Response of calcareous Nagawicka Lake, Wisconsin, to changes in phosphorus loading

Nagawicka Lake is a 400-ha, phosphorus (P)-limited, calcareous lake (hardness of 300 mg/L CaCO3) in Wisconsin. Because of concern over potential degradation in water quality associated with urban development in the watershed, a study was conducted to determine the effects of past and future changes in P loading on the lake's water quality through the use of empirical eutrophication models. Six existing empirical P models consistently overestimated total P (TP) concentrations in the lake by a factor of about 2 over a range in external P loading because the models do not account for the unique properties of calcareous lakes: co-precipitation of P with calcite and negligible release of P from the deep sediments. Confirmation of the calcite mechanism was proven by analysis of sediment cores. Once the results were adjusted for the consistent biases, other conventional empirical models fairly accurately predicted the measured chlorophyll a concentrations (CHL) and Secchi depths in the lake. The models, adjusted for the consistent overestimation of TP, were then used to predict the effects of increases and decreases in P loading. Total P and CHL were predicted to decrease or increase by a % similar to the % change in P loading to the lake; however, these relations may become very nonlinear with increases in P loading > 100%. Because the natural buffering capacity resists eutrophication caused by P loading, roughly twice the P loading can be permitted in oligotrophic calcareous lakes than in noncalcareous lakes before eutrophication thresholds are exceeded.