A simple nutrient-dependence mechanism for predicting the stoichiometry of marine ecosystems

Significance The elemental ratios of nitrogen, phosphorus, and carbon in marine phytoplankton can diverge significantly from the “Redfield ratio,” but the underlying reasons have been hard to elucidate. As a result, global biogeochemical models often ignore this stoichiometric variability. Here we show that, hidden within the noise of a large dataset of particulate measurements, a surprisingly consistent relationship exists between community P:C and dissolved phosphate concentrations. The plasticity of ecosystem stoichiometry in the face of nutrient scarcity, with greater plasticity for P relative to N, appears to explain the main divergences from the Redfield ratio. When included in a simple model, the relationship implies a more important role for low latitude nutrient cycling in the biological pump than is commonly assumed. It is widely recognized that the stoichiometry of nutrient elements in phytoplankton varies within the ocean. However, there are many conflicting mechanistic explanations for this variability, and it is often ignored in global biogeochemical models and carbon cycle simulations. Here we show that globally distributed particulate P:C varies as a linear function of ambient phosphate concentrations, whereas the N:C varies with ambient nitrate concentrations, but only when nitrate is most scarce. This observation is consistent with the adjustment of the phytoplankton community to local nutrient availability, with greater flexibility of phytoplankton P:C because P is a less abundant cellular component than N. This simple relationship is shown to predict the large-scale, long-term average composition of surface particles throughout large parts of the ocean remarkably well. The relationship implies that most of the observed variation in N:P actually arises from a greater plasticity in the cellular P:C content, relative to N:C, such that as overall macronutrient concentrations decrease, N:P rises. Although other mechanisms are certainly also relevant, this simple relationship can be applied as a first-order basis for predicting organic matter stoichiometry in large-scale biogeochemical models, as illustrated using a simple box model. The results show that including variable P:C makes atmospheric CO2 more sensitive to changes in low latitude export and ocean circulation than a fixed-stoichiometry model. In addition, variable P:C weakens the relationship between preformed phosphate and atmospheric CO2 while implying a more important role for the nitrogen cycle.