A stoichiometric analysis of the zooplankton-phytoplankton interaction in marine and freshwater ecosystems

In the 35 years since A. C. Redfield's classic paper¹, the use of elemental ratios has become widespread in marine and freshwater phytoplankton studies^2,3. But nutrient ratios have only recently been studied elsewhere in pelagic ecosystems, such as the producer-consumer interface ^4,5. Here we report the results of the first study, to our knowledge, of N:P ratios in pelagic producers and consumers (phytoplankton and zooplankton) in lacustrine and marine habitats. The N:P ratio of phytoplankton was higher in lakes than in marine sites; however, N:P ratios were higher in marine zooplankton than in freshwater zooplankton. The elemental imbalance of the phytoplankton-zooplankton interaction (N:P_food-N:P _consumers) in lakes was positive and exceeded the negative imbal-ance in marine sites; thus P-deficient food may limit zooplankton growth in lakes but not in oceans. Stoichiometric calculations⁶ indicated that consumer-driven nutrient recycling ratios in lakes may be ^4-6 times higher than in marine systems. Consistent with this difference, phytoplankton P-limitation was more prevalent in lakes than in marine sites. Thus, the ecological stoichiometry of the zooplankton-phytoplankton interaction differs qualitatively in freshwater and marine ecosystems.