| Zusammenfassung: | ABSTRACT: 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), chlorophylla(chla), 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 chlaconcentrations (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 relationships 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 concentrations. 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.
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