TRENDS IN NUTRIENT CONCENTRATIONS IN HATCHERY BAY, WESTERN LAKE ERIE, BEFORE AND AFTER DREISSENA-POLYMORPHA

Concentrations of soluble reactive phosphorus, ammonium-nitrogen, nitrate-nitrogen, silica, and chloride have all increased since the establishment of the zebra mussel (Dreissena polymorpha) in Hatchery Bay, western Lake Erie, in 1988. Total phosphorus concentrations have changed little. These results are from 188 samples collected weekly and year round before the establishment of Dreissena (1984-1987) and 192 samples post-Dreissena (1990-1993). The mean annual total phosphorus concentration for the three complete post-Dreissena years was 35 mu g . L(-1), strikingly similar to the concentration of 36 mu g . L(-1), which in 1959 helped to define the waters of Lake Erie as eutrophic. The relative steadiness in total phosphorus may reflect sediment reflux, because Hatchery Bay is a polymictic system. The slight increase in the biologically conservative ion, chloride, in the 1990s, is probably due to the increased precipitation and runoff in the western Lake Erie watershed. Decreased phytoplankton and associated increased water clarity caused by efficient filtering by D. polymorpha, have lessened symptoms of eutrophication and produced a situation where nutrients are not fully utilized, i.e., biological oligotrophy.