Spatial distribution of pCO2, ΔO2/Ar and dimethylsulfide (DMS) in polynya waters and the sea ice zone of the Amundsen Sea, Antarctica

Abstract

We report the first simultaneous measurements of surface water pCO2, biological oxygen saturation (ΔO2/Ar) and dimethylsulfide (DMS) concentrations in polynya waters and the sea ice zone of the Amundsen Sea, Antarctica. Across our survey region, we observed large spatial variability in surface water gas concentrations, tied to strong gradients in hydrography and phytoplankton biomass. Variability in sea surface temperature and salinity was attributed to the interacting effects of surface ocean circulation and heat fluxes, sea ice melt and the upwelling of relatively warm, saline and nutrient-rich modified circumpolar deep water. Phytoplankton biomass ranged from <1μgL−1 to ∼40μgL−1 chlorophyll a (Chl a), with the highest values observed in regions of shallow mixed layer depths. Phytoplankton assemblages were dominated by the colonial haptophyte Phaeocystis antarctica at most sampling stations, with lesser abundances of diatoms found throughout the polynya and sea ice zone. Cryptophytes were abundant at a few stations along the continental shelf break. Across the open polynya waters and sea ice zone, ΔO2/Ar ranged from ∼−40% to 40% (mean 8.6%), pCO2 ranged from 80 to 530μatm (mean 250μatm) and DMS concentrations varied from <1nM to ∼350nM. Strong gradients in gas concentrations were observed over short (i.e. <10km) spatial scales. The distribution of pCO2 and ΔO2/Ar across our survey transects reflected the balance between deep water entrainment and net community production, with a significant imprint of air–sea exchange. Chl a concentrations were significantly correlated to both pCO2 and ΔO2/Ar, and the slope of the ΔO2/Ar vs. pCO2 relationship was consistent with photosynthetic stoichiometry (∼1.25molO2 evolved per mol DIC removed). DMS and Chl a concentrations were also correlated in surface waters, but the spatial distribution of DMS was often uncoupled from pCO2 and ΔO2/Ar, likely due to complex microbial cycling processes. Sea surface temperature and salinity were related to surface gas concentrations through their effects on mixed layer depth and as tracers of upwelling. During the time of our cruise, the Amundsen Sea acted as an overall net sink for CO2 (mean sea–air flux=−15.9mmolm−2d−1) and a DMS source (mean sea–air flux=23.1μmolm−2d−1). Sea–air CO2 and DMS fluxes were more than 2-fold higher in open polynya waters relative to the overall cruise means. Simple calculations suggest that the Amundsen Sea contributes ∼5% of total Southern Ocean CO2 fluxes, and ∼1% of DMS fluxes.