Recent Changes in the Near-Shore Phytoplankton of Lake Erie's Western Basin at Kingsville,Ontario

A weekly net phytoplankton monitoring programme begun in 1967 at the Union water treatment plant, Kingsville, Ontario, was expanded in 1976 to include weekly chemical analyses of western Lake Erie water samples. Data from the Union intake monitoring programme have been examined in light of the significant reductions, since 1971, in phosphorus loading from the Detroit River. Total phosphorus concentrations in the “raw water” declined to an average of 30-35 μg P/L during 1977-78 from a pre-P control average of 63 μg P/L during 1967-70. Average annual net phytoplankton density during the pre-P control years averaged about 5000 Areal Standard Units per mL (A.S.U./mL), decreased steadily between 1971 and 1976 to a low of 2900 A.S.U./mL, but then increased again to 4490 A.S.U./mL by 1978. The recent increase was attributed almost entirely to massive mid-summer development of Fragilaria crotonensis. Winter densities of net phytoplankton during 1977 and 1978 were the lowest on record for the 12-year period. Similarly, average densities of green and blue-green algae showed a pre-P control density regime which was clearly higher than the post 1971 densities. Annual average diatom density was negatively correlated with annual average lake levels over the 12-year period and recent anomalous increases in Fragilaria may have been related to the observed drop in lake level which influenced turbulence and resuspension of this diatom.