Water clarity in Lake Mendota since 1900: Responses to differing levels of nutrients and herbivory

Water transparency, phosphorus concentrations, and herbivory in eutrophic 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 during 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 levels of herbivory on the basis of data independent of Secchi readings to avoid circularity in our analyses. For each seasonal period, we tested for differences in Secchi readings attributable to nutrients, herbivory, and their interaction. Our results indicate that in years with high herbivory, water clarity was significantly greater during all periods including summer when blue-green algal blooms frequently occurred. Higher nutrient levels only led to decreased water clarity during the 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 influence water clarity in large eutrophic lakes, and that the greatest improvements in summer water clarity in those lakes may be achieved by combining phosphorus reduction programs with biomanipulation techniques to increase herbivory.