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Enrichment of nutrients, exopolymeric substances and microorganisms in newly formed sea ice on the Mackenzie shelf

Newly formed sea ice was sampled at 32 stations on the Mackenzie shelf, between 30 September and 19 November 2003. At each station, sea ice and surface waters were analysed to assess the concentration and enrichment of nutrients, exopolymeric substances (EPS, measured with Alcian blue), chlorophyll a (chl a), autotrophic and heterotrophic protists, and heterotrophic bacteria. Dark incubations were also conducted to estimate net heterotrophic NH4 regeneration rates in sea ice <5 cm thick. Large (≥5 µm) autotrophs were selectively enriched during sea-ice formation, having the highest average enrichment index (I S = 62), although heterotrophic protists (I S = 19), EPS (I S = 17), bacteria (I S = 6) and dissolved inorganic nitrogen (I S = 3 to 5) were also significantly enriched in the sea ice. Significant relationships were observed between sea-ice EPS and total chl a concentrations (r = 0.59, p < 0.001) and between sea-ice EPS and ≥5 µm autotroph enrichment indices (r = 0.48, p < 0.01), suggesting that EPS were actively produced by algae entrapped in the sea ice. These relation- ships also suggest that the presence of EPS may enhance the selective enrichment of large autotrophs. Heterotrophic regeneration contributed to the observed enrichment of NH4 in the sea ice, with an average regeneration rate of 0.48 µM d -1 , contributing 67% of the sea-ice NH4 concentra- tions. In the newly formed ice, NH4 regeneration was coupled to NO3 and Si(OH)4 consumption and was significantly related to EPS concentrations (r = 0.87, p < 0.05). Our data suggest that EPS enhance NH4 regeneration by acting as a carbon source for sea-ice heterotrophs or a substrate for sea-ice bacteria.