BGD Biogeosciences Discussions BGD Biogeosciences Discuss. 1810-6285 Göttingen, Germany 10.5194/bgd-10-11255-2013 Temperature and phytoplankton cell size regulate carbon uptake and carbon overconsumption in the ocean Craig S. E. https://orcid.org/0000-0002-8963-0951 1 Thomas H. https://orcid.org/0000-0002-6720-8434 1 Jones C. T. 1 Li W. K. W. 2 Greenan B. J. W. 2 Shadwick E. H. 3 Burt W. J. 1 Department of Oceanography, Dalhousie University, 1355 Oxford Street, P.O. Box 15000, Halifax, Nova Scotia, B3H 4R2, Canada Bedford Institute of Oceanography, Department of Fisheries and Oceans, Dartmouth, Nova Scotia, B2Y 4A2, Canada Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania, Australia 08 07 2013 10 7 11255 11282 Copyright: © 2013 S. E. Craig et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/preprints/10/11255/2013/bgd-10-11255-2013.html The full text article is available as a PDF file from https://bg.copernicus.org/preprints/10/11255/2013/bgd-10-11255-2013.pdf

Phytoplankton plays a critical role in the uptake of atmospheric carbon dioxide by the ocean, and is comprised of a spectrum of cell sizes that are strongly associated with different oceanographic conditions. Studies suggest that the ocean will become increasingly stratified in response to a warming climate, limiting nutrient exchange to the upper sunlit ocean and favouring small cells able to grow in warmer, nutrient poor conditions. Here we show that, in a temperate shelf sea, a summertime population of numerically abundant small cells accounts for approximately 20% of annual carbon uptake. These small cells are not well represented by chlorophyll <i>a</i> – the ubiquitously used proxy of phytoplankton biomass – but rather, are strongly correlated with surface water temperature. Given the persistent near-zero nutrient concentrations during the summer, it appears that small cells drive carbon overconsumption, and suggest that their role in carbon fixation will become increasingly important in a warming ocean.

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