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Of the hundreds of species of submersed macrophytes, only a few, such as wigeongrass, Potamogeton pectinatus, and Zostera spp., are of nearly worldwide importance as foods of waterfowl and other aquatic wildlife. Traditional access to these few foods probably determines the routes, migration chronologies, and wintering areas for many species of waterfowl. Thus, the priority of conservationists should be to determine the historic range of wigeongrass and then conduct systematic surveys to inventory existing natural stands so that plans to restore and protect their populations can be formulated.

Perhaps the greatest challenge for waterfowl managers is to find economical methods to restore populations of wigeongrass and other desirable waterfowl food plants to natural and impounded wetlands, where conditions have become so unsuitable that these plants have mostly disappeared or no longer can survive. Death of the submersed macrophyte community can also drastically deplete populations of macroinvertebrates that are also choice waterfowl foods (Davies 1982).

Problems of low wigeongrass production usually are related to excessive turbidity. Butler and Hanson (1990, unpublished data) showed that the causes of turbidity vary seasonally and are often difficult to determine. In the relatively large, eutrophic prairie lake they studied, turbidity was related to resuspension of fine particulates from wave action, precipitation of calcite in the water column, growths of planktonic algae, and, to a much lesser extent, resuspension of sediments by rough fish. They found that the removal of planktivorous fish resulted in greatly increased populations of filter-feeding zooplankton that fed heavily on planktonic algae. Within 1 year, great increases in water clarity resulted, not only from the decreases in algae, but from the removal of detrital and inorganic particulates. Wigeongrass and other submersed angiosperms grew vigorously in response to the much improved light climate. By the second and third years, luxurious beds of these plants were effective in further increasing water clarity, either through protecting sediments from resuspension, limiting phytoplankton populations by competition for light or other resources, or by producing compounds toxic to algae. Further research and monitoring of this lake will be required as it responds to these physical and biological changes. More such biomanipulation experiments are needed to discover methods to ameliorate light-limiting turbidity; to understand trophic interactions between benthic omnivorous fish, planktivorous fish, zooplankton, and phytoplankton; and to determine their effects on water chemistry and vascular plant communities. As suggested by Spencer and King (1984), can manipulation of fish populations through stocking or removal - or indirectly through altering their prey or habitat - economically increase wigeongrass and other valuable waterfowl food plants in a variety of wetland types?

Managers are often asked that impoundments be managed simultaneously for irrigation, flood control, recreational boating, sport fishing, and waterfowl hunting. Undue water-level fluctuations and water shortages are common features of many of these wetlands. Therefore, it is especially important that managers be able to predict the effects of water level manipulations on submersed macrophytes at various times of the year and across a wide range of environmental settings.

In coastal areas, impoundments are common wherein water regimes can be controlled for the single purpose of establishing and maintaining wigeongrass and other valuable waterfowl foods; and fairly sophisticated techniques have been developed to manage these wetlands. But in other regions, research will be required to develop techniques to control turbidity, excessive emergent vegetation, undesirable fish, and siltation in such impoundments. Cooke (1980) called for research to determine proper dewatering intervals, effects the season of dewatering has on such intervals, and effects of dewatering on sediment and water-column chemistry. He also suggested enhancing the efficacy of dewatering techniques by combining them with other plant management methods. Finally, he emphasized the need to develop better methods of evaluating techniques used to manipulate vegetation. This approach should lead to the development of standardized methods applicable to different regions or wetland types and would allow managers to compare their results.

To determine factors that lower productivity and species diversity, we need controlled experiments to simulate the effects of human developments on a wide range of wetland types. In heavily populated areas, common problems of eutrophication, disturbance, and siltation are often complicated by the effects of special industrial effluents, thermal pollution from electrical power plants, and hydrological changes resulting from dredging and filling operations. Other areas are affected by oil spills, and irrigation wastewater, and increased use of complex agricultural chemicals. We need major advances in pollution and soil erosion control technology to solve these problems.

Research should intensify studies of genetic adaptations of wigeongrass because it is disposed to survival with changes in environmental factors. Van Wijk (1988, 1989) and Van Wijk et al. (1988) describe genetic adaptations to salinity and other habitat factors that determine whether a submersed macrophyte reproduces sexually or asexually. Through reciprocal transplant experiments, the studies could include factors such as substrate type, nutrient availability, or water level fluctuation. The information could identify Ruppia genotypes that could be used to revegetate seriously altered or disturbed aquatic ecosystems.

Finally, it must be emphasized that our ability to manage wetlands for wigeongrass production or predict the fate of standing crops cannot be fully realized until we understand the basic patterns of energy flow, resource partitioning, and community dynamics in littoral ecosystems. For example, our poor understanding of factors influencing algal and macrophyte productivity greatly limits our ability to address basic questions, such as whether herbivory or detritivory fuel secondary production in these ecosystems (Murkin 1989). Similarly, much research is needed to determine whether herbivory on standing stocks of submersed plants by organisms other than waterfowl is an important factor regulating their seasonal abundance (Sheldon 1987). Competition between Ruppia and Potamogeton pectinatus has been stated to occur in certain brackish waters (Howard-Williams and Liptrot 1980), but the basic question of whether competition is an important factor in determining the distribution of submersed plants in general remains unanswered (Rorslett 1987).