Water as the Critical Factor

Water is a critical factor in all marshes. The duration of flooding is the most important factor determining the kind of wetland that occurs. Water can arrive as short pulses of

Figure 3 Sedges, grasses, and forbs compose this marsh on the leeward side of a narrow peninsula projecting into one of the Great Lakes (Lake Michigan), Michigan, USA. Courtesy of Cathy Keddy.
Figure 4 Extensive marshes of bulltongue (Sagittaria lancifolia) and American bulrush (Schoenoplectus americanus) now occur in coastal Louisiana, USA, where logging destroyed baldcypress forest. Courtesy of Paul Keddy.

flooding by rivers, as rainfall, or as slow and steady seepage. Each mode of arrival produces different kinds of wetlands. In order to better understand marshes, let us consider four examples of wetlands with very different flooding regimes.

Floodplains. Wetlands along rivers are often flooded by annual pulses of water (see Riparian Wetlands). These pulses may deposit thick layers of sediment or dissolved nutrients that stimulate plant growth. In floodplains (see Floodplains), animal life cycles are often precisely determined by the timing of the flood. Fish may depend upon feeding and breeding in the shallow warm pools left by retreating floodwaters. Birds may time their nesting to be able to feed their young on the fish and amphibians left

Figure 5 The largest wetland in the world occurs in the Western Siberian Lowland. Although much of this is peatland, marshes occur along the watercourses, particularly in the southern areas. Courtesy of M. Teliatnikov.
Figure 6 Southern marshes on the coastal plain of North America may be dominated by a single grass, maidencane (Panicum hemitomon). This marsh occupies an opening within a baldcypress swamp, Louisiana, USA. Courtesy of Cathy Keddy.

behind by receding water. Marshes are often intermixed with swamps, depending upon the duration of flooding (Figure 6). Early human civilizations developed in this type of habitat, along the Nile, Indus, Euphrates and Hwang Ho, where the annual flooding provided fertilized soil and free irrigation.

Peat bogs. Some peat bogs receive water only as rainfall. As a consequence, the water moves slowly, if at all, and contains very few nutrients. Hence, these types of wetlands often are dominated by slow growing mosses and evergreen plants (see Peatlands). Most such wetlands occur in the far north in glaciated landscapes. Humans have developed a number of uses for the peat - in Ireland, the peat is cut into blocks and used for fuel. In Canada, the peat is harvested and bagged for sale to gardeners. In Russia, peat is used to fuel electrical plants. Marshes may form on the edges of bogs where nutrients accumulate from runoff, or along river courses where nutrients are more available.

Seepage wetlands. In gently sloping landscapes water can seep slowly through the soil. In northern glaciated landscapes, such seepage can produce fens, which have distinctive species of mosses and plants, and may develop in distinctive parallel ridges. In more southern landscapes, seepage can produce pitcher plant savannas or wet prairies. Often these seepage areas are rather small (only a few hectares in extent) but are locally important because of the rare plants and animals they support. Seepage areas can be larger, and when the water flow is sufficiently abundant, shallow water can move across a landscape in a phenomenon known as sheet flow. The vast Everglades, with its distinctive animals, is a product of sheet flow of water from Lake Okeechobee in south central Florida southward to the ocean.

Temporary wetlands. In many parts of the world, small temporary (or ephemeral) pools form after heavy rain or when snow melts. These pools can go by a variety of local names including vernal pools, woodland ponds, playas or potholes (see Temporary Waters). The aquatic life in these pools is forced to adopt a life cycle that is closely tied to the water levels. Many species of frogs and salamanders breed in such pools, and the young must metamorphose before the pond dries up. Wetland plants may produce large numbers of seeds that remain dormant until rain refills the pond.

Since water has such a critical effect on wetlands, where water levels change, plant and animal communities will change as well. A typical shoreline marsh will often show distinct bands of vegetation ('zonation'), with each kind of plant occupying a narrow range of water depths (Figure 7). Most kinds of animals, including frogs and birds, also have their own set of preferred water depths. Wading birds (egrets, ibis, herons) may feed in different depths of water depending upon the length of their legs. Ducks, geese, and swans can feed at different water depths depending upon the length of their necks. Some water birds (Northern Shoveler, flamingos) strain microorganism from shallow water, while others (cormorants, loons) dive to feed further below the surface. Some ducks prefer wetlands that are densely vegetated, while others prefer more open water. Hence, even small changes in the duration of flooding or depth of water can produce very different plant and animal communities.

Many marsh plants adapt to flooding by producing hollow shoots, which allow oxygen to be transmitted to the rooting zone. The tissue that allows the flow of oxygen is known as aerenchyma. Not only can oxygen move by diffusion, but there are a number of methods in which oxygen moves more rapidly through large clones of plants, entering at one shoot and leaving at another. Consequently, plants can play an important role in oxidizing the soil

Figure 7 Different marsh plants tolerate different water levels. Hence, as the water level changes from shallow water (left; seasonally flooded) to deeper water (right; permanently flooded), the plants appear to occur in different zones. Courtesy of Rochelle Lawson.

Figure 8 The Amazon water lily has the largest floating leaves of any wetland plant. Note the prominent ribs on the underside of the leaf. Courtesy of Corbis.

around their rhizomes, allowing distinctive microbial communities to form. Some marsh plants also have floating leaves (e.g., water lilies) or even float entirely on the surface (e.g., duckweeds). The largest floating leaves in the world (Figure 8) are those of the Amazon water lily (Victoria amazónica). The gargantuan leaves can be 2 m in diameter with an elevated lip around the circumference. There are two gaps in the lip to allow water to drain, and large spines to protect the underwater sections of the foliage.

Other Environmental Factors Affecting Marshes

Nutrients

The main nutrients that affect the growth of marsh plants, and plants in general, are nitrogen and phosphorus. As described above, flood pulses that carry sediment down river courses can produce particularly fertile and productive marshes. Floodplains can therefore be thought of as one natural extreme along a gradient of nutrient supply. At the other end of the gradient lie peat bogs, which depend partly or entirely upon rainfall, and which therefore receive few nutrients. Sphagnum moss is well adapted to peatlands, and often comprises a large portion of the peat. In between the natural extremes of river floodplains (high nutrients) and peat bogs (low nutrients), one can arrange most other types of wetlands. The type of plants, and their rate of growth, will depend where along this gradient they occur, but most marshes generally occur in more fertile conditions.

While nutrients enhance productivity, paradoxically they can often reduce the diversity of plants and animals. Often, the high productivity is channeled into a few dominant species. One finds large numbers of common species, while the rarer species disappear. Humans often increase nutrient levels in watersheds and wetlands, thereby changing the species present and reducing their diversity. Carnivorous plants are known for tolerating low nutrient levels, because they can obtain added nutrients from their prey. Common examples include pitcher plants (Sarracenia spp.), bladder-worts (Utricularia spp.), and butterworts (Pinguicula spp.). Cattails (Typhaspp.) and certain grasses (Phalarisarundinacea) are particularly well known for rapid growth and an ability to dominate marshes at higher nutrient levels.

Disturbance

A disturbance can be narrowly defined as any factor that removes biomass from a plant. In marshes, sources of disturbance may include waves in lakes, fire, grazing, or (in the north) scouring by winter ice. One of the principal effects of disturbances is the creation of gaps in the vegetation, allowing new kinds of plants to establish from buried seeds. Most marshes have large densities of buried seeds, often more than 1000 seeds m~2. After disturbance, marsh plants can also re-emerge from buried rhizomes. Hence, cycles of disturbance play an important role in creating marshes.

Although the presence of fire in wetlands may seem paradoxical, fire can often occur during periods of drought. Northern peatlands, cattail marshes on lake-shores, wet prairies, and seepage areas in savannas can burn under the appropriate conditions. In northern peat-lands, a fire can remove thousands of years of peat accumulation in a few days, even uncovering boulders and rock ridges that were buried beneath the peat. In marshes, fire can selectively remove shrubs and small trees, preventing the marsh from turning into a swamp. In the Everglades, burning can create depressions that then cause marshes to revert to aquatic conditions.

Animals that feed upon plants often cause only small and local effects. Think of a moose grazing on water lilies, a muskrat feeding on grasses, or a hippopotamus feeding on water hyacinth. Often the small patch of removed foliage is quickly replaced by new growth. But when herbivores become overly abundant, they can destroy the marsh vegetation entirely. In northern North America along Hudson Bay, Canada geese (Branta canadensis) are now so abundant that they remove all vegetation from expanses of coastal marsh. In southern North America, along the Gulf of Mexico, an introduced mammal, nutria (Myocastor coypus), similarly can strip marsh vegetation to coastal mudflats. To some extent, disturbance by herbivores is a natural phenomenon, one that has occurred cyclically throughout history. However, in the above two examples, one suspects humans may be the ultimate cause of the large-scale overgrazing (see the next section).

Periodic droughts may at times function like a natural disturbance by killing adult plants, and allowing new species to re-establish from buried seeds. Vernal pools and prairie potholes both have plant and animal species that are adapted to this kind of cyclical disturbance.

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