Seasonal particle and carbon dynamics in the eastern Bering Sea

The ocean margins of high-latitude seas, such as the eastern Bering Sea, are recognized as important areas for the enhanced scavenging removal of particle reactive chemicals and for the potential to sequester atmospheric carbon dioxide via photosynthetic conversion to biogenic particles and subsequent downward particle transport to deeper waters. This region is expected to warm in the future, and with the warming will come a reduction in the extent and duration of seasonal sea-ice. This physical process exerts an important control on the timing, location, and magnitude of the spring primary production bloom. Within the context that 2008-2010 are characterized as cold years in the eastern Bering Sea, establishing the mechanistic link between primary production and the seasonal progression in particle export represents a significant challenge because these processes exert a control on organic matter transport from the surface ocean and on the success of economically and culturally important animals. During spring and summer cruises in 2009 and 2010, distributions of 234 Th (t1/2 = 24.1 days) in the water column and sediments were measured at ~60 stations over the middle and outer regions of the shelf and at the shelf break. The inventory of excess 234 Th ( 234 Th which is not produced in the sediments) in shelf sediments was ~1/3 of the total deficit of 234 Th ( 234 Th scavenged by sinking particles) in the overlying water column leading to an average focusing factor of 0.34 ± 0.23. Further, 234 Th export from the shelf was determined to ~30% of the total production of this radionuclide by 238 U decay based on a 234 Th budget. These results, taken together with elevated focusing factors in the offshelf region, suggest that the shelf sediments and lateral transport of particles from the shelf represent significant sinks for biogenic particles produced over the shelf in the spring and summer. The elevated focusing factors at the shelf break are attributed to enhanced particle flux from blooms of primary production in this region, an area commonly referred to as the ‘Green Belt’ for its exceptionally high rates of primary production. Thus, seasonal export of particles from elevated rates of primary production at the shelf break may transfer a significant amount of particulate organic carbon (POC) from the surface waters. POC export in this region demonstrates a clear seasonal progression with low fluxes in the early spring that increase by late spring and early summer. Rates of net primary production (NPP) were high and export fluxes relatively low near the ice-edge in spring, leading to export ratios (e-ratio = POC export/NPP) <0.25. In early summer, POC export exceeded NPP individual stations leading e-ratios >1, which is attributed to a temporal lag, or offset, between the high rates of primary production in spring and export as POC during the early summer. Using a water column-sediment 234 Th budget, the export of POC from the outer shelf to slope water was estimated to be 24±35 mmol C m -2 d -1 , which represents an off-shelf e-ratio of 0.07-0.52 for contemporaneous seasonally averaged and historical monthly averaged daily rates of NPP. In addition to the vertical POC fluxes measured at the shelf break, the imputed off-shelf export flux and e-ratios further suggests that there may be a significant transfer of shelf-derived particles to the slope waters. The high e-ratios and particles fluxes determined at the shelf may be a result of the biological response to the timing of physical processes in spring and summer of cold years. In spring, total chlorophyll a concentrations are generally low; however, localized phytoplankton blooms near the marginal ice zone (MIZ) lead to elevated spring average chlorophyll a concentrations, relative to summer, over the shelf and at the shelf break. Diatoms represent the greatest contribution to total chlorophyll a in spring and summer of cold years. This algal class also represents the majority of total chlorophyll a in particles sinking through the water column. Further, the relatively high proportion of pheophorbide a in sediment trap material indicates that sinking of zooplankton fecal pellets facilitates the export of particles through the water column. In cold years, the emergence of large diatom blooms in the spring MIZ supports the production of abundant large zooplankton. Large zooplankton are a primary food source for juvenile pelagic fishes of economically important species. Therefore, these cold year specific processes may be essential for the transfer of POC from the surface waters and the success of the economically important pelagic fishery. A consequence of a warmer Bering Sea in the coming decades is a reduction in seasonal sea-ice extent and duration. A change in seaice cover may alter the timing and magnitude of spring primary production and the flow of energy through the lower trophic levels.