Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 3 2007 10.5194/bg-4-353-2007 http://www.biogeosciences.net/4/353/2007/ http://www.biogeosciences.net/4/353/2007/bg-4-353-2007.html http://www.biogeosciences.net/4/353/2007/bg-4-353-2007.pdf 353 367 2007-06-20 Biotic stoichiometric controls on the deep ocean N:P ratio T. M. Lenton t.lenton@uea.ac.uk C. A. Klausmeier School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK W. K. Kellogg Biological Station, Michigan State University, Hickory Corners, MI 49060, USA We re-examine what controls the deep ocean N:P ratio in the light of recent findings that the C:N:P stoichiometry of phytoplankton varies with growth rate, nutrient and light limitation, species and phylum, and that N₂-fixation may be limited by Fe, temperature and/or light in large parts of the world ocean. In particular, we assess whether a systematic change in phytoplankton stoichiometry can alter the deep ocean N:P ratio. To do this we adapt recent models to include non-Redfieldian stoichiometry of phytoplankton and restriction of N₂-fixers to a fraction of the surface ocean. We show that a systematic change in phytoplankton C:N:P can alter the concentrations of NO₃ and PO₄ in the deep ocean but cannot greatly alter their ratio, unless it also alters the N:P threshold for N₂-fixation. This occurs if competitive dynamics set the N:P threshold for N₂-fixation, in which case it remains close to the N:P requirement of non-fixers (rather than that of N₂-fixers) and consequently so does the deep ocean N:P ratio. Then, even if N₂-fixers are restricted to a fraction of the surface ocean, they reach higher densities there, minimising variations in deep ocean N:P. 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