Phytoplankton photosynthesis at two index stations in Lake Ontario 1987-1992: Assessment of the long-term response to phosphorus control

Phytoplankton photosynthesis was measured from May to late October at two stations in Lake Ontario from 1987 to 1992, and published data from the International Field Year for the Great Lakes (IFYGL) in 1972 were reanalyzed. Our measurements indicate that seasonal phytoplankton photosynthesis (May 1 - October 31) may have declined about 30% since IFYGL. An empirical relationship between seasonal phytoplankton photosynthesis and mean phosphorus concentration indicates that the estimate of offshore seasonal PP by the Lake Ontario Nutrient Assessment Study (LONAS) in 1982 was unusually low (44%) compared with that predicted from phosphorus concentration. Accurate assessment of long-term change in productivity has been hampered by lack of adequate base-line data from the past. Annual variability in seasonal phytoplankton photosynthesis was remarkably low at both stations during our study with the exception of anomalously low results in 1988. Long-term changes in phytoplankton photosynthesis are probably associated more with declining summer chlorophyll levels than with changes in photosynthetic capacity or its response to temperature. The importance of subepilimnetic photosynthesis, interstation differences in transparency, and seasonal changes in photosynthetic capacity as controlling factors for areal phytoplankton photosynthesis is presented. The implication of lower lake productivity for fisheries and lake management is also discussed.