Long term changes in nutrients, chloride, and phytoplankton density in the nearshore waters of Lake Erie

Monitoring at five municipal water treatment plant intakes revealed several significant trends in Lake Erie's nearshore water quality since the 1960s and 1970s. Phytoplankton densities, in particular the abundance of diatoms, decreased following early phosphorus loading controls and the establishment of dreissenid mussels, while silica concentrations increased. There were notable reductions and breakpoints in the trends in phytoplankton cell densities in consecutive years from the west to the east of the lake (western basin in 1987, central in 1988, and eastern in 1989) coinciding with the reported timing of dreissenid mussel invasion and establishment. There have also been shifts in phytoplankton species composition that were related to variations in total phosphorus, chloride, silica, and nitrate concentrations. Annual mean phosphorus concentrations have decreased significantly and remained low since the early 1990s in the eastern basin, showed no consistent trend in the central basin, and decreased in the western basin. Concentrations have increased in the western basin since the mid-1990s, although not to the levels seen during the early 1980s. Recently, chlorophyll a concentrations have also increased in the western and eastern basins of the lake.