Research_Bering

Introduction to the scientific question:

The Bering Sea and other sub-Arctic and Arctic seas are predicted to be among the regions most severely affected by global warming (e.g., Sarmiento et al. 2004; Meier et al., 2005; Overpeck et al., 2005), as relatively small changes in the heat content of the water colu mn can have a disproportionately large effect on the spatial distribution and dynamics of sea ice. Indeed, recent evidence indicates that the eastern Bering Sea is warming and the extent and duration of seasonal sea ice cover is diminishing (Overland and Stabeno 2004), leading to a reduction in benthic biomass and metabolic rates (Grebmeier et al. 2006). From these observations, we pose the following key question: Has climate-driven interannual variability in sea ice extent altered the magnitude of gross and net primary production, its autotrophic community structure, and subsequently, carbon export, and degree of pelagic-benthic coupling in the eastern Bering Sea? Based upon a conceptual model described below (Figure 1), we present the following central 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.

    We propose to investigate how the production and partitioning of spring bloom organic carbon, phytoplankton community structure, export, and water column-benthic coupling varies spatially (North - South) and temporally (seasonally and from year to year), as a function of sea ice extent. These spatial a nd temporal patterns are hypothesized to affect the lower trophic levels (primary producers and zooplankton) as well as upper-trophic organisms (fish, marine birds, mammals) exploited by commercial fisheries and subsistence hunters (Hunt et al. 2002). Specifically, 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 our 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 us to test our hypothesis that the autotrophic level switches from a diatom-dominated, high export system in the MIZ, to a flagellate dominated, lower export system in open water bloom scenarios. Estimates of export production made in deeper waters, removed from potentially biased resuspension areas, will be used to test our hypothesis that pelagic-benthic coupling is markedly different in open water and MIZ blooms, for which there is some recent compelling evidence though without direct contemporaneous measures of primary and export production, as proposed here.

    The proposed observations will provide critical information for our understanding of how changes in sea ice extent impacts regional variability in productivity, and in turn how this alters the ecosystem structure, from diatom to flagellate dominated blooms, and attenuates the export flux of organic carbon fuelling benthic metabolism. Moreover, the proposed measurements will provide data central to ecosystem models of the seasonally productive BEST study region that are necessary to constrain the relationship between ice extent and carbon cycling, focusing on lower trophic levels. These measurements will be integrated with upper trophic levels and models, such as proposed by the NPRB and outlined in the Best Science Plan (2004).

    We present a conceptual ecosystem model that is characterized by two scenarios with respect to the primary producer community (Figure 1). In scenario 1, open-water blooms occur under significantly reduced sea-ice conditions, as was observed from 2001-2005 (Rodionov et al. 2007). We hypothesize that these open water blooms are characterized by flagellated species (i.e., non-diatom spp.), an overall lower integrated biomass but higher growth rates, such that gross primary productivity is similar to MIZ blooms. With this autotrophic community, we hypothesize that a larger fraction of primary productivity is retained in the pelagic zone and rapidly recycled through the microbial loop and microzooplankton grazers. In scenario 2, we hypothesize that the MIZ blooms occur under increased sea-ice cover (e.g., in 2006, and earlier decades before warming was observed in the Bering Sea). We propose that such MIZ blooms are dominated by large, chain-forming diatoms, which results in significant biomass accumulation but at slower growth rates, again maintaining gross primary production comparable to that of open-water blooms. We further hypothesize that this autotrophic community shunts most of its organic carbon to the benthos, resulting in high rates of NCP (i.e., new ≈ export production), tight pelagic-benthic coupling, and reduced microbial and zooplankton standing stocks.

    This conceptual model of climate-driven changes in sea-ice extent and ecosystem response serves as the basis for our research project and testing of our central hypothesis. Embedded within our model and central hypothesis, variability in the extent of sea-ice is mechanistically linked to:

        A. shifts between high-biomass MIZ blooms and high growth rate open-water blooms;
        B. shifts between diatom-dominated MIZ blooms and flagellate-dominated open-water blooms, and;
        C. shifts between carbon export to the benthos and carbon retention within the pelagic ecosystem.

Sea ice and phytoplankton community composition.

Differences in water column stability, mean water temperatures and sea-ice extent in the eastern Bering Sea will also have an impact on the dominant phytoplankton species. Different phytoplankton functional groups (e.g., diatoms vs. flagellates) have a unique set of physiological traits that match particular environmental niches. A key environmental variable, and one that is likely to change significantly under a change in climate, is the mixed layer depth (and its relationship to the euphotic zone). Currently, MIZ blooms are dominated by large diatoms, whereas in open water blooms diatom growth potential is limited by the stability of the mixed layer and smaller non-diatom species, for example coccolithophores (Figure 2), dominate (e.g., Olson and Strom 2002; Jin et al. 2006).

We propose a combination of methods to assess all functional groups within the phytoplankton community, with redundancy in some methods, providing a more robust assessment of community structure. These methods include flow cytometry for identification and quantification of picoplankton, HPLC pigment analysis for chemo-taxonomic identification, and microscopy for morphological identification and quantification of dominant functional groups. We propose to document these changes in community structure along transects roughly parallel to the receding ice edge and away from the ice edge. The inherent natural variability in sea-ice extent provides a wide range of conditions, thus improving our ability to test our hypothesis that stratified water columns, even early in the year when diatoms historically bloom, favor blooms of small flagellated phytoplankton.