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We re-examine what controls the deep ocean N:P ratio in the light ofrecent findings that the C:N:P stoichiometry of phytoplankton varieswith growth rate, nutrient and light limitation, species and phylum,and that N2-fixation may be limited by Fe, temperature and/orlight in large parts of the world ocean. In particular, we assesswhether a systematic change in phytoplankton stoichiometry can alterthe deep ocean N:P ratio. To do this we adapt recent models toinclude non-Redfieldian stoichiometry of phytoplankton andrestriction of N2-fixers to a fraction of the surface ocean. Weshow that a systematic change in phytoplankton C:N:P can alter theconcentrations of NO3 and PO4 in the deep ocean but cannotgreatly alter their ratio, unless it also alters the N:P thresholdfor N2-fixation. This occurs if competitive dynamics set theN:P threshold for N2-fixation, in which case it remains closeto the N:P requirement of non-fixers (rather than that ofN2-fixers) and consequently so does the deep ocean N:P ratio.Then, even if N2-fixers are restricted to a fraction of thesurface ocean, they reach higher densities there, minimisingvariations in deep ocean N:P. Theoretical limits on the N:Prequirements of phytoplankton suggest that whilst the deep ocean hasbeen well oxygenated (i.e. since the Neoproterozoic, with thepossible exception of Oceanic Anoxic Events), its N:P ratio isunlikely to have varied by more than a factor of two in eitherdirection. Within these bounds, evolutionary changes inphytoplankton composition, and increased phosphorus weathering dueto the biological colonisation of the land surface, are predicted tohave driven long-term changes in ocean composition

Biotic stoichiometric controls on the deep ocean N:P ratio