Growth and Nutrition of Submersed Macrophytes in a Eutrophic Wisconsin Impoundment

The annual growth of submersed macrophytes in Eau Galle Reservoir, Wisconsin, was measured during 1981. Ceratophyllum demersum L. and Pota-mogeton pectinatus L. collectively comprised approximately 94% of maximum macrophyte standing crop. Much of the macrophyte standing crop, accrued during the summer, senesced during the early fall. However, C. demersum, persisted into the winter. Major senescence in this species occurred prior to spring thaw. Epiphyte biomass comprised a large portion (up to 33%) of macrophyte biomass, and in combination with phytoplankton, may have limited the growth of submersed macrophytes by light attenuation. Areal densities (g m−2) of N, P, and K in macrophyte tissue changed primarily as a function of change in standing crop rather than due to changes in respective tissue nutrient concentrations. Release of tissue nutrients associated with macrophyte decay potentially occurred as two separate annual events of senescence. The significance of the submersed macrophyte community of Eau Galle reservoir in nutrient cycling appears to be relatively minor, because of the persistence of C. demersum into the winter, but more importantly due to the periodically riverine nature of this system. Nutritional conditions in most reservoirs are probably much less influenced by internal loading associated with macrophyte decay than in natural lakes.