Oceanic nitrogen reservoir regulated by plankton diversity and ocean circulation

The average nitrogen-to-phosphorus ratio of marine phytoplankton (16N:1P) is closely matched to the nutrient content of mean ocean waters (14.3N:1P). This condition is thought to arise from biological control over the ocean’s nitrogen budget, in which removal of bioavailable nitrogen by denitrifying bacteria ensures widespread selection for diazotrophic phytoplankton that replenish this essential nutrient when it limits the growth of other species. Here we show that in the context of a realistic ocean circulation model, and a uniform N:P ratio of plankton biomass, this feedback mechanism yields an oceanic nitrate deficit more than double its observed value. The critical missing phenomenon is diversity in the metabolic N:P requirement of phytoplankton, which has recently been shown to exhibit large-scale patterns associated with species composition. When we model these variations, such that diazotrophs compete with high N:P communities in subtropical regions, the ocean nitrogen inventory rises and may even exceed the average N:P ratio of plankton. The latter condition, previously considered impossible, is prevented in the modern ocean by shallow circulations that communicate stoichiometric signals from remote biomes dominated by diatoms with low N:P ratios. Large-scale patterns of plankton diversity and the circulation pathways connecting them are thus key factors determining the availability of fixed nitrogen in the ocean.