Biogeochemistry of a Southern Ocean plankton ecosystem: Using natural variability in community composition to study the role of metazooplankton in carbon and nitrogen cycles

The pelagic ecosystem around the island of South Georgia is subject to significant interannual variability, and changes in zooplankton community composition can be used as natural ecosystem experiments to examine biogeochemical cycles. The biomass of the large euphausiid Antarctic krill may range from ca. 2 to 150 g fresh mass (FM) m−2. When krill biomass is low, copepod biomass may be correspondingly higher and overall zooplankton biomass remains more or less unchanged. Krill are omnivorous, feeding facultatively either as grazers on microplankton or as predators on smaller zooplankton. This leads to complex feedbacks within the plankton. A simple model of the phytoplankton–copepod–krill system is used to simulate two scenarios of zooplankton composition. For the “low krill-high copepod” scenario, the model predicts higher phytoplankton biomass and production, lower mixed layer (ML) ammonium, nitrate and silicate concentrations, and higher detrital carbon production than in the “high krill-low copepod” scenario. Nitrogen cycling provides the most explicit demonstration of the differences between the scenarios. For the “low krill-high copepod” scenario, ML ammonium concentration decreased by 25% over 20 days, but excretion by metazooplankton supplied 30% of phytoplankton nitrogen demand. In the “high krill-low copepod” scenario, ML ammonium only declined by 10% over 20 days, but metazooplankton excretion was much lower, at 10% of phytoplankton N demand. These predictions are compared with data from several surveys covering krill biomass in the range 10–55 g FM m−2. Phytoplankton chlorophyll biomass is negatively related to krill biomass, and ML nutrients are positively correlated with krill biomass in these data. Both observations and model results suggest that variation in biogeochemical carbon and nitrogen cycles in the South Georgia pelagic ecosystem is determined largely by changes in zooplankton community composition and its impact on phytoplankton dynamics