A LAKEWIDE COMPARISON STUDY OF PHYTOPLANKTON BIOMASS AND ITS SPECIES COMPOSITION IN LAKE HURON, 1971 TO 1985

Phytoplankton were collected at offshore sites during 23 cruises in the spring, summer and autumn of 1983, 1984, and 1985. Forty common species and varieties from a total of 448 taxa accounted for 89.8% of the total abundance and 88.1% of the total biomass. The average phytoplankton density (mean +/- S.E.) from April through December was as follows: 1985 - 2,020 +/- 113 cells/mL, 1984 - 2,918 +/- 196 cells/mL, 1983 - 2,567 +/- 178 cells/mL. Average biomass for the same period was: 1985 - 0.34 +/- 0.021 g/m3, 1984 - 0.41 +/- 0.44 g/m3, 1983 - 0.37 +/- 0.040 g/m3. The following predominant diatoms were observed in 1974, 1983, 1984, and 1985: Asterionella formosa, Cyclotella comensis, C. comta, C. ocellata, Fragilaria crotonensis, Melosira islandica, Tabellaria flocculosa, and Rhizosolenia spp. The historical consistency of the mesotrophic-eutrophic diatom ratio, the similarity of dominant species, the predominance of oligotrophic and mesotrophic diatom indicator species, and the similarity in phytoplankton biomass suggest little change in the trophic status of the offshore waters of Lake Huron from 1971 to 1982-85.