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Aquatic macrophytes serve as a substrate for epiphytic algae. Thus seasonal changes in macrophyte abundance can influence the abundance and distribution of epiphytic algae as well as epipelic and epilithic algae that are often shaded by the macrophytes.

The role of algae in the primary production of prairie wetlands is not well defined (see Chapter 4).

Patterns of algal growth in seasonally flooded wetlands are influenced by hydroperiods and the apparent uptake and leaching of nitrogen and phosphorus associated with shallow marsh vegetation (LaBaugh et al. 1987). Wetland T8 in the Cottonwood Lake study area (Winter and Carr 1980) functions as a ground-water recharge area and dries up by midsummer (LaBaugh et al. 1987). Following snowmelt and flooding of dead shallow-marsh vegetation, dominated by Carex atherodes and Polygonum coccineum, decomposition begins. Shallow-marsh vegetation decomposes rapidly when flooded and releases nitrogen and phosphorus that are available for use by algae because this wetland has few submerged vascular plants competing with the algae for nutrients. When the water recedes, filamentous algae dominate the wetland surface until complete drawdown. New growth of shallow marsh vegetation and leaching of nutrients from the decomposing algae complete the cycle.

Semipermanently flooded wetlands also establish seasonal patterns of algal growth that tend to alternate with the growth of submerged vascular plants and periodic drawdowns. Wetland Pll in the Cottonwood Lake study area functions as a groundwater discharge area and is dominated by open water and beds of the submersed macrophyte Potamogeton pectinatus. During spring the shallow water in this wetland is turbid (Secchi disc depth=10 cm) and dominated by planktonic algae. As the summer progresses, epipelic and epiphytic algae and P. pectinatus stabilize the flocculent substrate and the water clears. During fall, when the P. pectinatus and benthic algae break down, the water again becomes turbid and planktonic algae dominate. Waterfowl contribute to the onset of turbid conditions by excavating the substrate in search of P. pectinatus turions.

Aquatic invertebrates, which are important foods of laying female dabbling ducks and their broods, tend to graze on periphyton or filter phytoplankton, bacteria, and organic detritus from the water column.

Phytoplankton species occupying a wetland complex in the Cottonwood Lake area of North Dakota are presented in Appendix A (LaBaugh and Swanson 1988). Wetland T8 is seasonally flooded and functions as a ground-water recharge area. Wetland T3 is seasonally flooded and functions as a through-flow system. Wetland P1 is a semipermanent wetland that does not have a surface outlet, but functions as a throughflow system with respect to ground water. Wetland P8 is a semipermanent wetland that contains a surface outlet and functions as a groundwater discharge area. Wetland P11 is a semipermanent wetland that does not have a surface outlet, functions as a ground-water discharge area, and concentrates salts through loss of water to the atmosphere.

Of the 306 taxa listed in Appendix A, 103 (34%) were Cyanophyta, 83 (27%) were Bacillariophyta, and 66 (22%) were Chlorophyta. Of the taxa 159 (52%) were found in seasonally flooded wetlands and 245 (80%) were found in semipermanently flooded wetlands. The more saline wetland Pll supported only 40% of the total taxa found on semipermanent wetlands, whereas the fresher wetlands P1 and P8 supported 81% (198 taxa).