Abstract

  Lake Erie has a long history of natural and cultural perturbations ranging from glacial origins, arrival of Europeans, exploration-early colonization, degradation, exotic invasion, and phosphorus reduction to its recent recovery. Is Lake Erie a resilient ecosystem responding to phosphorus abatement and exotic invasion? It is believed that Erie was an oligotrophic system when glaciers receded followed by a long period of mesotrophic conditions. It has been classified from mesotrophic to eutrophic ecosystem during the past three decades. In the 1970s the Great Lakes Water Quality Agreement was signed between Canada and the United States and steps were taken to reduce the phosphorus loading to the Great Lakes including Lake Erie. Total phosphorus and chlorophyll a levels in the eutrophic west have dropped from 41 µg L-1 and 13.8 µg L-1 M the 1970s to <20 µg L-1 and 5.6 µg L-1 in the 1990s. Similarly a sign significant decrease in phytoplankton biomass was recorded from 1970 to 1992 in the western basin. During the same period Diatomeae decreased markedly from 55% to 10% whereas Chlorophyta increased from 8% to 55%. Similar trends were evident in the other biota. Primary production rates in the 1990s were dominated by small sized organisms (Pico plankton and nanoplankton) similar to Lake Superior-a pristine oligotrophic ecosystem. Based on several criteria such as reduction of biomass and primary production, high species diversity, decrease of eutrophic and increase of mesotrophic-oligotrophic species and prevalence of picoplankton-nanoplankton, Lake Erie appears to be a rapidly changing and resilient ecosystem altering from eutrophic to meso-oligotrophic conditions. These observations are also supported by the response of other biota such as zooplankton and benthos. For example during 1993 the non-zebra mussel benthic biomass in the western basin had returned to a similar composition observed earlier in 1952 including the recovery of the mayfly. On the other hand eastern basin benthos has not shown the same extent of recovery as the west Fish community trends are very complex, but the return of the walleye and whitefish in the western and eastern basins respectively are encouraging signs of recovery. The changes observed at various trophic levels are indicative of a meso-oligotrophic environment.

Keywords: recovery, phosphorus, phytoplankton, species, exotics, zebra mussels