Water clarity responses to phosphorus and daphnia in Lake Mendota
Factors that affect algal densities and hence water clarity in eutrophic Lake Mendota (3,985 ha, 25.3 m maximum depth) were identified from analyses of long-term data. In Chapter 1, 71 years of Secchi disk records (1900-93) were analyzed using ANOV A to determine the effects of nutrient and herbivory levels on water clarity for 6 seasonal periods. Results indicated that herbivory increases water clarity in all seasons including the summer months when blue-green algal blooms normally occur. Nutrients reduce water clarity only in the summer and destratification periods. In Chapter 2, a 21-year phosphorus (P) loading dataset (1975-96) was analyzed using a mass balance model expressed as difference equation with an annual time step incremented from mid-April. From that analysis, the steady-state distribution of April P concentrations was predicted. New distributions of April P were also predicted from scenarios of altered P inputs due to changes in management practices. Those results were then linked to the probability of summer blue-green algal blooms in the lake. For no P load reduction, the probability of a bloom ( \textgreater 2 mg algae L-1 ) on any summer day is about 60\%. The probability decreases to 20\% with a load reduction of 50\%. This approach illustrates how the frequency of extreme events like blooms can be considered. In Chapter 3, a 20-year dataset (1976-95) of external P loadings, in-lake P concentrations, Daphnia biomasses and lake mixing indices was analyzed using stepwise linear regression to determine which factors affect summer water clarity. June water clarity was mostly affected by food web dynamics. July-August water clarity was negatively related to spring P concentrations, and positively related to July-August Daphnia biomasses and water column stabilities. Results from the three chapters indicate that P levels, food web dynamics and climate have important effects on water clarity in Lake Mendota. While all three factors have implications for lake management, future improvements in summer water clarity will be mainly benefited by P control from a nonpoint pollution abatement program currently being implemented. This is because the other factors are difficult to manipulate and because P reductions below the recent drought-induced minimum levels should occur if pollution abatement is significant.