Temperature Dependence of Freshwater Phytoplankton Growth Rates and Zooplankton Grazing Rates

Phytoplankton growth rates and zooplankton grazing rates were estimated on 16 occasions over a period of 17 months in University Lake, a highly eutrophic lake on the campus of Louisiana State University. Phytoplankton growth rates and chlorophyll a concentrations averaged 1.0 +/- 0.2 d(-1) and 240 +/- 120 mg m(-3), respectively. Chlorophyll a concentrations were at or above the inflection point of the Holling type I curve that described the relationship between zooplankton grazing rates and chlorophyll a concentrations. In most cases, it was necessary to dilute lake water by more than a factor of 4 before zooplankton grazing rates became sensitive to chlorophyll a concentrations. Chlorophyll a concentrations were positively correlated with temperature and were roughly fourfold higher at 30 degrees C than at 15 degrees C. An analysis of the temperature dependence of the growth rates and grazing rates in this study and 87 other paired estimates of limnetic phytoplankton growth rates and zooplankton grazing rates revealed virtually identical temperature dependences of growth rates and grazing rates that were very similar to the temperature dependence predicted by the metabolic theory of ecology. Phytoplankton growth rates exceeded zooplankton grazing rates by 0.13 +/- 0.05 d(-1) at all temperatures over a temperature range of 8.5-31.5 degrees C. The Q(10) for both phytoplankton growth rates and zooplankton grazing rates was 1.5 over that temperature range.

Automated summary

Phytoplankton growth rates and zooplankton grazing rates were estimated in University Lake, showing a positive correlation between chlorophyll a concentrations and temperature. Dilution of lake water by more than a factor of 4 was necessary for zooplankton grazing rates to become sensitive to chlorophyll a concentrations. The temperature dependence of growth rates and grazing rates revealed similarities to the metabolic theory of ecology.