Internal phosphorus loading in Lake Mendota: response to external loads and weather

Abstract

We quantified summer internal phosphorus (P) loads and all the major P fluxes in Lake Mendota, Wisconsin, a deep stratified eutrophic lake, during two summers of contrasting external loads to determine ( i) whether internal loading by entrainment (mixing at the thermocline) is a significant part of the epilimnetic P budget and ( ii) what factors lead to interannual variability in internal loading. We estimated variables for the P budget weekly (standing stock, sedimentation) or daily (outflow, inflow, and entrainment) during a summer of average runoff (1992) and one of higher than average runoff (1993). Entrainment, estimated by calculating the amount of P transported into the epilimnion after the thermocline deepened following storms, was about 10 times higher than external loading during 1992, but was about equal to external loading during 1993. When entrainment was included, the epilimnetic P budget balanced. Interannual variability in internal loading appears to be due to a combination of water column stability, weather, and the P levels accumulated in the metalimnion. External loads to the epilimnion during summer 1993 were much higher than in 1992 (7.0 and 0.9 mg P·m -2 ·day -1 , respectively); however, total loads (internal + external) were more similar (12.5 and 7.8 mg P·m -2 ·day -1 in 1993 and 1992, respectively). Although summer chlorophyll concentrations were similar in the two summers, blue-green algal biomass in 1993 was about double that in 1992.