PHYTOPLANKTON BIOMASS AND SPECIES COMPOSITION IN LAKE ERIE, 1970 TO 1987

Phytoplankton were collected at offshore sites during 33 cruises in the spring, summer, and autumn from 1983 to 1987. Forty-nine common species and varieties accounted for 83.3% of the total abundance and 83.1% of the biomass. Over the 5-year study period, the phytoplankton biomass (mean +/- S.E.) of the western, central and eastern basins averaged 1.88 +/- 0.12 g/m3, 1.04 +/- 0.075 g/m3, and 0.63 + 0.071 g/m3, respectively. Depending on the basin, a 52 to 89% reduction in mean weighted algal biomass had occurred in the offshore waters of Lake Erie between 1970 and 1983-87. The historically highly productive western basin had a consistent decrease in biomass from 1958 to 1987. In general, occurrences of common species in 1970 and 1983-87 were similar. However, dramatic decreases in the abundances of many nuisance and eutrophic species were evident. A 70 to 98% reduction in biomass of Stephanodiscus binderanus, S. niagarae, S. tenuis, Aphanizomenon flos-aquae, and Rhodomonas minuta was observed. The decrease in biomass of the nuisance and eutrophic indicator species, the reappearance of mesotrophic species, such as Asterionella formosa and Rhizosolenia eriensis, common in the 1940s and 1950s, and the decrease in total phytoplankton biomass suggest a major improvement in the offshore waters of Lake Erie.