NUTRIENT ELEMENT LIMITATION OF ZOOPLANKTON PRODUCTION

Freshwater crustacean zooplankton species show pronounced interspecific variability but approach intraspecific homeostasis in their stoichiometry. It is especially interesting that members of the dominating herbivores in freshwater systems, such as Daphnia species, show consistently high P:C ratios. Sestonic P:C ratios in epilimnetic waters are usually far below that encountered in the zooplankton, which suggests that grazers may approach a direct phosphorus limitation in many localities. This view was supported by a regional study of 47 lakes, in which most independent variables including phytoplankton biomass and chlorophyll were scarcely correlated with zooplankton biomass but particulate P (>0.45 mum) could explain a major part of the variance of zooplankton biomass. Of the various taxa, daphnids gave the best correlation with particulate P, cyclopids and rotifers the worst. These observations suggest that phytoplankton biomass per se may not be the major determinant of zooplankton biomass but that the nutritional status of phytoplankton (P cell quotas) rather than biomass in terms of carbon is most important, or that zooplankton suffer a direct P limitation in many lakes. The latter view is supported by model predictions given in this article. Also, both freshwater and marine copepod grazers could correspondingly be growth limited by an N:C ratio of ingested food that is too low. The possibility of direct nutrient element limitation has wide implications for the regulation of herbivore production, competition, and overall cycling of nutrients in epilimnetic waters.