Funds are provided for an analysis of the impact of changing sea ice conditions on planktonic food web structure, focusing on microzooplankton and mesozooplankton trophic linkages and the fate of phytoplankton blooms in the Bering Sea during spring sea ice conditions. To this end, the principal investigators plan to conduct experiments over a range of ice, ice edge, and open water conditions, and, particularly, will consider the importance of detached ice algae, when present, as a food source for the secondary consumers. As part of this effort, they will determine standing stock biomass, composition, and size structures of phytoplankton, microzooplankton and mesozooplankton assemblages, measure microzooplankton growth and mesozooplankton reproduction rates, and measure grazing rates and prey preferences of heterotrophic protists and dominant species of mesozooplankton. They will also determine the fine scale vertical distribution of plankton, especially of fragile forms such as colonial phytoplankton, to identify thin layers of plankton/ particles in association with hydrographic features. Their collaborative study will explicitly address trophic linkages previously unexplored in this region of the subarctic. They hypothesize that changing ecosystem structure due to global warming, e.g. decrease in seasonal sea ice, will alter these trophic interactions, and thus the ultimate fates of algal production in this region. They plan to participate in each of the three annual cruises proposed for spring sea-ice conditions during the BEST field program. They will carry out a full set of analyses (standing stock determinations and rate measurements) at designated stations along transects within the BEST study area. Abundances and rate measures will be combined to determine relative microzooplankton and mesozooplankton grazing impacts.
While the direct measurements and observations to be derived from this program will allow the principal investigators to describe the microzooplankton and mesozooplankton of the eastern Bering Sea shelf and their grazing impacts with a detail heretofore unavailable, the numbers they derive will also provide strong constraints on the ecosystem models to be developed as part of BEST. These models will serve as an important tool during the synthesis of the collective understanding derived by the BEST investigators.
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 food resource for the fish and seabirds in the Bering Sea. Our results also suggest that there is plenty of plant food available for the animals that live on the seafloor such as the king crabs. During the period of the project, we provided opportunities for training and professional development for a K-12 teacher who participated in our cruise in 2009 and a graduate student and four undergraduate students from URI who assisted in sample analysis (4/5 of the students were female). We also provided opportunities for G. Rosenwaks (2008) and C. Linder and H. Fields (2009, funded through NSF grant) to join us on the cruise and document the Bering Sea Project science through photography and written materials. Linder and Fields also disseminated information live from the cruise via teleconferences. Significant local community communication also was conducted before (meetings, pre-cruise presentations), during (daily communication via email regarding cruise plans and findings, visit to Gambell, AK on St. Lawrence Island), and after (dissemination of cruise plans, post-cruise presentations).
