Seasonal Phosphate Demand for Lake Erie Plankton

From April to mid-October 1979, in the eastern and central basins of Lake Erie, the pattern for phosphate demand was assessed by four independent methods: conventional monitoring, measurements of polyphosphate, phosphate turnover time, and a newly developed phosphate deficiency index. Thermal stratification and P limitation occurred faster in the central than in the eastern basin and the plankton remained P limited throughout most of the stratified season. This condition relaxed as the thermocline deepened in the central basin, exposing the epilimnion to a greater area of sediments. In September, when mixing of the entire water column occurred, the plankton were no longer P limited. In contrast, as the thermocline deepened in the eastern basin, metalimnetic water was entrained but the plankton remained P limited until mid-October. These observations support the view that a reduction of phosphorus in Lake Erie would probably affect the phytoplankton biomass during the periods of P limitation.