Summer water clarity responses to phosphorus, Daphnia grazing, and internal mixing in Lake Mendota

Linear models were developed for predicting mean Secchi disk depth readings as a measure of water clarity for the summer months in Lake Mendota, Wisco nsin. The 20-yr (1976-1995) data set also included external phosphorus (P) loadings and in-lake April P concentrations as indices of lake nutrient sta tus, and monthly (28 d) water column stabilities and Daphnia biomasses as i ndices of lake mixing and algal grazing potentials, respectively. June Secc hi depths were mostly controlled by food web dynamics, which dictated wheth er the larger-bodied Daphnia pulicaria or the smaller-bodied D. galeata men dotae dominated during the spring clear-water phase. May Daphnia biomasses were significantly greater in D. pulicaria-dominated years than in D. galea ta-dominated years; D. pulicaria-year biomasses were also greater during th e summer months. The model for the midsummer (July-August) months indicated that Secchi depths were inversely related to April P concentrations and po sitively related to midsummer Daphnia biomasses and lake stabilities. Scena rios for midsummer Secchi depths were tested using the observed minimum and maximum values for each predictor variate. While holding two variates cons tant, April P, Daphnia biomass, and lake stability each resulted in relativ ely similar Secchi ranges (0.81, 0.81, and 1.17 m, respectively). Our resul ts suggest that summer water clarity in eutrophic Lake Mendota is controlle d by interacting ecosystem processes linked to land use activities, lake fo od web dynamics, and climate.