Funds are provided to use the diverse and comprehensive dataset collected during the BEST/BSIERP field program of 2007-2010 to carefully structure and parameterize a new, system- and life-stage specific biophysical model. The PIs? approach differs from traditional NPZ type ecosystem modeling because of the inclusion of life-stage specific zooplankton components and will permit them to better understand the impact of the changing environment on plankton phenologies and on carbon cycling. They will focus on spring conditions, when sea ice is present or retreating, to understand the linkages and functional relationships between different components of the plankton ecosystem and to understand the interaction of zooplankton life histories with the timing and extent of sea ice and stratification of the water column. Syntheses of biological rate processes measured at sparsely distributed process stations with standing stocks and physical and chemical data from the broadly distributed survey stations will identify which components and processes are most important in controlling the structure and function of the system and will independently derive a data-based mechanistic synthesis against which the model's internal pathways can be tested. They will use model-based scenario testing to examine how projected future change in environmental conditions, especially sea ice, may impact planktonic ecosystem structure and function, and to identify the key trophic and physiological mechanisms to which this ecosystem response is most sensitive.

The timing and extent of sea ice cover in the eastern Bering Sea influence the timing of plankton production, the composition and abundance of the zooplankton, and ultimately the availability of food for upper trophic levels including commercially important fish species such as pollock. The PIs propose to gain a greater understanding of the potential impact of ongoing climate change on this planktonic ecosystem. Specifically, they ask: How will climate change, and the anticipated earlier retreat in seasonal sea ice, warmer water temperatures, and reduced sea ice extent alter the structure and function of the planktonic food web during spring, and thus the ultimate fate of algal production in the Bering Sea, the utilization and cycling of carbon, and the production and availability of zooplankton as prey for upper trophic levels, including commercially important fish species?

Project Report

The Bering Sea ecosystem supports one of the richest fisheries in the world’s oceans and provides almost half of the total U.S fish catch annually. The success of this highly productive fishery can be largely attributed to the massive blooms of tiny single celled plants that occur each spring when increasing light and abundant nutrients enable these plants to grow and flourish both in the ice (ice algae) and in the water column (phytoplankton) as the sea ice retreats. These blooms in turn support a diverse zooplankton community awaking from a period of relative inactivity during the long, dark, cold winter. This community is made up of unicellular microzooplankton, often no larger than the plant cells they consume, and larger, mesozooplankton dominated by copepods and krill. The zooplankton community in response to the highly productive spring bloom dramatically increases its numbers and biomass at this time providing an abundant, highly nutritious food source for seabirds, mammals, and fish, including commercially valuable species. This project involved the integration and synthesis of data collected on research cruises during an earlier phase of the BEST-BSIERP program, and the development of a new ecosystem model for the Bering Sea. This was done to try to better understand the importance of the spring bloom to the Bering Sea ecosystem and to predict how these blooms might be altered for better or worse in a future warmer Bering Sea, with the associated implications for the success of the important fisheries. Research cruises were undertaken in the first phase of the program to describe and quantify the food web dynamics of the planktonic ecosystem during spring sea ice conditions. Samples were collected over a large region of the shelf using ice cores, water samplers, and various net systems to identify, enumerate, and quantify the biomass of the various planktonic ecosystem components. Shipboard experiments were conducted to measure important biological rate processes such as primary productivity and zooplankton feeding, growth, and reproductive rates. During this project, data sets collected during the research cruises were integrated and synthesized to determine regional patterns and year to year variability in the biomass, productivity, and consumption rates of different planktonic components, and to develop mathematical equations describing these processes for use in the model. A new planktonic ecosystem model was constructed and validated against the BEST field observations. Both hindcasts (1971–2012) and future projections (to 2040’s) were conducted with the model. We found the spring ice associated bloom to be of vital importance to the productivity of the Bering Sea. It not only begins the growing season in the plankton but it supplies a large and dependable food source to which the life cycles of many of the important zooplankton species are timed. The spring bloom provides an almost inexhaustible food supply, enough so that the copepods are able to increase their biomass by up to 10-fold between early spring and summer. Even so, the zooplankton community leaves much of the spring bloom production un-grazed, but this excess productivity by no means goes to waste. It is available for export to the benthic community – the animals that live on the bottom- such as the king crab. What happens if a future warmer ocean upsets this balance? Currently, we believe that one reason that the spring bloom is so productive is that it gets a jump on the zooplankton grazers, which are just waking up from the long winter and are not very abundant. A warmer ocean could very well result in a tighter coupling between the planktonic producers and consumers with detrimental consequences for the benthic community that depends on the spring bloom as well. We used the ecosystem model to better understand the current dynamics and to ask important questions about how the system will respond to climate change. Results from the model suggest that climate driven changes in the magnitude of the spring bloom are not large, but changes to bloom timing may have a substantial impact on zooplankton and higher trophic levels as their life cycles are timed to the spring bloom. Future projections (2040s) suggest that in the north ice cover may be reduced but physical and plankton dynamics will fall within the range of that observed today. The southern Bering Sea, however, will have much less ice (similar to present day warm years), surface water temperatures outside of present variability, and shifts in the timing of physical (e.g., onset of stratification) and biological (e.g., spring bloom) components that may have deleterious effects on the abundance of large zooplankton. Our synthesis and modeling activities have increased our understanding of ecosystem structure and function in the Bering Sea. Results of modeling efforts such as these can be used in predictive models focusing on ecosystem status and health and in management decisions on commercially important fish and crab species.

National Science Foundation (NSF)
Division of Polar Programs (PLR)
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William J. Wiseman, Jr.
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University of Washington
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