Spatial variability and temporal dynamics of surface water pCO 2, ΔO 2/Ar and dimethylsulfide in the Ross Sea, Antarctica

Abstract

We report results from two surveys of pCO 2, biological O 2 saturation (ΔO 2/Ar) and dimethylsulfide (DMS) in surface waters of the Ross Sea polynya. Measurements were made during early spring (November 2006–December 2006) and mid-summer (December 2005–January 2006) using ship-board membrane inlet mass spectrometry (MIMS) for high spatial resolution ( i.e. sub-km) analysis. During the early spring survey, the polynya was in the initial stages of development and exhibited a rapid increase in open water area and phytoplankton biomass over the course of our ∼3 week occupation. We observed a rapid transition from a net heterotrophic ice-covered system (supersaturated pCO 2 and undersaturated O 2) to a high productivity regime associated with a Phaeocystis-dominated phytoplankton bloom. The timing of the early spring phytoplankton bloom was closely tied to increasing sea surface temperature across the polynya, as well as reduced wind speeds and ice cover, leading to enhanced vertical stratification. There was a strong correlation between pCO 2, ΔO 2/Ar, DMS and chlorophyll a (Chl a) during the spring phytoplankton bloom, indicating a strong biological imprint on gas distributions. Box model calculations suggest that pCO 2 drawdown was largely attributable to net community production, while gas exchange and shoaling mixed layers also exerted a strong control on the re-equilibration of mixed layer Ο 2 with the overlying atmosphere. DMS concentrations were closely coupled to Phaeocystis biomass across the early spring polynya, with maximum concentrations exceeding 100 nM. During the summer cruise, we sampled a large net autotrophic polynya, shortly after the seasonal peak in phytoplankton productivity. Both diatoms and Phaeocystis were abundant in the phytoplankton assemblages during this time. Minimum pCO 2 was less than 100 ppm, while ΔO 2/Ar exceeded 30% in some regions. Mean DMS concentrations were ∼2-fold lower than during the spring, although the range of concentrations was similar between the two surveys. There was a significant correlation between pCO 2, ΔO 2/Ar and Chl a across the summer polynya, but the strength of these correlations and the slope of O 2 vs. CO 2 relationship were significantly lower than during the early spring. Summertime DMS concentrations were not significantly correlated to phytoplankton biomass (Chl a), pCO 2 or ΔO 2/Ar. In contrast to the early spring time, there were no clear temporal trends in summertime gas concentrations. Rather, small-scale spatial variability, likely resulting from mixing and localized sea-ice melt, was clearly evident in surface gas distributions across the polynya. Analysis of length-scale dependent variability demonstrated that much of the spatial variance in surface water gases occurred at scales of <20 km, suggesting that high resolution analysis is needed to fully capture biogeochemical heterogeneity in this system.