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.
Results from this project provide new insights related to the larger question of the time-scale and fate of organic carbon and particle production, cycling, and transport in continental shelf ecosystems. There are only a few studies that have used our radiochemical approach to constrain the production and export of particulate organic carbon in a complex and dynamic shelf environment, in particular on a seasonal and interannual time-scale and further with sufficient spatial and temporal resolution as was afforded in the BEST project. With regard to high-latitude shelf systems, it is fair to say that this project is one of the most comprehensive in terms of data coverage and in constraining the key issue of carbon and particle balance over a seasonally ice-covered shelf. These data will surely provide a valuable baseline for future studies that more broadly attempt to address questions related to the fate and transport of organic carbon in high-latitude shelf systems in the face of future global warming. With regard to intellectual merit, a general conclusion from our Th budget analysis is that the ocean margin of the eastern Bering Sea may serve as an accumulation area for particles and associated reactive chemicals. On the other hand, when taken in context of the carbon budget, a general implication is that off-shelf POC transport may represent a significant seasonal net sink for CO2 in this and other polar shelf regions. Finally, our research reveals that the autotrophic community is dominated by the presence of diatoms species in spring and summer in both the water column and in sinking particulate matter. Overall these results provide new evidence for the importance of zooplankton grazing control of export production over the shelf. In addition, our work provides direct field observational data that is consistent with the shelf export hypothesis of Walsh et al.'s model analysis. Specifically, the Walsh model predicts up to 48% of net primary production is exported off the outer shelf, which compares remarkably well with our three-year observational data that suggests approximately 20-30% of NPP may be exported. Apart from the close agreement of these two very different approaches to this issue (model vs. observations), we believe our work raises again the issue of shelf export of organic carbon as a possible sink for atmospheric CO2. The added twist to this long-standing research question is that such off-shelf carbon export may be pronounced on a seasonal basis in high latitudes, due to the seasonal drawback of shelf sea ice, which is predicted to be more pronounced with future global warming. In terms of the broader impacts, results of this research have been disseminated at several national ocean science meetings noted in the Products section of this report, and to the public through dissemination in the Bering Sea highlights report due for publication in the coming months. This project has also supported the doctoral research one graduate student and has engaging a number of undergraduate students in laboratory and field sampling preparations.
