Evidence for a Trophic Cascade Effect on North-shore Western Lake Erie Phytoplankton Prior to the Zebra Mussel Invasion

Abstract

Increasing summer total phosphorus (TP) concentrations measured in samples from a municipal water intake off the north shore of western Lake Erie during 1976 to 1983 were inconsistent with TP loads to the western basin of Lake Erie and with phytoplankton densities in the intake samples, both of which declined over the same time interval. The long-term (1976 to 1988) summer TP data were inversely correlated (r = −0.858) with summer average maximum daily wind velocities, suggesting that low wind velocities contributed to anoxia at the sediment-water interface and high sediment TP release rates in summer. While TP loading reductions in the late 1960s and early 1970s likely contributed to phytoplankton declines, continued phytoplankton declines during the late 1970s to early 1980s could not have been caused by continued reductions in TP loadings while TP concentrations increased. The phytoplankton declines of the 1980s are more likely attributable to changes in the trophic cascade associated with dramatic declines in some species of zooplanktivorous fish during the 1970s and 1980s as a result of a restored walleye population. Long term phytoplankton densities were fit (R 2 = 0.902) to a multiple regression model with western Lake Erie TP loads and an index of zooplanktivore density as independent variables; the zooplanktivore component of this model was the most significant contributor to the prediction of phytoplankton density. The implications of these findings for maintenance of good lake water quality include the need to maintain strong piscivore populations as well as reduced phosphorus loads.