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High turnover rates indicated by changes in the fixed N forms and their stable isotopes in Antarctic landfast sea ice

Author(s): Fripiat, François; Sigman, Daniel M; Massé, Guillaume; Tison, Jean-Louis

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Abstract: We report concentration and nitrogen and oxygen isotopic measurements of nitrate, total dissolved nitrogen, and particulate nitrogen from Antarctic landfast sea ice, covering almost the complete seasonal cycle of sea ice growth and decay (from April to November). When sea ice forms in autumn, ice algae growth depletes nitrate and accumulates organic N within the ice. Subsequent low biological activity in winter imposes minor variations in the partitioning of fixed N. In early spring, the coupling between nitrate assimilation and brine convection at the sea ice bottom traps a large amount of fixed N within sea ice, up to 20 times higher than in the underlying seawater. At this time, remineralization and nitrification also accelerate, yielding nitrate concentrations up to 5 times higher than in seawater. Nitrate δ15N and δ18O are both elevated, indicating a near‐balance between nitrification and nitrate assimilation. These findings require high microbially mediated turnover rates for the large fixed N pools, including nitrate. When sea ice warms in the spring, ice algae grow through the full thickness of the ice. The warming stratifies the brine network, which limits the exchange with seawater, causing the once‐elevated nitrate pool to be nearly completely depleted. The nitrate isotope data point to light limitation at the base of landfast ice as a central characteristic of the environment, affecting its N cycling (e.g., allowing for nitrification) and impacting algal physiology (e.g., as reflected in the N and O isotope effects of nitrate assimilation).
Publication Date: 31-Mar-2015
Citation: Fripiat, François, Daniel M. Sigman, Guillaume Massé, and Jean‐Louis Tison. "High turnover rates indicated by changes in the fixed N forms and their stable isotopes in Antarctic landfast sea ice." Journal of Geophysical Research: Oceans 120, no. 4 (2015): 3079-3097. doi:10.1002/2014JC010583.
DOI: doi:10.1002/2014JC010583
EISSN: 2169-9291
Pages: 3079 - 3097
Type of Material: Journal Article
Journal/Proceeding Title: Journal of Geophysical Research: Oceans
Version: Final published version. Article is made available in OAR by the publisher's permission or policy.

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