Decoupling Between N:P Ratios of Particulate Organic Matter and Seawater

Globally averaged N:P ratios in particles and seawater are both about 16:1, as found by Alfred Redfield in the 1930s. Since then this ratio has defined our understanding of marine biogeochemistry. Today we know that particulate N:P exhibits strong spatial variations and the average is also different from the dissolved N:P ratio, yet the question how particulate and dissolved inorganic N:P ratios are coupled remains unresolved. Here we show how particulate N:P ratio (pN:P) varies in relation to dissolved N:P (dN:P) in a 3-D global model with an optimality-based non-Redfield plankton. In our sensitivity analysis, dN2P varies by a factor of 7, whereas pN:P only varies by a factor of 2. pN:P is positively correlated with dN:P when dN:P ranges from 14 to 25. However, pN:P does not change when dN:P is less than 14 or greater than 25. Trade-offs between growth rate and nitrogen uptake by phytoplankton set the upper limit of pN:P when dN:P > 25, while nitrogen fixation by diazotrophs prevents pN:P from decreasing when dN:P < 14. The coupling between pN:P and dN:P is even weaker when adaptation of phytoplankton is considered. Our simulations suggest that a shift from N limitation to P limitation of marine primary production is unlikely and that we may observe an increase in globally averaged pN:P when we have a lower nitrate concentration in the future.