Decadal, Interannual, and Seasonal Variability in Enrichment Bioassays at Lake Tahoe, California–Nevada, USA

The response of Lake Tahoe water to macronutrient supplementation has been assayed with inorganic 14 C uptake since the 1960s. On the decadal scale, a change in bioassay response to macronutrient enrichment took place around 1980, with a decrease in the frequency of N stimulation and an increase in the frequency of P stimulation. On the annual scale, an effect of spring mixing depth on the size of the bioassay response could be observed: stronger mixing events resulted in a more positive response. On the seasonal scale, the response magnitude was affected by stratification, but the significance of the response was independent of season. In a subset of the bioassays, in vivo fluorescence and chlorophyll a were measured in addition to inorganic 14 C uptake; bioassay results were essentially the same, regardless of which of these three bioassay response indicators was used.