Role of nutrients and zooplankton grazing on phytoplankton growth in a temperate reservoir in New South Wales, Australia

The role of nitrogen (N) and phosphorus (P) and zooplankton grazing on the growth of a phytoplankton community was investigated at different times in the Ben Chifley reservoir. In situ nutrient enrichment bioassays (n=12) indicated that phytoplankton growth was limited by P in 33% of experiments, by both N and P in 25% of experiments and no limitation was found in 42%. The hypothesis that N or P limitation occurred when ambient N: P ratios were different from the Redfield ratio was supported in 33% of bioassay experiments, suggesting that ambient N: P ratios do not always correctly indicate if N or P is limiting. Grazing rates of the reservoir zooplankton (>150 mum in size) ranged from 0.023-0.199 day(-1) (mean: 0.078 day(-1), n=8). The grazing efficiency, as measured by a weight-specific clearance rate, ranged from 0.049-0.743 mL mug dry wt(-1) day(-1), and was positively correlated with the relative biomass of Daphnia in the community. The nutrient-stimulated growth of phytoplankton ranged from 0.085-1.031 day(-1) (mean: 0.461 day(-1), n=10). The effect of nutrient enrichment exceeded that of zooplankton grazing in 62% of experiments. Further study is necessary to understand a qualitative effect of nutrients and zooplankton grazing on the phytoplankton community structure in the Ben Chifley reservoir.