ocellata.
In several basins, when higher trophic conditions set in, Cyclotella species were
replaced by Stephanodiscus species (S. parvus, S. minutulus, S. hantzschii),
another genus of Centrales which, unlike Cyclotella, prefers habitats with
higher nutrient contents.
The alternation of these two genera was clearly highlighted by palaeolimnological research carried out on volcanic lakes in Latium, which
reconstructed trophic evolution by analysing diatom frustules in lake
sediments. The research also identified the community of the diatom
components in lakes Nemi and Albano since the Late Pleistocene, and
revealed variations of diatoms in eutrophic and oligotrophic waters in periods
even prior to human activity. They were probably due both to climatic events
that caused the water temperature to rise, and to periods of geothermal
activity which influenced the availability of algal nutrients. Unfortunately,
palaeo-limnological information about the structure of past phytoplanktonic
associations can only be obtained from algae that leave fossil traces, i.e.,
diatoms and golden algae (Chrysophyceae). According to palaeolimnological data about recent algal communities, the latter count only a few
species in volcanic lakes.
Generally speaking, both diatoms and golden algae are spring
phytoplankters, not only in volcanic lakes. Environmental conditions change
greatly between spring and summer, when thermic stratification occurs. The
nutrients that had started to mix in late winter and had supported the spring
development of diatoms, have almost completely disappeared from the
epilimnion.
Cyanophyceae
iIndividuals/ml
48
Diatoms
Cryptophyceae
Chlorophyceae
60000
50000
40000
30000
20000
10000
0
J
Stephanodiscus minutulus
F
M
A
M
J
J
A
Seasonal trend of the main phytoplanktonic groups in Lake Nemi (1982)
S
O
N
D
49
The blue-green alga Planktothrix rubescens
The species Planktothrix rubescens
typically forms dense deposits in the
lake thermocline, i.e., the layer of
water in which the highest
temperature gradient is measured
during summer water stratification
(temperature declines at least 1°C
with each metre of increase in
depth).
P. rubescens is a frigo-stenotherm
(resisting only slight changes in
temperature) which thrives at around
15°C. It is also sciaphilous, and
therefore prefers dim, poorly
illuminated areas of the lake.
From the trophic viewpoint, P.
rubescens lives in environments rich
in nutrients.
Chlorophyll (µgl-1)
0
0,5
1
0
10
Giuseppe Morabito
The combination of cold, dim and
nutrient-rich water occurs in the
thermocline of deep lakes, between
10 and 15 metres in depth, where
light has already diminished, and the
temperature is quite low.
The temperature gradient causes
differences in water density, which
favour the accumulation of organic
matter in the thermocline, where it is
decomposed and mineralised.
P. rubescens is also favoured by
special photosynthetic pigments,
which colour it red and enable it to
make efficient use of even low light
radiation. It uses its intracellular gas
vacuoles to regulate its position in
the water column.
1,5
2
2,5
15
20
25
0
Depth (m)
50
5
10
15
20
25
30
35
40
45
°C
5
Planktothrix
Temperature
Relationship between Planktothrix rubescens and water temperature as depth increases
Nonetheless, the thermocline - where
organic matter remains for some time,
due to temperature and density
gradients - can store nitrogen and
phosphorus. Differences in water
density in the thermocline are physical
barriers to the sedimentation of
particle-sized organic matter, which is
mineralised as it moves through this
layer of water, and can therefore
produce deposits of nutrients used by
growing algae.
Obviously, not all algae can use these
reserves and, in this season, the
numbers of diatoms decrease for three
reasons: firstly, because of thermic
stratification and the ceasation of the
Planktothrix rubescens
turbulence which kept them afloat;
secondly, because the silica used to form their frustules has been consumed
and the lack of this element now limits their growth; and thirdly, because
they have become prey to herbivorous zooplankton, especially water fleas
(Cladocera), which drastically reduce spring algae, giving rise to a seasonal
phenomenon whereby water is particularly clear, due to algal removal by
zooplankton.
Diatoms therefore leave the water free for the development of other algae but which? In the 1930s and ’40s, research on the pelagic (open water) flora
of lakes Albano and Nemi revealed that the most successful algae in
summer phytoplanktonic associations in volcanic lakes are dinoflagellates.
In particular, one of the most common is Ceratium hirundinella, whose
fascinating shape is typically found in lake phytoplankton. There are two key
factors giving it competitive advantage, one being its flagella, which enable
it to swim in the water column in search of the best conditions for its
development.
Thanks to this capacity for independent movement, Ceratium can migrate
through well-illuminated surface water and carry out photosynthesis, and
then move to the thermocline to feed. The other decisive factor lies in its
size, which enables it to store large amounts of nutrients whenever it can.
This is particularly helpful in environments poor in nutrients, like Italian lakes
prior to the 1960s.
51
52
Volcanic lakes in which trophic
conditions have deteriorated show a
decline in the total numbers of
dinoflagellates, which are replaced by
cyanobacteria (blue-green algae) in
summer algal communities.
The species which are most
successful in volcanic lakes depend
on the type of environment. Deep
volcanic lakes have almost exclusively
been colonised by Planktothrix
rubescens, and shallow lakes host
taxa of chroococcoid cyanobacteria
(for istance Microcystis, Woronichinia,
Merismopedia).
Planktothrix rubescens is a bluegreen alga that gathers in filamentous
Cyanobacteria of the genus Woronichinia
colonies, and is one of the bestknown species, due to its occurrence worldwide. Its blooms, which turn lake
waters red, are also very well-known because they are the clearest mark of
deteriorated trophic conditions in many European deep lakes. Its ecophysiological characteristics therefore also allowed it to colonise Italian
deep lakes successfully (see p. 50). A water column that is clear down to
great depths is the optimal ecological niche for the development of P.
rubescens.
The transparency of volcanic lake water guarantees an euphotic zone deep
enough to include the thermocline, which further enhances colonisation by
this species in volcanic lakes like Albano and Nemi. Shallow volcanic lakes
seldom have a stable thermocline and great transparency because they are
subjected to total overturn by strong winds. Turbulence can affect the
deepest areas of the lake, sediments may move to the upper layers of water,
and the water becomes more turbid. In addition, shallow lakes may have
high water temperatures even deep down.
These conditions therefore hinder the growth of Planktothrix and favour a
particular type of cyanobacteria, the Chroococcaceae, which form colonies
of irregular shape (Microcystis), round (Woronichinia), or rectangular
(Merismopedia). Their cells are contained in a gelatinous matrix, enabling
them to float easily and return to the surface after water overturn. Most
Chroococcaceae can withstand high temperatures and radiation, and may
therefore colonise the upper layers of
water very successfully. As these
bodies of water are quite shallow,
when
water
overturns,
great
quantities of nutrients sedimented at
the bottom float to the surface, giving
rise to cyanobacterial blooms that
look like large mats. Blooms can often
be seen in the Lago Grande of
Monticchio.
The development of cyanobacteria in
volcanic lakes has also affected the
food-chain, bringing, for example,
Eudiaptomus padanus etruscus - a
frequent copepod in these habitats
before eutrophication - to complete
extinction.
Microcystis wesenbergii
Let us now summarise the main
topics analysed in this section. Firstly, the dynamics of phytoplanktonic
communities in lakes is regulated by several physical, chemical and biotic
factors, the relative importance of which depends on the seasons and the
environment.
Among the variables determining the composition of algal species, recent
analyses on phytoplankton ecology have shown that physical factors are the
most important in determining species alternation. This is why the
description of phytoplankton in volcanic lakes focused on the adaptations of
algal species to light and temperature variations. However, this is not the
only reason, because the physical environment of volcanic lakes has
characteristics that distinguish these habitats from other lakes and make
them suitable for colonisation by algae which, nonetheless, are not exclusive
to this type of freshwater.
The scientific literature on the phytoplankton of volcanic lakes has highlighted
the fact that many of these environments are very vulnerable to man’s
influence. Their slow hydrological recharge favours the quick accumulation of
algal nutrients, and modified chemical characteristics give rise to sometimes
profound changes in the structure of phytoplanktonic communities, with the
development of species that may severely compromise how humans use
these waters. Typical examples are the blooms of potentially toxic
cyanobacteria which have recently occurred in volcanic lakes in Latium.
53