Biogeochemical observations during the winter―spring transition in East Antarctic sea ice: Evidence of iron and exopolysaccharide controls

This paper documents the spatial distribution and concentration of exopolysaccharides at 9 discrete sea ice sites, consisting of first year sea ice, and relates this information to physical (ice temperature, texture) and biogeochemical (oxygen stable isotopic composition of the ice, salinity, macronutrients, dissolved iron, particulate organic carbon, dissolved organic carbon and Chlorophyll a) variables. The sampling was carried out over a transition from austral winter to early spring conditions as part of the Sea Ice Physics and Ecosystems experiment (SIPEX), during September/October 2007 in the 110°-130° E region off East Antarctica. Exopolysaccharide concentrations in sea ice varied by 3 orders of magnitude from 2.8 to 2690 μg xanthan equivalent (xeq.)l―1; basal ice mean 493 μg xeq.l―1. Exopolysaccharides correlated significantly with particulate organic carbon and Chlorophyll a but not with dissolved iron, dissolved organic carbon or macronutrient data, indicating that exopolysaccharides are most likely produced in situ by autotrophic sea ice biota, superimposed over fossil organics. We observed increased exopolysaccharide per unit biomass in the colder surface to intermediate ice at three stations, supporting the theory that exopolysaccharides may be used as a cryoprotectant. Mean bulk ice dissolved iron (depth integrated) across all ice cores was 2.37 nM (range 0.23 to 14.4 nM). Sea ice dissolved iron concentration was always elevated relative to seawater. Apparent dissolved iron and estimates of cellular carbon to iron ratios suggest that the sea ice microbial biota was not limited by dissolved iron but may have been by NO2 + NO3 or Si(OH)4. Conversely, under ice seawater algal communities may have been limited by dissolved iron and/or light and grazing at the time of sampling. We observed a significant inverse correlation between dissolved Fe and Chlorophyll a in the basal layers of the ice, most likely indicating the active drawdown of dissolved Fe by the sea ice biota, combined with some fraction lost to the water column or converted to the particulate fraction.