Zooplankton are small or microscopic animals that are found in the water of all ecosystems and that are important links in the pelagic food chain between the primary producers (phytoplankton and, in polar regions, algae that grow on the underside of sea ice) and the larger fish, seabirds, and whales.  In the Bering Sea, large copepods and krill (euphausiids) are important prey for a range of predators including the commercially important pollock.  Smaller microzooplankton also eat primary producers as well as being potential prey for the copepods and krill.  If zooplankton do not eat all of the primary production from the phytoplankton and ice algae, this carbon will fall uneaten to the seafloor where it will feed the benthic animals found there including important invertebrates such as king and snow crabs. 

This project studied the types (phytoplankton, ice algae, microzooplankton) and amounts of prey eaten by the dominant copepods and krill in the Bering Sea, quantified the proportion of the primary production and phytoplankton standing stock that is eaten by the copepods, krill, and microzooplankton, and studied the ecology and biology of dominant copepods and krill to understand how environmental variability is associated with changes in reproduction, size, metabolic activity, and condition of those animals. Together, this information contributes to our understanding of how interannual and climatically-induced environmental variability (sea ice, water temperature, prey availability) might affect the success and biomass of the zooplankton. This in turn can have important consequences for the animals that feed on them, including the commercially important pollock.  

We worked on three 6-week cruises to the Bering Sea during spring 2008-2010.  On the cruises, we collected samples of the zooplankton communities using plankton nets.  We also did grazing experiments with the dominant copepods and krill at selected locations to measure the grazing rates of these animals, the prey preferences (phytoplankton or ice algae or microzooplankton) of the animals, and, by combining individual grazing rates (amount of food eaten per animal) with the total numbers of animals, the relative quantity of the total amount of available food that was eaten by the zooplankton community. We also measured the rate at which eggs were produced by a dominant copepod species that was reproductively active to determine if there are relationships between available food for the copepods and their egg production rate.  Finally we measured the carbon content of the copepods and krill to determine if they are actively storing fat and if they are a high quality prey for fish, seabirds, and the like. 

It turns out that zooplankton follow a range of dietary habits and most are omnivores.  Some eat phytoplankton and microzooplankton in about the same proportion as they find them in the sea.  Others seem to prefer microzooplankton, eating proportionally far more of them then are found in the prey field, and thus are much more carnivorous.  Many also eat the ice algae that grows on the underside of the sea ice.  In the spring, the larger zooplankton cannot eat all of the primary production or microzooplankton, leaving plenty of phytoplankton and ice algae primary production available for the animals on the seafloor and exerting no control on the population size of the microzooplankton.  Reproduction was closely tied to the availability of food in the dominant copepod except during late spring when the females apparently had built up enough lipid (fat) that they could produce eggs in part using this as an energy source. Our results indicate that zooplankton, at least in years with environmental characteristics similar to those of 2008-2010, are not food limited, and thus should flourish and provide a plentiful ...

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