| Summary: | Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Blackwell Publishing for personal use, not for redistribution. The definitive version was published in Global Change Biology 16 (2010): 1137-1144, doi:10.1111/j.1365-2486.2009.01960.x. The macroecological relationships between marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 μm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature-size relationship with the allometric size-scaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the -0.6 to 22ºC range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift towards smaller primary producers in a warmer ocean. Since the fate of photosynthesized organic carbon largely depends on phytoplankton size we anticipate future alterations in the functioning of oceanic ecosystems. X.A.G.M., A.C.-D. and Á.L.-U. acknowledge the financial support of research grants VARIPLACA (REN2001-0345/MAR), PERPLAN (CTM2006-04854/MAR) and the RADIALES time-series program of the Instituto Español de Oceanografía. W.K.W.L. was supported by the Canadian Department of Fisheries and Oceans Strategic Science Fund in the Ocean Climate Program and the Atlantic Zone Monitoring Program. This work was partially funded by Theme 6 of the EU Seventh Framework Programme through the Marine Ecosystem Evolution in a Changing Environment ...
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