There are many different kinds of wetlands that are characterized by differences in-

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.

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.

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 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.

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.

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..

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.

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.