The relationship between sea ice break-up, water mass variation, chlorophyll biomass, and sedimentation in the northern Bering Sea

摘要

The northern Bering Sea shelf is dominated by soft-bottom infauna and ecologically significant epifauna that are matched by few other marine ecosystems in biomass. The likely basis for this high benthic biomass is the intense spring bloom, but few studies have followed the direct sedimentation of organic material during the bloom peak in May. Satellite imagery, water column chlorophyll concentrations and surface sediment chlorophyll inventories were used to document the dynamics of sedimentation to the sea floor in both 2006 and 2007, as well as to compare to existing data from the spring bloom in 1994. An atmospherically-derived radionuclide, 7~Be, that is deposited in surface sediments as ice cover retreats was used to supplement these observations, as were studies of light penetration and nutrient depletion in the water column as the bloom progressed. Chlorophyll biomass as sea ice melted differed significantly among the three years studied (1994, 2006, 2007). The lowest chlorophyll biomass was observed in 2006, after strong northerly and easterly winds had distributed relatively low nutrient water from near the Alaskan coast westward across the shelf prior to ice retreat. By contrast, in 1994 and 2007, northerly winds had less northeasterly vectors prior to sea ice retreat, which reduced the westward extent of low-nutrient waters across the shelf. Additional possible impacts on chlorophyll biomass include the timing of sea-ice retreat in 1994 and 2007, which occurred several weeks earlier than in 2006 in waters with the highest nutrient content. Late winter brine formation and associated water column mixing may also have impacts on productivity that have not been previously recognized. These observations suggest that interconnected complexities will prevent straightforward predictions of the influence of earlier ice retreat in the northern Bering Sea upon water column productivity and any resulting benthic ecosystem re-structuring as seasonal sea ice retreats in the northern Bering Sea.