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Sago Pondweed (Potamogeton pectinatus L.):
A Literature Review

Research Needs

Only a few of the hundreds of species of submersed macrophytes are important foods of waterfowl and other aquatic birds. A single sago-dominated lake can support a large percentage of the continental population of an important waterfowl species for a month during fall staging period (Kantrud 1986a), and I suspect that the migration routes of some mostly herbivorous waterfowl are determined by traditional access to a relatively small variety of submersed plant foods. Yet, the effects of herbivory on standing stocks of these economically important plants and their potential competitors are just beginning to be revealed. For example, recent work by Sheldon (1987) shows that snails can consume up to 13% of the total leaf area of submersed plants in Minnesota lakes. The basic question of whether herbivory or detritivory form the basis for secondary production in littoral ecosystems is unanswered, because algal and macrophyte productivity and the factors influencing them are so poorly understood (Murkin 1989).

Further research seems warranted on the importance of genetic adaptation of sago to various basic environmental factors because modern society has created many threats to natural plant communities. Recent work by Van Wijk (1988, 1989) and Van Wijk et al. (1988) on genetic adaptations to salinity and the habitat factors that determine whether sago reproduces sexually or asexually likely could be, perhaps with reciprocal transplant experiments, expanded to include factors such as substrate type, nutrient availability, or water level fluctuation. Such information could help identify and protect sago genotypes suitable for future use to revegetate seriously altered or disturbed aquatic ecosystems.

Davies (1982) showed that the loss of sago in a brackish wetland drastically depleted the invertebrate community, and that the replacement primary producers did not provide enough shelter, food, or surface area for attachment to maintain large standing stocks of the primary and secondary consumers. Howard-Williams and Liptrot (1980) pointed out that such ecosystems may be especially vulnerable to catastrophic losses of primary production, because so few submersed macrophytes are able to tolerate brackish waters that show wide fluctuations in salinity. In heavily populated areas, commonly observed problems of eutrophication and siltation are often complicated by the effects of special industrial efffluents, thermal pollution from electrical power plants, and hydrological changes resulting from dredging and filling operations. Other areas show increased use of sophisticated agricultural chemicals and pesticides. Controlled experiments that simulate the effects of human developments on a wide range of wetland types are needed to determine how to prevent productivity losses and maintain species diversity.

From a wildlife standpoint, corrective actions require that biologists understand the mechanisms responsible for the deterioration of sago and other high quality waterfowl food plants in waterfowl staging, migration, and wintering areas. These places usually are deeper wetlands that also support fish populations. Especially needed are experiments to identify the causes of light-limiting turbidity. Similarly, experiments must address the trophic interactions between benthic omnivorous fish, planktivorous fish, zooplankton, and phytoplankton and their effects on water chemistry and vascular plant communities, in wetlands managed primarily for waterfowl. An important unanswered question for waterfowl managers is whether fish removal, manipulation of existing populations, or altering the predator-prey relations of fish populations through stocking can economically and permanently increase the abundance of sago and other valuable waterfowl food plants (Spencer and King 1984). Such research is currently under way in a Minnesota lake where it is suspected that increased water levels and introductions of rough fish initiated a complex series of events leading to light-limiting calcite accumulations in the water column (Butler and Hanson 1985, 1986, 1988, unpublished).

There is a need to greatly improve our ability to predict the response of sago and other hydrophytes to vegetation management in the shallow prairie wetlands that are the breeding areas for most of North America's waterfowl (Kantrud 1986b). In most of these wetlands, water control structures are not present, and managers are limited to only a few tools (prescribed burning, grazing, mechanical treatments, and herbicide applications). The usual goal here is to maintain mixtures of stands of emergents--used for nest sites and escape cover-- and beds of submergents--used for feeding areas. Prairie wetlands lie in basins of extremely diverse hydrological setting, so methods must be developed to manage hydrophytes across wide gradients of water chemistry and water permanency in a region where long-term cycles of drought and excess precipitation occur.

Much needs to be learned on how to establish and maintain sago and other submersed hydrophytes that are valuable waterfowl foods in manmade and natural wetlands where water control structures allow dewaterings or significant water level manipulations. Light-limiting turbidity, rough fish, excessive emergent vegetation, sedimentation, and untoward water level fluctuations are common features of many of these wetlands. Personnel are often asked to manage these areas simultaneously as flood control reservoirs, recreational boating areas, sport fisheries, and waterfowl hunting areas. It is especially important that managers know the dewatering schedules and rates of water level increase that would allow sago and other hydrophytes to germinate and grow. Gibbs (1973) suggested that much more information would be required to explain the little-understood cycles between dominance by phytoplankton, macroalgae (Chara), and Potamogeton in wetlands important to waterfowl and fish. Cooke (1980) called for research to determine proper dewatering intervals, effects of season of dewatering on such intervals, effects of dewatering on sediment and water column chemistry, and if the efficacy of the technique could be enhanced by combining it with other plant management methods. Finally, he emphasized the need to develop better evaluation techniques when vegetation manipulations are undertaken.

For sago control, Spencer (1986a,b) suggested pursuing research into management techniques that would disrupt turion formation sufficiently to result in the production of smaller turions. This could result in smaller plants more susceptible to environmental stress. Also recommended was the delaying of control techniques until after carbohydrate reserves in turions are exhausted.

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