Bermuda Biological Station for Research, Inc.
Funds are provided to test the following core hypothesis: climate-driven interannual variability in sea-ice extent and duration shifts the eastern Bering Sea autotrophic community between one of two states; marginal ice-zone (MIZ) blooms vs. open-water blooms. The MIZ bloom state is characterized by high biomass, diatom-dominated blooms, high pelagic export and tight pelagic-benthic coupling, whereas the open-water bloom state is characterized by lower biomass, flagellate blooms, low pelagic export, and reduced pelagic-benthic coupling. This project will generate measurements of primary production using traditional 14C, 13C methods, and use the innovative triple oxygen isotope technique and dissolved oxygen concentrations to estimate gross and net primary production, respectively. This combination of productivity measurements will be used to test the hypothesis that while gross primary production does not change with sea-ice extent, net production does, and is inversely related to sea-ice extent.
Phytoplankton community structure measurements will allow the PIs to test their hypothesis that the autotrophic community switches from a diatom-dominated, high export system in the MIZ, to a flagellate-dominated, lower export, system in open water blooms.
This project is part of a larger program designed to develop understanding of the integrated ecosystem of the eastern Bering Sea shelf, a highly productive region of US coastal waters. This ecosystem is home to a major portion of the commercial fisheries of the US and also provides significant resources to subsistence hunters and fisherman of Alaska. Characterization of rates of primary production by phytoplankton and the varying structure of the phytoplankton community in response to changing sea ice conditions will provide information about changes at the base of the food chain that will constrain models of the ecosystem. This information will be essential to a successful integrated ecosystem modeling protocol for the region.
The Bering Sea and other sub-Arctic and Arctic seas are predicted to be among those regions most severely affected by global warming, as relatively small changes in the heat content of the water can have a disproportionately large effect on the presence of seasonal sea ice. Over the last decade, a wealth of data has become available for the eastern Bering Sea from continued monitoring and multiple large-scale, comprehensive research programs. From these efforts, it is becoming increasingly evident that the presence or absence of sea-ice in spring is perhaps the single most important component determining the physical and biological structure of this critically important ecosystem, not only in spring, but for the remainder of the warm season. The Bering Sea Ecosystem Study (BEST) project, was designed to not only build upon previous research, but also to expand the research effort up to and including the human dimension. The BEST project involved the intensive study of most of the Bering Sea shelf in spring and summer from 2008 to 2010. The Bering Sea shelf ecosystem is one of the most productive, in terms of fisheries yield, in the world. These high levels of fisheries production are supported by even higher levels of productivity at lower trophic levels, namely the zooplankton (animal plankton) that are fed upon by fish, and the phytoplankton (plant plankton) that are fed upon by the zooplankton. Our specific component of the BEST project focused on how the presence or absence of spring sea ice impacted both the total productivity by phytoplankton as well as the phytoplankton assemblage composition. In addition, our project focused on how changes in phytoplankton productivity and composition may alter the fate of that productivity between the water column fisheries (e.g., Pollock) and bottom dwelling fisheries (e.g., king crab). We observed that as water temperatures increased, there was in general, a decrease in the total amount of phytoplankton present and that their productivity decreased as well. In addition, as water temperatures increased there was a substantial shift in phytoplankton assemblage composition from larger phytoplankton cells, such as diatoms, to smaller, flagellated phytoplankton. This shift in cell size was also associated with a decrease in the fraction of phytoplankton productivity that was channeled to the bottom-dwelling communities. These observed changes in phytoplankton productivity, abundance and composition that co-vary with increasing water temperatures are very similar to results observed in temperate latitude systems throughout the northern hemisphere. These coordinated changes in phytoplankton will likely shunt a larger proportion production and energy to smaller zooplankton, resulting in potentially lower overall fisheries harvests as energy entering the system via phytoplankton takes a more circuitous path to higher trophic levels and more energy is lost to respiration. In addition to the flow of particulate matter from phytoplankton to higher trophic levels, our data show that as water temperatures increase, a larger fraction of phytoplankton production also is entering the dissolved organic carbon pool. Detrimental to the highly productive fisheries, the release of dissolved organic carbon likely represents a ‘dead end’ for the flow of carbon and energy through the ecosystem. These changes in phytoplankton productivity and its fate, linked to increasing temperature, will undoubtedly be associated with reductions in carbon and energy available to higher trophic level fisheries.
