Lake Erie 1993, Western, West Central and Eastern Basins: Change in Trophic Status, and Assessment of the Abundance, Biomass and Production of the Lower Trophic Levels

Abstract

The western and west central basins were mesotrophic and the eastern basin was oligotrophic, based on many biological and chemical parameters measured in 1993. Gradients were observed for most parameters, with chlorophyll a, nitrogen, phosphorus, silica, and light extinction decreasing from west to east. In the western basin, phytoplankton biomass declined by 51% from 1983-85. Phytoplankton photosynthesis (g C·m-2), predicted from total phosphorus (TP) using a relationship developed in other offshore productivity studies in Lake Ontario, declined by 35% in 1993, without a corresponding decline in phosphorus (P) loading or TP. Diatoms decreased and there was a shift towards smaller phytoplankton species. These changes were attributed to zebra mussel filtration, but were not of sufficient magnitude to reduce zooplankton biomass. In the west central basin, the reductions in phytoplankton biomass were modest. Photosynthesis (g C·m-2) in 1993, was in line with that predicted by TP and the empirical relationship developed in other offshore studies. Limited mussel populations in the west central basin, resulting from low hypolimnetic oxygen concentrations, caused little change in the phytoplankton. There also were no reductions in mean biomass of zooplankton from 1984-87. In the eastern basin, phytoplankton biomass declined by 49% from 1983-85. Photosynthesis (g C·m-2) declined by 50% from the value predicted, from TP and the empirical relationship developed for other studies, for 1983-85, without a decline in P-leading. TP was lower in 1993 and was attributed to filtering by Dreissena and subsequent redirection of pelagic material to the sediments. Phytoplankton species indicative of eutrophy were reduced and there was an overall shift towards smaller species. Zooplankton biomass was also reduced. Mean zooplankton community size and the loss of Daphnia sp. suggest that predation by planktivores as well as a reduced food supply, affected zooplankton biomass in 1993. The Dreissena population also affected the benthic community structure as Diporeia were virtually eliminated from the eastern basin and Gammarus increased in all basins. Benthic biomass was 40% higher on average than in 1979. Dreissena dominated benthic production at all stations except offshore in the west-central basin.