With the acknowledgement that biological monitoring was fundamental to charting ecosystem health (Great Lakes Water Quality Agreement 1978), EPA's program was developed for Lakes Erie, Huron and Michigan to: 1) monitor seasonal patterns, ranges of abundance and, in general, structure of the phytoplankton and zooplankton communities; 2) relate the biological components to variations in the physical, nutrient and biological environment; and 3) assess the annual variance to allow better long-term assessments of trophic structure and state. Several offshore stations (9-11) on several cruises (9-11) during the spring, summer and autumn of 1984 and winter of 1985 were sampled.
By examining changes in the phytoplankton and zooplankton in relation to water chemistry, evidence was found suggesting little change in the trophic status of Lakes Huron and Michigan while an improvement in the trophic status of Lake Erie was evident. The offshore region of Lake Michigan is experiencing changes in phytoplankton and zooplankton composition consistent with nutrient control and top-down control by fish. Even so, the biomass of phytoplankton and zooplankton and the trophic status of the lake have not changed significantly. The appearance and establishment of Daphnia pulicaria in offshore waters of Lake Huron suggest a change in the forage fish base. With the exception of the resurgence of Asterionella formosa in Lake Erie, plankton composition has changed little since the 60's. However, dramatic reductions in biomass of nuisance and eutrophic indicator species have occurred. These changes are consistent with expectations of long-term nutrient control. However, a change in piscivory is evident that has apparently allowed the establishment of the large cladoceran Daphnia pulicaria.
Makarewicz, Joseph C., "Phytoplankton and Zooplankton: In Lakes Erie, Lake Huron and Lake Michigan: 1984" (1987). Technical Reports. 122.