Download Wetland

Classification & physical attributes of wetlands
including pollutant impact in coastal zones
Georg Umgiesser and Roberto Zonta
National Research Council, Institute of Marine Sciences,
Venice, Italy
NEAR

curriculum in natural environmental science, vol. 2, 2010
Coastal zones are essential
for the equilibrium of the
whole marine life, since the
large amount of organic
matter that is produced in
their waters is fundamental
for the maintenance of the
food chain.
Despite accounting for no more than 15% of the surface and
0.5% of the total volume of the ocean, about 90% of marine
living resources come from the coastal waters.
On the other hand, more than 60% of the world’s population
live within 60 km of the sea. Human activities are changing
rapidly the fluxes of matter and related pollutants from the
continent to the coastal zones.
These systems are therefore subjected to a very strong and
increasing pressure that induces degradation phenomena, in
term of loss of habitats for living organisms and biodiversity,
water and sediment pollution, eutrophication and landscape
deterioration.
European coasts, in particular, are most affected, with some
80% being at risk.
The global awareness related to the need of a comprehensive
protection of the marine environment and coastal resources
has increased since the 1972 United Nations Conference on
Human Environment held in Stockholm.
International agreements were established and different policy
actions were taken toward a sustainable and integrated
coastal management.
Contemporarily, scientific investigations are needed to acquire
an adequate understanding of coastal area conditions and
related environmental processes, to counteract the effects of
stressing factors, and to permit the forecasting of
consequences of management options.
Freshwater discharged by rivers, municipal and industrial
effluents, is quite generally the main source of pollutants for
coastal zones, since most of the pollution load to the ocean
derives from land-based activities, including the releases into
the atmosphere.
This places the quality of freshwater delivered to the sea as a
topic of world-wide importance.
Wetlands - Nature’s Filter
Wetlands are areas of land where the water table is
usually at or near the surface, or the land is intermittently
or permanently inundated by shallow water. Wetlands are
not only attractive landscape features, but also provide
habitats for native wildlife as well as potential sources of
reusable water. There may also be the potential for
sporting and recreational facilities, and agriculture and
aquaculture farming within artificially managed wetland
systems.
Wetlands are classified according to their depth, period of
inundation and salinity. The role of wetlands as biological
filters and treatment mechanisms to remove and convert
catchment sourced contaminants in runoff is of prime
importance.
The Players
salt water
fresh water
sediment/particles
pollutants
SEA
coastal waters
sediment
Possible Classification
Schemes of Wetlands
•
•
•
•
•
•
The Ramsar Convention
Functional Classification
Salinity Stratification
Flow rates
Topographic Classification
Lagoons and water exchange
The Ramsar Convention
definition of “Wetland"
Under the Convention on Wetlands (Ramsar, Iran,
1971) "wetlands" are defined as shown below:
– "For the purpose of this Convention wetlands are areas of
marsh, fen, peatland or water, whether natural or artificial,
permanent or temporary, with water that is static or
flowing, fresh, brackish or salt, including areas of marine
water the depth of which at low tide does not exceed six
metres.“
– Wetlands "may incorporate riparian and coastal zones
adjacent to the wetlands, and islands or bodies of marine
water deeper than six metres at low tide lying within the
wetlands".
Marine/Coastal Wetlands 1/2
• A -- Permanent shallow marine waters in most cases less than six metres
deep at low tide; includes sea bays and straits.
• B -- Marine subtidal aquatic beds; includes kelp beds, sea-grass beds,
tropical marine meadows.
• C -- Coral reefs.
• D -- Rocky marine shores; includes rocky offshore islands, sea cliffs.
• E -- Sand, shingle or pebble shores; includes sand bars, spits and sandy
islets; includes dune systems and humid dune slacks.
• F -- Estuarine waters; permanent water of estuaries and estuarine systems
of deltas.
Marine/Coastal Wetlands 2/2
• G -- Intertidal mud, sand or salt flats.
• H -- Intertidal marshes; includes salt marshes, salt meadows, saltings,
raised salt marshes; includes tidal brackish and freshwater marshes.
• I -- Intertidal forested wetlands; includes mangrove swamps, nipah
swamps and tidal freshwater swamp forests.
• J -- Coastal brackish/saline lagoons; brackish to saline lagoons with at
least one relatively narrow connection to the sea.
• K -- Coastal freshwater lagoons; includes freshwater delta lagoons.
• Zk(a) – Karst and other subterranean hydrological systems,
marine/coastal
Inland Wetlands 1/2
•
•
•
•
•
•
•
•
•
•
•
L -- Permanent inland deltas.
M -- Permanent rivers/streams/creeks; includes waterfalls.
N -- Seasonal/intermittent/irregular rivers/streams/creeks.
O -- Permanent freshwater lakes (over 8 ha); include large oxbow lakes.
P -- Seasonal/intermittent freshwater lakes (over 8 ha); include floodplain lakes.
Q -- Permanent saline/brackish/alkaline lakes.
R -- Seasonal/intermittent saline/brackish/alkaline lakes and flats.
Sp -- Permanent saline/brackish/alkaline marshes/pools.
Ss -- Seasonal/intermittent saline/brackish/alkaline marshes/pools.
Tp -- Permanent freshwater marshes/pools; ponds (below 8 ha), marshes and
swamps on inorganic soils; with emergent vegetation water-logged for at least most
of the growing season.
Ts -- Seasonal/intermittent freshwater marshes/pools on inorganic soils;
includes sloughs, potholes, seasonally flooded meadows, sedge marshes.
Inland Wetlands 2/2
•
•
•
•
•
•
•
•
•
U -- Non-forested peatlands; includes shrub or open bogs, swamps, fens.
Va -- Alpine wetlands; includes alpine meadows, temporary waters from
snowmelt.
Vt -- Tundra wetlands; includes tundra pools, temporary waters from snowmelt.
W -- Shrub-dominated wetlands; shrub swamps, shrub-dominated freshwater
marshes, shrub carr, alder thicket on inorganic soils.
Xf -- Freshwater, tree-dominated wetlands; includes freshwater swamp forests,
seasonally flooded forests, wooded swamps on inorganic soils.
Xp -- Forested peatlands; peatswamp forests.
Y -- Freshwater springs; oases.
Zg -- Geothermal wetlands
Zk(b) – Karst and other subterranean hydrological systems, inland
Human-made wetlands
•
•
•
•
•
•
•
•
•
•
1 -- Aquaculture (e.g., fish/shrimp) ponds
2 -- Ponds; includes farm ponds, stock ponds, small tanks; (generally below 8 ha).
3 -- Irrigated land; includes irrigation channels and rice fields.
4 -- Seasonally flooded agricultural land (including intensively managed or
grazed wet meadow or pasture).
5 -- Salt exploitation sites; salt pans, salines, etc.
6 -- Water storage areas; reservoirs/barrages/dams/impoundments (generally over
8 ha).
7 -- Excavations; gravel/brick/clay pits; borrow pits, mining pools.
8 -- Wastewater treatment areas; sewage farms, settling ponds, oxidation basins,
etc.
9 -- Canals and drainage channels, ditches.
Zk(c) – Karst and other subterranean hydrological systems, human-made
Functional Classification of
Wetlands
Inlet zone: This is a transitional zone between the waterways draining the
catchment and the wetland . The purpose of this zone is to reduce the velocity of
the inflowing water and to enable the larger particles to settle and sink. Aquatic
plants can also be grown in the edges of this area as they will generally be able to
withstand high velocity inflows during severe storm events.
Macrophyte zone: This area is usually occupied by emergent and submerged
aquatic plants. The plants and the coatings of solids and micro-organisms on their
stems plus the sediments in which these plants grow, take up or convert the
nutrients and thus assist in the treatment of stormwater. There is decomposition
of organic matter within this zone.
Open water zone: This is a deeper area that allows time for finer particles to settle
and sink to the bed, plus allowing sunlight to kill bacteria. Due to sunlight and
nutrient availability, periodic algal growth may also occur in this zone, which will
also trap dissolved nutrients and allow them to either enter the food chain or
settle to the bottom of the pond.
Definition of estuaries and
classification
• An estuary is a semi-enclosed coastal body of water which has a free
connection with the open sea and within which sea water is measurably
diluted with fresh water derived from land rainage. (Cameron and
Pritchard, 1963)
Estuaries described by this definition are known as positive estuaries.
Another definition that can be given eliminates this problem:
• An estuary is a narrow, semi-enclosed coastal body of water which has a
free connection with the open sea at least intermittently and within which
the salinity of the water is measurably different from the salinity in the
open ocean. (Tomczak, 1996)
Examples of Estuaries
•
•
•
•
•
•
Highly stratified estuaries (high flow, low tides)
Slightly stratified estuaries (moderate flow and tides)
Vertically mixed estuaries (low flow, high tides)
Inverse estuaries (hypersaline lagoons)
Salt-plug estuaries (temperature stratification)
Sub-estuaries
Highly stratified
estuary
Highly stratified: A
highly stratified, salt
wedge type estuary is
one in which the outgoing
lighter fresh water
overrides a dense
incoming salt layer. The
dense salt wedge will
advance along the bottom
until the fresh-water flow
forces can no longer be
overcome
Well mixed estuary
Well-mixed: In estuaries
where tidal flow is much
larger than river flow and
bottom friction large
enough to mix the entire
water column, a vertically
homogeneous (well-mixed)
estuary results. If the
estuary is wide, Coriolis
force may form a horizontal
flow separation; and in the
northern hemisphere, the
seaward flow would occur
on the right side (looking
downstream), while the
compensating landward
flow would be on the left.
Partially mixed estuary
Partially mixed: A partially mixed
estuary is one in which tidal
energy is dissipated by bottom
friction produced turbulence.
These turbulent eddies mix salt
water upward and fresh water
downward with a net upward flow
of saline water. As the salinity of
the surface water is increased,
the outgoing surface flow is
correspondingly increased to
maintain river flow plus the
additional upward-mixed saline
water. This causes a
compensating incoming flow
along the bottom. This welldefined, two-layer flow is typical of
partially mixed estuaries
Inverse estuaries
(hypersaline lagoons)
Inverse estuaries are
a feature of hot arid
climates. They
include shallow
regions with a large
surface area such as
hypersaline lagoons,
but also gulfs of
significant depth and
extent. Evaporation
becomes a dominant
factor in the salt
budget of the gulfs,
and the salinity
increases from the
mouth to the head.
Stratification Numbers
•
•
•
Simmons Ratio: Simmons (1955) related flow ratio (the ratio of riverflow per tidal
cycle to the tidal prism) to estuary type. When the ratio is 1.0 or greater, the estuary
is highly stratified. When the flow ratio is 0.2 to 0.5, the estuary is partially mixed,
and when less than 0.1, a well-mixed condition exists.
Ippen Number: Using the tidal properties of amplitude and phase, Ippen and
Harleman (1961) developed a relationship between energy and estuary mixing. This
stratification number is a measure of the amount of energy lost by the tidal wave
relative to that used in mixing the water column. Increasing values of the
stratification number indicate increasingly well-mixed conditions, and low numbers
indicate highly stratified conditions.
Hansen Parameter: Hansen and Rattray (1966) chose the parameters of salinity
and velocity to develop a means of estuary classification and comparison. Two
dimensionless parameters, stratification (ratio of the surface to bottom salinity
difference divided by the mean cross-sectional salinity) and circulation (ratio of the
net surface current to the mean cross-sectional velocity) are used to construct a
diagram.
River Flow, Tides, and Waves
Dronkers (1988) proposed an estuarine classification that distinguished
various types of estuarine ecosystems based on water exchange processes
(e.g., river flow, tides, and waves) that greatly affect energy and material
fluxes including mixing. This classification suggests that river flow in partially
mixed estuaries is essentially neutral, but its variation relative to
hydrodynamic residence time can be important in interpreting propertysalinity diagrams (Cifuentes et al. 1990). River flow in the partially mixed
mainstem of the Chesapeake Bay is seasonally important.
+
+
+
-
+
+
+
+
-
+
+
+
Topographic classification
Pritchard (1952) has suggested a topographic
classification in several groups:
• Coastal plain estuaries
– Classical estuaries
– Salt marsh estuaries
• Fjords
• Bar-built structures (lagoons)
• Other (tectonically caused)
Coastal Plain Estuaries:
Classical and Salt Marsh
Both subclasses are characterized by well-developed longitudinal salinity
gradients that influence development of biological communities. Examples of
the classical type include the Chesapeake Bay (the largest estuary of this
type), Delaware Bay, and Charleston Harbor, SC. Vertically stratified systems
with relatively long residence times (e.g., Chesapeake Bay) tend to be
susceptible to hypoxia formation.
The salt marsh estuary lacks a major river source and is characterized by a
well-defined tidal drainage network, dendritically intersecting the extensive
coastal salt marshes (Day et al. 1989). Exchange with the ocean occurs
through narrow tidal inlets, which are subject to closure and migration following
major storms (e.g., Outer Banks, NC). Consequently, salt marsh estuarine
circulation is dominated by freshwater inflow, especially groundwater, and
tides. The drainage channels, which seldom exceed a depth of 10 m, usually
constitute less than 20% of the estuary, with the majority consisting of
subaerial and intertidal salt marsh.
Fjords and Fjordlike Estuaries
Classical fjords typically are several hundred meters deep
and have a sill at their mouth that greatly impedes flushing.
Hypoxia/anoxia is often a natural feature but anthropogenic
nutrient loading can severely exacerbate the problem.
Examples of classical fjords on the North American continent
can be found in Alaska and Washington State (Puget Sound).
Some other estuaries were also formed by glacial scouring of
the coast, but in regions with less spectacular continental
relief and more extensive continental shelves. Examples of
these much shallower, fjordlike estuaries can be found along
the Maine coast.
Lagoons
Lagoons are characterized by narrow tidal inlets and are
uniformly shallow (i.e., less than 2 m deep) open-water areas.
The shallow nature enhances sediment–water nutrient cycling.
Flushing is typically of long duration. Most lagoonal estuaries are
primarily wind-dominated and have a subaqueous drainage
channel network that is not as well drained as the salt marsh
estuary. Lagoons fringe the coast of the Gulf of Mexico and
include the mid-Atlantic back bays; Pamlico Sound, NC; and
Indian River Lagoon, FL. Although these systems are typically
shallow, they may have pockets of hypoxic water subject to
spatial variability because of freshwater pulsing and wind effects.
Some lagoonal systems have relatively strong vertical
stratifications near the freshwater river mouth and may be
subject to hypoxia formation (e.g., Perdido Bay, AL/FL;
Livingston 2001a).
Tectonically Caused
Estuaries
Tectonically caused estuaries were created by faulting,
graben formation (i.e., bottom block-faults downward),
landslide, or volcanic eruption. They are highly variable
and may resemble coastal plain estuaries, lagoons.
Classification of lagoons
using water exchange
Lagoons are classified into 3 main types: leaky
lagoons, choked lagoons, and restricted lagoons
(Kjerfve 1986).
– Leaky lagoons have wide tidal channels, fast currents and unimpaired exchange
of water with the ocean.
– Choked lagoons occur along high energy coastlines and have one or more long
narrow channels which restrict water exchange with the ocean. Circulation
within this type of lagoon is dominated by wind.
– Restricted lagoons have multiple channels, well defined exchange with the
ocean, and tend to show a net seaward transport of water. Wind patterns in
restricted lagoons can also cause surface currents to develop, thus helping to
transport large volumes of water downwind
Lagoons
Tidal Amplitude - A Dominant
Physical Factor
Tidal amplitude provides a means to broadly classify estuaries
relative to their sensitivity to nutrient supplies. Monbet (1992)
analyzed phytoplankton biomass in 40 estuaries and concluded
that macrotidal estuaries (mean tidal range 2 m) generally exhibit
a tolerance to nitrogen pollution despite high loadings originating
from freshwater outflows. These systems generally exhibit lower
concentrations of chlorophyll a than do systems with lower tidal
energy, even when they have comparable concentrations of
nitrogen compounds. Estuaries with mean annual tidal ranges 2 m
seem more sensitive to dissolved nitrogen, although some overlap
occurs with macrotidal estuaries.
Tide induced exchanges
Simulate transport
processes and dispersion
of tracers and pollutants
•
• Estimate the renewal time
of the basin
• Characterize water
masses with the help of
time dependent
parameters
• Correlate physical,
biological and chemical
characteristics between
each other
Water Residence Time
Water residence time, the average length of time that a parcel of water
remains in an estuary, influences a wide range of biological responses
to nutrient loading. The residence time of water directly affects the
residence time of nutrients in estuaries, and therefore the nutrient
concentration for a given loading rate, the amount of nutrient that is lost
to internal processes (e.g., burial in sediments and denitrification), and
the amount exported to downstream receiving waters (Dettmann in
press, Nixon et al. 1996). Residence times shorter than the doubling
time of algae will inhibit bloom formation because algal blooms are
exported from the system before growing to significant numbers.
Residence time can also influence the degree of recruitment of species
reproducing within the estuary (Jay et al. 2000).
There are a number of definitions of water residence time, including
freshwater residence time and estuarine residence time (Hagy et al.
2000; Miller and McPherson 1991), each with its own interpretation and
utility.
Definition of flushing time
• Flushing time: tF = VF / R
• By salinity: tF = (fw V) / R
– fresh water volume fw is the integral of f
– fresh water fraction f = (S0 - S) / S0
• Tidal Prism method: tF = T V / (VT + VR)
• Knudsen: tF = V / R (1 - Stop/Sbottom)
S (t )
• Zimmermann:
F  
dt
S0
Dealing with residence times
• Residence time is an
indicator for the renewal
capability of a basin
• Residence time is
controlled through fresh
water fluxes and
exchange with the open
sea
Dealing with residence times
Residence Time and
Stratification
Residence Time
Estuaries that flush rapidly (i.e., have a short residence time) will export
nutrients more rapidly than those that flush more slowly, resulting in lower
nutrient concentrations in the estuary. Dettmann has derived a theoretical
relationship between the mean residence time of freshwater in an estuary
and the increase in the average annual concentration of total nitrogen in the
estuary as a result of inputs from the watershed and atmosphere. In addition,
estuaries with residence times shorter than the doubling time of algal cells
will inhibit formation of algal blooms.
Stratification
Highly stratified systems are more prone to hypoxia than are vertically mixed
systems. Stratification not only limits downward transport of oxygen from
atmospheric reaeration, it also retains nutrients in the photic zone, making
them more available to phytoplankton. In stratified systems, it may be more
appropriate to estimate the dilution potential of the estuary using the volume
above the pycnocline rather than the entire volume of the estuary.
Sediment processes in estuaries
When the freshwater of continental origin meets the sea water (mixing zone), a
series of physical and physico-chemical processes occur (mixing processes), due
to the different characteristics of the two waters, in term of density, temperature,
ionic field, suspended particles concentration and so on.
In particular, finer suspended particles (and organic matter) transported by the
fresh water flow tend to increase their weight by aggregating (flocculation,
precipitation), so increasing their motion component toward the bottom.
sediment
On the other hand, turbulence occurring in the water column (and particularly in
the lower layers close to the bottom) determines the re-suspension of particles.
The overall result is quite often the presence of a (maximum) turbidity zones.
salt water
salt water
(mainly)
strong
influence of the
tide
fresh and salt
water mixing
the extension of
the middle
estuary varies
with the fresh
water discharge
and the strenght
of the tide
fresh water
fresh water
tide still
influences the
water dynamics
here the water
motion is
always toward
the sea
maximum
turbidity zone
estuary
sea
low
middle
river
upper
The Cona Marsh
Finer suspended particles (and organic matter) are the preferential carriers of
pollutants from the land to the sea. As a consequence, high levels of pollutants
are found in the zones where finer particles accumulate, i.e. slack dynamics
zones such as shallow lateral areas of the estuary, lagoons, and bays.
This is for instance the example of
the Cona Marsh, a shallow water
area of the Venice Lagoon.
It is the lateral area of the Dese
River (the main tributary of the
lagoon) estuary, where pollutants
accumulate, creating a zoning in the
concentration distribution in surface
sediments.
Sediments and pollution
Moreover, these locations where polluted sediments accumulate could be also
subjected to the presence of other pollutant sources, such as cities, industrial
districts, wastewater treatment plants, and so on.
Sediments can, in turn, behave as secondary sources of pollutants for the water
column. As a consequence, the monitoring of sediment condition is important to
describe and control the ecological status of the coastal zone.
Investigations in the estuarine environment furnish fundamental information on the
evolution trends of pollutant processes, as well as for the maintaining of
biodiversity, and the control of the impact of human activities on aquatic living
organism.
By studying water circulation and matter transport in these environments we
obtain basic indications for the management of the freshwater bodies and to plan
actions for the safeguard of the coastal zone.
Acknowledgements
We are grateful to the following authors for the possibility to
use the material from their web sites:
Jonathan Sharples & Joanna Waniek & Cesar Ribeiro: Quantifying The Variability Of Vertical
Turbulent Mixing And Its Role In Controlling Stability And Mass Transport In A Partially-Mixed
Estuary http://www.soes.soton.ac.uk/research/groups/soton_water/index.html
EPA, US-842-B-06-003: Volunteer Estuary Monitoring A Methods Manual
http://www.epa.gov/owow/estuaries/monitor/
Catherin Coriou: IFREMER
ftp://ftp.ifremer.fr/ifremer/delao/plazure/ENSAR2005/doc/
Tim Wool, U.S. Environmental Protection Agency Region 4: Water Quality Analysis Simulation Program
(WASP)
http://www.cwemf.org/Agendas/tmdlmodelingwrkshp.htm
Matthias Tomczak: An Introduction to Physical Oceanography
http://www.es.flinders.edu.au/~mattom/IntroOc/index.html