Spring Bloom Dynamics of the Eastern Bering Sea Shelf as Estimated from Oxygen/Argon Ratios and Triple Oxygen Isotopes

The Bering Sea's position at the end of the global ocean "conveyor belt" and shoaling of nutrient rich water masses onto its broad shelf make the Eastern Bering Sea shelf one of the most productive regions of the polar oceans, with reported annual primary production rates of 150 to 500 gC*m-2 yr-1. Much of this production occurs during spring blooms, which follow the inception of water column stratification after the retreat of sea-ice. The fate of the bloom biomass determines the amount of the export production available to higher trophic levels in the shelf ecosystem, but due to the hydrographic variability of the ice edge regime direct measurements of productivity rates are not easily extrapolated in space and time. Hence, a more integrative approach is needed. Here we report estimates of Net Community and Gross Photosynthetic Production rates (GPP and NCP) obtained from O2/Ar and triple oxygen isotope ratios measured as a part of the BEST (Bering Sea Ecosystem Study) project during six weeks of spring 2007. Under steady state conditions, NCP and new production should be stoichiometrically equivalent to net photosynthetic O2 production and can be estimated from oxygen air-sea exchange fluxes. In this study, O2/Ar ratios, used to distinguish between biological and physical components of oxygen flux, were measured continuously by a quadrupole mass spectrometer equipped with an Equilibrator Inlet (EIMS, modified from Kaiser et al., 2005). To calibrate EIMS results, discrete samples were collected from the ship's underway seawater system and from hydrocasts. Dissolved O2 and Ar were cryogenically isolated and extracted offline and analyzed on an IRMS at the Geosciences Dept, Princeton Univ. To estimate the rates of gross photosynthetic production, oxygen triple isotope ratios of dissolved O2 were measured on the same discrete samples. Ventilation rates (piston velocities) were calculated based on Quikscat wind speeds and the parameterization of Wanninkhof (1992). During the 12-day interval (22-23 April and 4-5 May 2007) between two repeat occupations of the same transect at 60°N, an extensive bloom developed at the edge of the retreating ice field. The biological oxygen fluxes increased from 20±60 mmol*m-2d-1 in April to a maximum of 1863±200 mmol*m-2d-1 in May. The rise in NCP rates was coincident with a shoaling of the mixed layer depth from > 30 m to ~ 8 m. Based on triple O isotope measurements, the fraction of NCP to GPP was 15 ± 20 % at the initial stages of the bloom, and increased substantially 12 days into the bloom. If we assume the bloom production increasing linearly from zero to 1863 mmolO2*m-2d-1 over the initial 12 days and then decreasing to zero over the next 12 days, this event would produce 185 gC*m-2 or about 40 % of the maximum annual total production previously estimated for the Eastern Bering Sea Shelf.