WATER CLARITY IN LAKE MENDOTA SINCE 1900 - RESPONSES TO DIFFERING LEVELS OF NUTRIENTS AND HERBIVORY

Water transparency, phosphorus concentrations, and herbivory in eutrop hic Lake Mendota have been highly variable this century. To assess the effects of both nutrients and herbivory on water clarity, mean Secchi disk readings were computed for six seasonal periods for each year du ring 1900-1993 that had readings (71 years but not in all periods). We assigned each year one of three levels of nutrients and one of two le vels of herbivory on the basis of data independent of Secchi readings to avoid circularity in our analyses. For each seasonal period, we tes ted for differences in Secchi readings attributable to nutrients, herb ivory, and their interaction. Our results indicate that in years with high herbivory, water clarity was significantly greater during all per iods including summer when blue-green algal blooms frequently occurred . Higher nutrient levels only led to decreased water clarity during th e summer and destratification periods; water clarity was not affected by nutrient levels during other periods. Our analysis of Lake Mendota' s long-term Secchi disk record indicates that herbivory can strongly i nfluence water clarity in large eutrophic lakes, and that the greatest improvements in summer water clarity in those lakes may be achieved b y combining phosphorus reduction programs with biomanipulation techniq ues to increase herbivory.