Phytoplankton nutrient limitation in a polymictic eutrophic lake: community versus species-specific responses

Phytoplankton nutrient limitation was examined in polymictic, eutrophic Lake Rotorua, New Zealand using three in situ incubation experiments of 4 to 6 days' duration in summer 2004. Two of the incubations were conducted during stratification, and one immediately after breakdown of stratification. Samples were enriched with ammonium (1 mg NH4-N L-1), phosphate (0.1 mg PO4-P L-1) or with both nutrients. A control containing no added nutrients was used for comparison. Phytoplankton responses to nutrient additions were determined at a species level from cell counts and at a community level from changes in chlorophyll-a concentration. Phytopiankton biomass generally responded to N plus P additions to a greater extent than with single nutrient additions, though results were often not statistically significant. Increase in community biomass was greater for P than N, and nutrient demand decreased after breakdown of stratification. Individual species responded differently to N and P additions, suggesting co-limitation within the phytoplankton species assemblage. A simple, dynamic phytoplankton growth limitation model, with time-varying responses to hourly inputs of temperature, nutrients and light, was applied to consider the interacting effects of phosphorus (P), nitrogen (N) and light. Model results indicate that light plays a major role in regulating phytoplankton biomass, although there were periods of P limitation. For the management of water quality in Lake Rotorua it is suggested that simultaneous reductions in external inputs of both N and P is mostly likely to reduce phytoplankton biomass, including cyanobacterial populations.