Changes in nutrient stoichiometry in responding to diatom growth in cyclonic eddies

Nutrient stoichiometry (e.g., nitrate + nitrite to soluble reactive phosphorus, refer to N + N/SRP, N/P hereafter) governs growth, competition and niche partitioning of phytoplankton in the illuminated oceans. The N/P, however, varies widely across the ocean and the underlying mechanisms remain unclear. Here, we report direct observations of significant variations in N/P in response to different life stages of two cyclonic eddies observed in the western South China Sea. High N/P (19.1 ± 6.9) values were observed around the nitracline in a mature-stage eddy, whereas a decay-stage eddy was characterized with low N/P (14.4 ± 4.1). The elevated N/P ratios accompanied by enriched fucoxanthin (pigment for diatom) and biogenic silica around the nitracline suggest that eddy pumping enhanced the growth of diatom which preferentially uptakes P relative to N in the mature stage of the eddy. Such high N/P ratios in the upper ocean could be reproduced if diatom uptake ratio was set between 10 and 16 in a data constrained numerical model. The preferential P uptake by enhanced diatom growth might reduce the P supply to the surface ocean, which is critical for N 2 -fixers. The transient changes in nutrient stoichiometry associated within the life cycle of cyclonic eddies also challenges the parameterization of physical–biogeochemical models with fixed phytoplankton uptake stoichiometry ratios, which could lead to bias of the model output for phytoplankton dynamics in oligotrophic ocean, where eddies frequently occur.