Seasonal succession of net primary productivity, particulate organic carbon export, and autotrophic community composition in the eastern Bering Sea

Abstract

Seasonal patterns in the partitioning of phytoplankton carbon during receding sea ice conditions in the eastern Bering Sea water column are presented using rates of 14C net primary productivity (NPP), phototrophic plankton carbon content, and POC export fluxes from shelf and slope waters in the spring (March 30–May 6) and summer (July 3–30) of 2008. At ice-covered and marginal ice zone (MIZ) stations on the inner and middle shelf in spring, NPP averaged 76±93mmolCm−2d−1, and in ice-free waters on the outer shelf NPP averaged 102±137mmolCm−2d−1. In summer, rates of NPP were more uniform across the entire shelf and averaged 43±23mmolCm−2d−1 over the entire shelf. A concomitant shift was observed in the phototrophic pico-, nano-, and microplankton community in the chlorophyll maximum, from a diatom dominated system (80±12% autotrophic C) in ice covered and MIZ waters in spring, to a microflagellate dominated system (71±31% autotrophic C) in summer. Sediment trap POC fluxes near the 1% PAR depth in ice-free slope waters increased by 70% from spring to summer, from 10±7mmolCm−2d−1 to 17±5mmolCm−2d−1, respectively. Over the shelf, under-ice trap fluxes at 20m were higher, averaging 43±17mmolCm−2d−1. POC export over the shelf and slope estimated from 234Th deficits averaged 11±5mmolCm−2d−1 in spring and 10±2mmolCm−2d−1 in summer. Average e-ratios calculated on a station-by-station basis decreased by ∼30% from spring to summer, from 0.46±0.48 in ice-covered and MIZ waters, to 0.33±0.26 in summer, though the high uncertainty prevents a statistical differentiation of these data.