Hydrographic controls on net community production and total organic carbon distributions in the eastern Bering Sea

Abstract

In order to assess spatial and temporal variability of net community production (NCP) in shelf areas of the eastern Bering Sea, seawater samples for dissolved inorganic carbon (DIC) and total organic carbon (TOC) were collected during BEST-BSIERP cruises in the spring, summer, and fall of 2009 and compared to prior measurements made in 2008. DIC and TOC data were used to estimate seasonal changes in rates of NCP and the balance of net autotrophy versus heterotrophy in different shelf areas. In 2009, springtime surface layer DIC concentrations were generally uniform across the shelf and averaged ∼2100μmolkg−1, although concentrations in northern shelf areas (under sea-ice cover) were slightly higher (∼2130μmolkg−1). Subsequently, surface layer DIC (∼1950μmolkg−1) decreased significantly by summertime with the largest drawdown of DIC observed in the Middle Domain between 57° and 61°N. In this area, high NCP rates of up to 92mmolCm−2d−1 were observed and were higher than those reported in 2008. Comparing 2008 and 2009, the shelfwide average drawdown of DIC in the upper 30m between spring and summer was greater by ∼16μmolkg−1. In both spring and summer of 2008 and 2009, concentrations of TOC generally decreased from the coast. TOC concentrations were tightly coupled to salinity, particularly in spring, and largely influenced by the discharge of the Yukon and Kuskokwim Rivers. TOC accumulation between spring and summer was relatively small. In nearshore regions of the shelf, negative rates of NCP observed in 2009 were indicative of net heterotrophy with remineralization of labile organic carbon from rivers likely contributing to the observed net respiration signal in this region. In contrast, net heterotrophy was not observed in 2008, when river discharge rates were 30% lower (likely with lower river transport of TOC). While 2009 rates of production were higher outside the coastal domain than those observed in 2008, integrated annual production over the shelf was fairly comparable between the two years (2008: 103TgCyr−1; 2009: 97.2TgCyr−1). DOC accumulation in the surface layer was also equivalent between the two years (∼12μmolkg−1), and in both years shelfwide export production was estimated to be ∼75% of total NCP.