Biotic regulation of the environment at global scales has been debated for several decades.An example is the similarity between deep-ocean and phytoplankton mean N:P ratios. N and P cycles are heavily altered by human activities, mainly through an increase in nutrient supply to the upper ocean. As phytoplankton only access nutrients in the upper ocean, it is critical to understand (1) to what extent phytoplankton are able to regulate N and P concentrations as well as their ratio in the deep, inaccessible layer and (2) what mechanisms control the value of the deepwater N:P ratio and the efficiency of its biotic regulation. With a model of N and P cycles in the global ocean separated in two layers, we show that the value of the deepwater N:P ratio is determined by nonfixer's N:P ratio, recycling, and denitrification. Our model predicts that although phytoplankton cannot efficiently regulate deep nutrient pools, they can maintain nearly constant ratios between nutrients because compensatory dynamics between nonfixers and nitrogen fixers allows a control of deepwater chemistry through nutrient recycling. This mechanism could explain the near constancy of the deepwater N:P ratio, in agreement with Redfield's (1934Redfield's ( , 1958 classical hypothesis. Surprisingly, N:P ratio of phytoplankton does not affect their ability to regulate the deepwater N:P ratio. Our model suggests that increased water column stratification as a result of global climate change may decrease the stability of the N:P ratio in the deep ocean over long temporal and spatial scales.

Global biogeochemical cycles are being profoundly affected by human activities; therefore, it is critical to understand the role played by organisms in their regulation. Autotrophic organisms can regulate nutrient abundance at local scales through resource consumption, but most resources are inaccessible to them at global scales, either because of physical barriers or because of the presence of non-assimilable chemical forms of nutrients. Here we present a generic model of resource access limitation and apply it to the oceanic cycles of iron, phosphorus, and silicon to examine whether phytoplankton can regulate the concentrations of these key nutrients. Our model predicts that autotrophs cannot at the same time strongly impact accessible nutrients and exert perfect regulation on inaccessible nutrients. We show that the ability of organisms to regulate inaccessible nutrient pools strongly depends on passive physical and chemical flows, and on the fraction of the system that is accessible to organisms. Components of global climate change such as increasing water column stratification might result in a further decrease of the biotic regulation of inaccessible nutrients in freshwater and marine systems.

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