High production going along with high respiration: impact of biofilm formation for sea-ice biogeochemistry

While representing less than 5% of the total ice cover around Antarctica, landfast sea ice is nevertheless an important habitat known to exhibit high biomass levels at the ocean/ice interface, with particulate organic carbon (POC) concentrations easily reaching 2000 μmol C L–1 during spring bloom. Surprisingly, together with the POC increase in bottom ice, fieldwork measurements performed in East Antarctica (Adélie Land 2011, McMurdo Sound 2012, Prydz Bay 2015) of nitrate and phosphate concentrations report a simultaneous increase with concentrations exceeding those of underlying seawater, suggesting an intense remineralization and nitrification processes within the ice. This goes against the classic view of nutrients being consumed during the growth season and regenerated after the height of the bloom. Regardless of the high nitrate levels available in the ice, increasing total nitrogen concentrations also suggest still more nitrogen from the underlying seawater was brought into the ice. Results of a NPZD-model indicates that a second nutrient pool, in addition to the brine pool, is essential to successfully model and reproduce field observations. The presence of a biofilm attached to the ice walls could act as a water-retaining substrate forming microenvironments with chemical gradients within the brine channels. The effect of biofilm on nitrogen dynamics (concentration and isotopic composition) in sea ice will be discussed as well as potential implications for other parameters (phosphate, carbon, oxygen). This calls for the integration of the biofilm concept into the current view of sea-ice biogeochemistry.