Funds are provided to examine the impact of sea ice on the distribution and abundance of zooplankton, and how they are partitioned among top predators. To this end, new and historical data will be used to test a series of hypotheses and answer questions relating to bottom-up and top-down control of large crustacean zooplankton and their impact on the flow of carbon and energy in the ecosystem. From the examination of these hypotheses, new mechanisms will be derived and old ones re-evaluated. Existing numerical models will be used to assess the relative importance of these mechanisms. Existing conceptual models will be tested, and new conceptual models of carbon and energy flow will be developed. Such a study of the eastern Bering Sea shelf ecosystem is timely. Some of the most direct effects of changing climate will be on sea-ice - its extent, duration, timing of retreat, and inter-annual variability. Sea ice in turn controls ocean temperature/salinity, timing of the spring phytoplankton bloom and thus the extent of export/remineralization of fixed carbon. Crustacean zooplankton can be a choke-point in the flow of carbon and energy through the food chain.

The Bering Sea ecosystem provides roughly half the commercial seafood landings in the United States, as well as significant resources for subsistence fishermen. Major fluctuations in these stocks have occurred in the past and been associated with climate variations. Improved management of these important fisheries should be enabled by the understanding derived from this project.

Project Report

We contributed to the "Collaborative Project: Impact of sea-ice on bottom-up and top-down controls on crustacean zooplankton and the mediation of carbon flow in the eastern Bering Sea" by synthesizing data on microzooplankton, providing insights into the role of microzooplankton in this ecosystem, and providing data to ecosystem modelers. The microzooplankton assemblage includes microscopic, usually one-celled, zooplankton that are dominant grazers on phytoplankton and are prey for larger crustacean zooplankton, which in turn, are prey for many forage fish as well as some sea birds and marine mammals. Thus they are an important link in food webs that support fisheries and wildlife in the Bering Sea. Microzooplankton were an important component of the plankton in the eastern Bering Sea; they consume much of the phytoplankton production in spring and summer. The microzooplankton assemblage was particularly important in summer when it contributed almost as much biomass as phytoplankton in the upper layers of the Bering Sea, particularly on the shelf. Thus, microzooplankters were a large part of the prey available to larger crustacean zooplankton. In summer, planktonic ciliates were not only an important part of the microzooplankton biomass but also contributed to the chlorophyll content of the water (Chlorophyll is a proxy for phytoplankton biomass; phytoplankton are important because they are at the base of the food web and are responsible for primary production). Many of the ciliates were mixotrophs that probably contributed to primary production as well as grazing in surface waters, particularly when the water column on the shelf was thermally stratified. Comparison of Bering Sea data to that from other Arctic and sub-Arctic Seas shows that microzooplankton are generally an important part of the Arctic and sub-Arctic plankton communities. Furthermore, mixotrophic ciliates are generally important components of the microzooplankton during stratification. These data suggest that the microzooplankton trophic link in food webs that supports larger zooplankton may be particularly efficient when mixotrophic ciliates are an important part of the plankton. Review of grazing data revealed that during both spring and summer, microzooplankton grazing experiments sometimes yielded negative estimates of grazing, particularly during blooms. This indicates that grazing rates may be under-estimated. Comparison of data from the Bering Sea to data from studies conducted in high latitude waters in the Atlantic indicated this may be a general phenomenon. It is possible that during blooms that microzooplankton grazing is underestimated. Depending on the magnitude of the under-estimate, this could influence modeling of carbon pump and of food web dynamics in high latitude seas. Besides directly contributing to a Bering Sea Synthesis project, we have also contributed to a broader international effort to understand the role of microzooplankton in high latitude seas, to a growing recognition of the role of mixotrophs in carbon cycling and production in surface waters, and to recognition of the effects of phytoplankton blooms on our ability to measure grazing. Material from the project has been used in a graduate course (Zooplankton Ecology) in the MEES program at University of Maryland and in open house events at UMCES, Horn Point Laboratory. The PI has contributed to international workshops and projects focused on high latitude phytoplankton and climate change as well as specifically to the Bering Sea Project.

National Science Foundation (NSF)
Division of Polar Programs (PLR)
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William J. Wiseman, Jr.
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University of Maryland Center for Environmental Sciences
United States
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