Major Wetland Types

Here we highlight several of the most extensive wetlands types, of the many that exist. Our interest is in those most likely to be affected by development activities.

Tidal Salt Marshes Salt marshes tend to form in protected areas such as in the shelter of spits, bars, or islands, in bays, and in estuaries. Marshes may grow with sedimentation supplied by rivers or tidal action and influenced by sediment-trapping effects of the biota. In some places, sea-level rise results in an increase in the extent of salt marshes. For example, sea level along the Atlantic coast of North America has been increasing 1 to 3 mm per year for about 4000 years. Marshes kept pace by accumulating peat, in fact increasing their extent by growing both landward and seaward.

Salt marshes are primarily nitrogen limited. The main available form is ammonium. However, nonnutrient stresses may also be significant. These can include salt stress and lack of drainage. If evaporation exceeds rainfall, portions of the marsh away from the water may accumulate salt. Even salt-tolerant plants must expend energy to obtain water against the osmotic gradient.

Cordgrass (S. alterniflora) tends to dominate in the lower marsh, near the water. S. alterniflora has stiff leaves and occurs in two forms. The tall form is 1 to 3 m tall and dominates in the extreme lower portions of the marsh. The short form is 17 to 80 cm tall and is found in higher salinity conditions, farther from the water. Closer to land, S. alterniflora gives way to the salt-tolerant species S. patens and Distichlis spicata (spike grass). At the high-tide level will be found stands of Juncus gerardi or J. Roemerianus. Finally, at the part of the marsh reached only by the highest spring tides, if rainfall exceeds evaporation, salt-intolerant species such as Panicum virgatum and Phragmites australis are found.

Salt marsh fauna may be grouped into one of three habitats. The aerial habitat consists of the parts of the plants that are almost always above the water. It is similar to a terrestrial habitat and is dominated by grazing insects and spiders that live by chewing leaves or sucking the sap. Many birds feed off these insects as well as crabs, worms, and fish. Wading birds such as egrets and herons, and waterfowl such as the black duck, are residents of the salt marsh. The benthic habitat supports the detrital food chain and includes fungi, bacteria, and invertebrates.

The aquatic habitat includes some of the benthic organisms, but by convention may be defined to refer mostly to aquatic vertebrates, especially fish. Most salt marsh fish live at least part of their life cycles outside the marsh. Many fish and shellfish spawn upstream or offshore. The juveniles then find food and shelter in the salt marsh, eventually continuing their journey to their adult range. Over 90% of the commercially valuable fish and shellfish harvested off the Gulf and southeastern Atlantic coast of the United States spend part of their life-cycles in the salt marsh. This fact is one of the major utilitarian motives behind wetlands protection.

Freshwater Marshes Freshwater marshes have a wide variety of types, so it is more difficult to make generalizations that apply to all. They range from tidal freshwater marshes to inland freshwater marshes. The latter include prairie potholes of the northern prairie of North America, the Everglades of Florida, the playas in arid areas of Texas and New Mexico, the Great Lakes marshes, and riverine marshes distributed in most regions. Hydrology is still a major controlling factor. The periodicity of inland marshes tends to be seasonal, in contrast to the tidal cycling of salt marshes.

The vegetation tends to grow in zones from upland to deep water (Figure 15.22). The edge that is periodically flooded is dominated by grasses. Where the water level is at or just above the soil, one can find sedges (Carex spp.), rushes (Juncus spp.), bulrush (Juncus spp.), and the broadleafed monocots, the arrowheads (Sagittaria spp.). In deeper water, cattails are common. The broadleafed cattail Typha latifolia is less floodresistant than the narrow-leafed cattail Typha angustifolia. The latter will grow in depths up to 1 m. Hardstem bulrushes (Scirpus acutus) occupies the deepest zone of emergent plants. At greater depth the floating plants are found. These include water lilies (Nymphaea tuberosa or T. Odorata) and water lotus (Nelumbo lutea). Also found at depths that preclude emergent vegetation are the submersed hydrophytes, including coontail (Ceratophyllum demersum), water millfoil (Myriophyllum spp.), pondweed (Potamogeton spp.) and waterweed (Elodea canadensis). Interestingly, many temperate aquatic ecosystems develop floating marshes, buoyed by trapped nitrogen and methane and plant material.

Waterlogged or Shallow Water

Emergent Macrophytes

Floating-Leaves and submersed Aquatics

Periodically Flooded

Waterlogged or Shallow Water

Emergent Macrophytes

Floating-Leaves and submersed Aquatics lowland grasses sedges (Carex) arrowhead (Sagittaria spp.)

cattalis {Typha) water lilies (Nymphaea spp.) bulrush (Scirpus) pond weeds (Potamogeton spp.)

bladder wort (¡Jtricularia spp.)

Figure 15.22 Zonation of vegetation in a freshwater marsh. (From Mitsch and Gosselink, 1993. Used with permission.)

Eichornia crassipes (water hyacinth) is an introduced species that grows very rapidly and has become a nuisance in the southeastern United States.

Phragmites and cattails are among the most productive wetlands plants, having photo-synthetic efficiency from 4 to 7%. Measurements of primary production ranges from 1000 to 6000 g/m2 • yr for Phragmites and 2,040 to 3,450 g/m2 • yr for Typha. However, the pattern differs for these two plants. Typha grows fastest early in the growing season, then slows down. Phragmites grows at a fairly constant rate throughout the season.

The most obvious invertebrates in the freshwater marsh are members of the insect order Diptera, the true flies. These include the mosquitoes as well as midges and crane flies. Many are herbivores, although many others are benthic in their larval stages. Most of the mammals are also herbivorous. One of the most common mammals is the muskrat (Ondatra zibethicus).

Omnivorous and herbivorous waterfowl are plentiful. They nest in northern marshes and migrate between them and southern marshes in the winter. The loon frequents deeper ponds where there may be fish. Some ducks nest in upland locations; those that fish by diving nest over the water. Geese, swans, and canvasback ducks are the major herbivores in the marsh. Wading birds nest in colonies in marshes and fish in shallow waters. Songbirds and swallows usually nest and perch in uplands near the marsh, and fly into the marsh to feed.

Marshes may provide shelter for reproduction of fish from adjacent lakes. Common carp (Cyprinus carpio) are able to withstand great fluctuations in water temperature and dissolved oxygen that occurs in shallow marshes. They graze and uproot the vegetation, increasing turbidity in the water.

Freshwater marshes tend to be both nitrogen and phosphorus limited. Vegetation plays a role in nutrient cycling similar to that in forests. They remove nutrients from the soil, only to return them as dead plant material is degraded.

Riparian Wetlands The riparian zone is the area adjacent to rivers that has a high water table and is regularly flooded by the river. Also called the floodplain, they are widely distributed, even in arid areas. They are open systems, with significant exchanges of energy and material with the adjacent upland and riverine ecosystems, as well as upstream and downstream areas. Because of their wide distribution and narrow geometry, they have been particularly vulnerable to draining and filling. Riparian wetlands provide water storage during flood stages of the river. The elimination of this "safety valve'' has contributed to downstream damage due to floods.

Western riparian wetlands of the United States tend to be narrow and have steep geo-morphology. The uplands tend to be clearly distinguished from the wetlands. Eastern riparian systems include those of the Mississippi valley, the southeastern and northeastern United States. They may have steep morphology along low-order streams near the high-elevation headwater. The higher-order streams at lower elevations tend to have gentler slopes, wider wetlands, and more gradual transition to uplands. A natural levee often forms by sediment deposition between the wetland and the river.

As flooding is intermittent, anoxic conditions in the soil occur only when inundated. The organic matter content is intermediate, about 2 to 5%. Clay deposits and import by flooding results in accumulation of a large quantity of nutrients.

Spatial and temporal scales are linked, as distance from the river (or actually, elevation) determines the frequency of flooding. Figure 15.23 shows an example of this in terms of a classification applied to southwestern U.S. bottomland forest wetlands. The type of vegetation correlates approximately with the zone. For example, among the oaks, overcup oak (Quercus lyrata) is typical of zone III, laurel oak (Quercus laurifolia) of zone IV, and water oak (Quercus nigra) in zone V.

In arid areas, riparian wetlands are easy to distinguish from upland ecosystems, since the latter are usually grasslands or deserts and the former are forested. They tend to form narrow strips along the river, snaking their way through the otherwise dry region. Obviously, they serve as a local oasis to animals and humans. These wetlands usually lack oak but include willow (Salix spp.) and cottonwood (Populus fremontii), which are also common throughout the United States. Western riparian wetlands also have sycamore (Platanus wrightii), ash (Fraxinus pennsylvanica velutina), and walnut (Juglans major), as well as alder (Alnus tenuifolia) at higher elevations.

Riparian wetlands often provide the only woodlands in an area, either because the uplands have been converted to agriculture or because it was that way in the first place, as in arid areas. This increases the importance of riparian wetlands for providing food and habitat, and as corridors for dispersal or migration.

Animal life in riparian wetlands is too diverse to describe here. Most are common to adjacent upland or aquatic systems as well. Some are strongly associated with wetlands, such as the beaver and the cottonmouth snake. In the western United States there are 88 species of birds that are strictly riparian. In southern California there are 45 riparian mammals. Some species of fish spawn in the floodplains during flood stage.

The intermittent nature of riparian flooding augments the ecosystem productivity by providing moisture without causing long periods of anoxic soil conditions, while providing nutrients and flushing wastes away. Typically, riparian ecosystems with regular wet and dry periods have an aboveground net biomass productivity exceeding 1000 g/m2 • yr.

Riparian ecosystems are open systems. Much of their productivity is exported to the adjacent waterway, and they can be sinks for nutrients from upland ecosystems.

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