Epilimnetic Phytoplankton and Zooplankton Biomass and Species Composition In Lake Ontario, 1986 to 1992.

Abstract

Results of the Food Web Workshop II (Hartig et al. 1991) indicated that Lake Ontario may be the next Great Lake after Lake Michigan to demonstrate the effects of changing nutrient levels and food web controls. Total phosphorus loads into the lake declined by 80% since 1972 and have approached the target loading set by the Great Lakes Water Quality Agreements between the United States and Canada. Spring total phosphorus levels declined from 25 to 14 IlglL between 1971 and the late 1980s and are currently below 10 Ilg/L. Although declines in chlorophyll-a were relatively low and transparency has not changed appreciably, there is some evidence that algal biomass has declined. Besides changes in nutrient concentrations, changes in fish abundance has occurred as alewife, slimy sculpin and smelt biomass have decreased, while stocking of coho and chinook salmon increased from 40,000 to 5.4 million from 1968 to 1984 (Hartig et al.. 1991). In 1993, the rate of salmonine stocking was reduced (Luckey 1994). These changes in nutrient status and in the food web of the lake, and the potential for further appreciable change in the biota of Lake Ontario, have directed attention to the long-term data sets of phytoplankton and zooplankton collected by the Great Lakes National Program Office of the U. S. Environmental Protection Agency as indicators of quantitative and compositional changes in plankton community structure. Phytoplankton, which have short carbon turnover rates, are sensitive to water quality conditions and to grazing by zooplankton and thus respond rapidly to perturbations of the lake ecosystem. The determination of phytoplankton abundance and species composition is one method to trace long-term changes in lakes (Munawar and Munawar 1982, Makarewicz 1993, Makarewicz and Bertram 1991). Similarly, whether aquatic ecosystems are perturbed by changes in the top predator fish that 2 cascade down the food web or by nutrients or by other stressors that are expressed from the first trophic level upward, the zooplankton are sensitive integrators of such changes (McNaught and Buzzard 1973). They have also proved useful for complementing phytoplankton data to assess the effects of water quality (Gannon and Sternberger 1978) and fish populations on biota (e.g. Brooks and Dodson 1965). The phytoplankton and zooplankton data sets collected by EPA's Great Lakes National Program Office provide such information and support the International J oint Commission's call for more and better information through monitoring and research on components of the Lake Ontario food web (Hartig et al. 1991). In this study, data about the 1986-92 spring and summer phytoplankton and zooplankton assemblages make it possible to examine the historical, geographic, and seasonal relationships prevailing in Lake Ontario and to compare them, where possible, to previous studies.