Classification
There are many different kinds of wetlands that are characterized by differences in-
- hydrology (source(s) of water and
duration, timing and depth of flooding);
- geomorphological setting (flats,
basins, slopes, channels, etc.);
- soil composition (mineral/peat) and
oxygen content ; the anaerobic soils distinguish
wetlands from terrestrial systems
- water chemistry (pH, Ca and salinity)
- vegetation uniquely adapted to the
above conditions (submersed aquatic beds,
emergents, mosses, shrubs, trees); the presence of vegetation dominated by trees,
shrubs, grasses, mosses, and other large plants distinguishes wetlands from aquatic
systems.
Hydrology
The hydrology of any wetland is a function of the various inputs of water (precipitation,
surface
inflows, groundwater inflows) and outputs of water (evapotranspiration, surface outflows,
groundwater outflows). These determine not only the amount of water in a wetland at any time, but
also the chemical characteristics of this water.
Environmental conditions experienced by organisms in wetlands are largely a function
of the
physical and chemical properties of water and of the sources and amounts of water entering, being
stored in, and leaving a given wetland. Important physical properties of water include its high
density, specific heat, transparency, and viscosity. Important chemical properties include its pH,
nutrient content, and oxygen content. Plant canopies significantly alter environmental conditions
(water temperatures, oxygen levels) creating a series of microhabitats within each wetland. The
overall hydrology of wetlands is described by its water budget. Within any given wetland, the
vegetation found at different elevations experiences different water regimes. Water regimes, the
duration of flooding, at any point in a wetland can be determined from long- term hydrographs, plots
of water depth over time.
Microorganisms
Because of their metabolic diversity, bacteria play a central role in nutrient cycling
in wetlands.
They are involved in the decomposition of organic matter and also in important chemical
transformations of nitrogen, sulphur, and other elements.
Wetland fungi are also involved in the decomposition of litter, especially the early
stages.
Algae are major primary producers in wetlands. There are a number of algal assemblages
in most
wetlands: plankton, epiphyton, epipelon, and metaphyton. The relative importance of each
assemblage varies from place to place within a wetland. Algae are important primary producers in
many wetlands and are an important source of food for many invertebrates.
Plants
The presence of plants is one of the defining features of wetlands. They have evolved
a variety of
adaptations to flooded conditions, especially anoxic soils. Plants have overcome these constraints
either by the internal redistribution of gasses or by the direct exchange of gasses between roots
and the atmosphere. Heterophylly and clonal growth allow plants to cope with fluctuating water
levels. They create a mosaic of microhabitats in wetlands that can be exploited by animals.
Wetlands are often a complex mix of vegetation types. The ever-changing vegetation along a
hydrologic or other gradient is called a coenodine.
Wetland vegetation is very dynamic and temporal changes in vegetation are common,
especially in
wetlands dominated by herbaceous species. Maturation is the increase in biomass from year to
year due to the plant growth. Fluctuation is a change in the abundance of species from year to year
caused by changes in environmental conditions. Micro-succession is a change in species
composition from year to year. The distribution of species along coenoclines is dependent on many
factors, including seed distribution, seed germination patterns, seedling survival, and adult flooding
tolerances. Permanent changes in wetland vegetation, macro- succession, can also occur due to
changes in environmental conditions.
Many wetland functions are a consequence either directly or indirectly of plant production.
Although
various algal assemblages can be major contributors to the overall primary production of wetlands,
especially when emergent vegetation is sparse, most of the living (biomass) and dead (litter)
standing crop in wetlands at any given time was produced by plants. Therefore much of the
physical structure of wetlands is produced by plants, for example, the plant canopies in which birds
nest and the living and dead stems colonized by algae, fungi, bacteria, and invertebrates.
Invertebrates
Invertebrates provide an essential link in wetland foodwebs between the primary producers
and
larger omnivorous (eg ducks and fish). With the partial exception of some protzoans, all
nvertebrates are heterostrophs that eat algae and each other or macrophytes. Different groups of
algae and invertebrates are found in the water w.ltcr column, growing on plants, and on or in the
substrate. Low levels of oxygen caused by the decomposition of litter can be a major problem for
some invertebrates, especially aquatic insects. Aquatic insects have a variety of behavioural,
morphological, and physiological adaptations that enable them to cope with low oxygen levels.
Birds
Wetland birds such as waterfowl and wading birds have a variety of anatomical, morphological,
and
physiological adaptations that enable them to feed in wetlands.
Although many fish avoid wetlands because of low oxygen levels in the water column
and
fluctuating water levels, some fish are able to use wetlands during the day as refuges from
predators. In subtraopical and tropical wetlands in Africa, Asia andSouth America, air breathing
fish are able to live in wetlands despite low oxygen levels in the water column.
Vertebrates
Amphibians and reptiles are common in wetlands, with some reptiles, alligators, and
crocodiles,
often being top predators. There are few large mammals that are restricted to wetlands with the
hippopotamus being a notable exception. Many small rodents and mustellids, however, can be
found in wetlands and some like the American muskrat can have a significant impact on wetland
vegetation when their populations are high. Alligators excavate holes in wetlands that contain
water during the dry season. These holes become refugia during the dry season for amphibians,
other reptiles, fishes, and many invertebrates. Beavers can turn riverine wetlands into palustrine
wetlands..
Nutrient cycles
Although there are both vertebrate and invertebrate grazers in wetlands, most of the
organisms that
are found in wetlands do not obtain their energy from them. The base of food chains in many
wetlands seems to be various assemblages of algae, especially phytoplankton, epiphyton, and
epipelon.
The decomposition of litter is the most important process in wetlands. Litter decomposition
has two
stages, leaching and microbial mineralization. The former removes readily soluble small molecules
such as sugars and amino acids and makes them available to microorganisms. The later requires
the colonization of the litter surface by microorganisms capable of breaking down various kinds of
organic molecules.
Fungi are able to hydrolyse complex, long-chain polymers.
Bacteria are able to metabolize smaller molecules.
If the litter is in an aerobic environment, it can be completely mineralized. The
large amount of litter
produced every year quickly exhausts the limited amount of oxygen at the soil surface of flooded
wetlands. Consequently, most litter decomposes in anaerobic environments. Under anaerobic
conditions, fermenters metabolize long-chain polymers to fatty acid, ethanol, or other short- chain
organic molecules. These molecules are further broken down by various bacteria capable of
anaerobic respiration. Many of these bacteria use oxidized forms of nitrogen (NO3, sulphur (SO42)
or carbon (CO2) as terminal electron acceptors in anaerobic respiration. Under very reduced
conditions, one of the products of litter decomposition is the greenhouse gas, methand (CH4).
Nutrient cycling in wetlands is either due to microbial cycles (carbon, nitrogen,
and sulphur) or
sedimentary cycles (phosphorus).
Microbial cycles involve a series of oxidation–reduction reactions carried out by
bacteria or
cyanobacteria. In microbial cycles, one or more of the products of these oxidation–reduction
reactions is a gas, CO2 and CH, in the carbon cycle, N2 and NO, in the nitrogen cycle, and H2S in
the sulphur cycle. Thus carbon, nitrogen, and sulphur compounds added to wetlands can
theoretically be permanently removed from them. In reality, because of spatial and temporal
heterogeneity, transport problems, and lags in the growth of microbial populations, these elements,
especially carbon, often can accumulate over time.
Phosphorus, however, does not undergo oxidation–reduction reactions. When added to
wetlands,
phosphorus is either adsorbed on soils or co- precipitated with carbonates under some conditions.
Phosphorus accumulates until the adsorption/precipitation capacity of the wetland is exceeded.
After that, it stays in solution and can potentially leave the wetland in surface or groundwater
outflows. Stirring up wetland sediments can release this phosphorus and put it back into solution,
especially under anaerobic conditions.
Human impact
The drainage of wetlands to create land suitable for crops is still a problem in some
parts of the
world as are water projects that divert water from wetlands. Laws to protect wetlands from drainage
and filling have successfully protected wetlands in some countries.
The Ramsar Convention, an international effort to protect wetlands, has provided protection
for
many large wetlands around the world.
Existing wetlands are being degraded by alterations in their hydrology and by increased
nutrient
inputs. These environmental changes have often been accompanied by major changes in
vegetation, often due to the rapid spread of invasive species that were previously not present.
System modelling suggests that hydrological changes caused by projected global climate
change
will also alter wetland vegetation. The carbon stored in peat-lands may be oxidized more rapidly
due to these hydrological changes and this may accelerate global climate change.
The restoration of wetlands promises to undo some of the wetland losses that have
occurred.