A case history: Effects of mixing regime on nutrient dynamics and community structure in Third Sister Lake, Michigan during late winter and early spring 2003

We studied the winter to spring transition in Third Sister Lake (TSL), a small glacial lake in southeastern Michigan, to determine the effect of mixing regime on nutrient dynamics and community structure in an urban temperate lake. After ice-off, the oxycline was mixed downward from 3 to 6 m depth, resulting in addition of 5 mg m(-2) P-SRP, 857 mg m(-2) N-NH4+, and 400 mg m(-2) N-NO3- to the epilimnion, but trapping 299 mg m(-2) P-SRP, 7877 mg m-(2) N-NH4+ and 36 mg m-2 N-NO3 in the bottom waters. Nutrients supplied by snow melt runoff (138 mg m(-2) P-SRP, 430 mg m(-2) N-NH4+, 596 mg m(-2) N-NO3-) were an order of magnitude greater than rain event loads (0.13 mg m(-2) P-SRP, 0.17 mg m(-2) N-NH4+, and 1.05 mg m(-2) N-NO3-) during the transition time from ice cover to open water. Reduced spring mixing did not have a large impact on N:P molar ratios, because external N:P ratios were low (7.5) compensating for reduced supply of P from the bottom waters. Bacterial production was greater in the hypolimnion than in the epilimnion, and mesocosm experiments showed that bacteria were P limited in the epilimnion but not in the hypolimnion. Total algal and zooplankton densities increased after ice-out, while Daphnia and Bosmina densities decreased. Increases in zooplankton grazing rates after ice-off were most dramatic in small-bodied zooplankton. Sediment core analysis showed that Asterionella relative abundance continues to increase, suggesting that the lake has become more brackish and oligotrophic. Our findings suggest that TSL has undergone a transition from dimictic to meromictic conditions, and that continued salt inputs have altered the structure and function of this ecosystem.