Long-Term (1927-1978) Changes in the Phytoplankton Community of Lake Michigan at Chicago

Abstract

Fifty-one years of phytoplankton data from the South District Water Intake of the Chicago Water Filtration Plant were analyzed to determine changes in the phytoplankton community related to the eutrophication of Lake Michigan. From 1930 to 1940, a net biomass (~100 mg C/m3) indicative of oligotrophic-mesotrophic conditions were implied by the net algal biomass. By 1961 net algal biomass was ~600 mg C/m3-a biomass indicative of a eutrophic lake. Much of the biomass increase is due to Tabellaria, Stephanodiscus tenuis and S. binderanus. Since the early 1970's, there has been a consistent general decrease in algal biomass to levels associated with oligotrophic-mesotrophic conditions. The decrease in net algal biomass, the decrease in abundance of eutrophic species, the small but general increase in genera that were decreasing in relative abundance until ~ 1972, and the increase in dissolved reactive silica concentrations in Lake Michigan suggest a reversal of cultural eutrophication of Lake Michigan near Chicago. Only the increase in the relative abundance (22% of the total community biomass in 1978) of blue-green algae, mostly Oscillatoria and Gomphosphaeria, argues for accelerated eutrophication.