7.3: Productivity in wetland ecosystems

Wetlands are among the most productive ecosystems in the world, comparable to rain forests and coral reefs. They also are a source of substantial biodiversity in supporting numerous species from all of the major groups of organisms — from microbes to mammals. Physical and chemical features such as climate, topography (landscape shape), geology, nutrients, and hydrology (the quantity and movement of water) help to determine the plants and animals that inhabit various wetlands. Wetlands in Texas, North Carolina, and Alaska, for example, differ substantially from one another because of their varying physical and biotic nature.

Wetlands can be thought of as "biological supermarkets." They produce great quantities of food that attract many animal species. The complex, dynamic feeding relationships among the organisms inhabiting wetland environments are referred to as food webs. The combination of shallow water, high levels of inorganic nutrients, and high rates of primary productivity (the synthesis of new plant biomass through photosynthesis) in many wetlands is ideal for the development of organisms that form the base of the food web —for example, many species of insects, mollusks, and crustaceans. Some animals consume the above-ground live vegetation (herbivore-carnivore food web); others utilize the dead plant leaves and stems, which break down in the water to form small, nutrient-enriched particles of organic material called detritus.

As the plant material continues to break down into smaller and smaller particles, it becomes increasingly enriched (nutritious) due to bacterial, fungal and protozoan activity. This enriched proteinaceous material, including the various microbes that colonize it, feeds many small aquatic invertebrates and small fish. Many of these invertebrates and fish then serve as food for larger predatory amphibians, reptiles, fish, birds, and mammals. Numerous species of birds and mammals rely on wetlands for food, water, and shelter, especially while migrating and breeding.

Many animals need wetlands for part or all of their life-cycles. In late winter and early spring, for example, adult tiger salamanders migrate from uplands to vernal pools for breeding and egg deposition. The gilled larvae resulting from their fertilized eggs then develop further, eventually producing lungs. Therefore, they must leave the vernal pools for adjacent upland, generally forested, habitat as adults, where they are mainly subterranean. In this instance, a complex of wetlands within a forest matrix is important as the life-cycle requirements of the tiger salamanders change. Thus, for the existence of the tiger salamander, both wetlands and uplands are important and essential. This can similarly be said of other amphibians like the spotted salamander as well as many other animals.

The diversity of habitats in a watershed or larger landscape unit is also important for other ecological functions associated with wetlands. One such function, biogeochemical cycling, involves the biologic, physical, and chemical transformations of various nutrients within the biota, soils, water, and air. Wetlands are very important in this regard, particularly relating to nitrogen, sulfur, and phosphorous. A good example of this occurs in anaerobic (non-oxygenated) and chemically reduced wetland soils and the muddy sediments of aquatic habitats like estuaries, lakes, and streams, which support microbes that function in nitrogen and sulfur cycling. Upon death and decay, the nitrogen and sulfur in plant and animal biomass is released through mineralization. Much of this is eventually transformed into gaseous forms and released into the atmosphere, where it once again becomes available to certain plants and their associated nitrogen-fixing bacteria in the soil. This is literally a major defense for mud, since it is the anaerobic and chemically reducing conditions in the substrate, in conjunction with various microbes, that ensure the gaseous release of the nitrogen and sulfur. On the other hand, phosphorous does not have a gaseous form, but vascular plants in wetlands transform inorganic forms of phosphorus (that might otherwise be shunted into undesirable algal blooms) into organic forms in their biomass as they grow. Thus, wetlands provide the conditions needed for the removal of both nitrogen and phosphorus from surface water.

Scientists also point out that atmospheric maintenance is an additional wetland function. Wetlands store carbon within their live and preserved (peat) plant biomass instead of releasing it to the atmosphere as carbon dioxide, a greenhouse gas affecting global climates. Therefore, wetlands world-wide help to moderate global climatic conditions. On the other hand, filling, clearing and draining wetlands releases carbon dioxide.

Wetlands also play an important role in the hydrologic cycle -- a cycle we all experience quite readily, for example, with the precipitation from a thunderstorm and the evaporation of ponded water from a puddle or bird bath. Wetlands can receive, store, and release water in various ways -- physically through ground water and surface water, as well as biologically through transpiration by vegetation -- and therefore function in this very important global cycle.

Fish and wildlife use wetlands to varying degrees depending upon the species involved. Some live only in wetlands for their entire lives; others require wetland habitat for at least part of their life cycle; still others use wetlands much less frequently, generally for feeding. In other words, for many species wetlands are primary habitats, meaning that these species depend on them for survival; for others, wetlands provide important seasonal habitats, where food, water, and cover are plentiful.

For example, wetlands are essentially the permanent habitat of the beaver, muskrat, wood duck, clapper rail, mud minnow, wild rice, cattail, broadleaf arrowhead and swamp rose. For other species, such as largemouth bass, chain pickerel, woodcock, hooded warbler, otter, black bear, raccoon, and meadow vole, wetlands provide important food, water, shelter, or nesting habitat.

Numerous birds —including certain shorebirds, wading birds, and raptors, and many songbirds— feed, nest, and/or raise their young in wetlands. Migratory waterfowl, including ducks, geese, and swans, use coastal and inland wetlands as resting, feeding, breeding, or nesting grounds for at least part of the year. For example, in the Chesapeake Bay Region (a major wintering area for waterfowl), coastal wetlands supported an annual average of nearly 79,000 wintering black ducks over a 45-year period (1950-1994); over the same period, it supported an annual average of about 14,000 wintering pintails. Most of these ducks rely on the prairie potholes (depressional wetlands) in upper mid-western United States and adjacent Canada and interior wetlands in northeastern North America for nesting. Indeed, an international agreement to protect wetlands of international importance was developed because some species of migratory birds are completely dependent on certain wetlands and would become extinct if those wetlands were destroyed (read on for the economic values associated with these resources.)

Because they produce so much plant biomass and invertebrate life, estuaries and their coastal marshes serve as important nursery areas for the young of many game (recreational) and commercial fish and shellfish. Menhaden, flounder, sea trout, spot, croaker, and striped bass are among the more familiar fish that depend on coastal wetlands. Such areas are also critical nursery habitat for young commercial shrimp along the Southeast and Gulf Coasts. Freshwater fish, such as the chain pickerel and northern pike, use well-flooded or ponded wetlands as breeding and nursery areas. Some fish, like the brown bullhead and mud minnow, even subsist in wetlands that have natural low dissolved oxygen concentrations that unadapted species cannot endure. In the Pacific Northwest, some wetlands release cooler water to salmon-bearing streams and rivers; in places this is critical to the health of coldwater fish populations.

Excerpted from:

Wetland Functions and Values, Watershed Academy Web, US Environmental Protection Agency. Accessed on December 2023 https://cfpub.epa.gov/watertrain/moduleFrame.cfm?parent_object_id=262