Seasonal variation of heterotrophic potential in Lake Rotoroa

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Abstract
A 20‐fold seasonal variation in heterotrophic potential (Vmax) for glucose uptake was observed in Lake Rotoroa during a 3‐year sampling programme. Peaks in activity coincided approximately with maximum surface water temperature, but the large variations suggest quantitative and/or qualitative changes in the heterotrophic microbial population. Vmax, values in subsurface samples taken from within dense beds of Elodea canadensis were on average nine times greater than those in surface samples from above the beds. Values for Vmax, ATP, and chlorophyll a (Chi a) decreased and nitrogen and phosphorus concentrations increased slightly after a drastic decline in the extent of Elodea beds occurred in 1974. The seasonal ranges of the above parameters were similar to published values for oligotrophic lakes: Vmax, less than 0.001–0.02 μg.L‐1 .h‐1; ATP, 0.01–0.09 μg.L‐1; Chi a, 0.3–1.4 μg.L‐1; total‐N, 0.03–0.22 mg.L‐1; total‐P, 0.001–0.013 mg.L‐1.