Lateral Transport of N‐Rich Dissolved Organic Matter Strengthens Phosphorus Deficiency in Western Subtropical North Atlantic

The ability of the subtropical North Atlantic to sustain export production despite the lack of available nutrients is fascinating. Subtropical gyres are expected to expand under a global warming scenario, so it is important to understand the mechanisms supplying the required nutrients. Current issues for the region concern the nutrient and metabolic balance, the origin of excess nitrogen and phosphorus shortage, and the maintenance of nitrogen fixation. We report data on the allocation of nitrogen and phosphorus in dissolved and suspended pools, the isotopic delta N-15 of suspended nitrogen, and the lability of dissolved organic phosphorus (DOP) along a section crossing the eastern seasonally stratified North Atlantic and the western subtropical North Atlantic (NASW). We find extreme P-deficiency in the NASW, with the highest dissolved inorganic N:P ratios located within the upperthermocline isopycnals (sigma(theta) = 26.3-26.8). Our data indicate an important role of the midlatitude northeast seasonally stratified North Atlantic bringing dissolved organic matter (DOM) to the thermocline of the North Atlantic. The mineralization of N-rich DOM contributes to the N excess (P deficit) of the upperthermocline of NASW. We find lower concentrations of more reactive DOP in the western than in the eastern part of the transect, indicating an active role of DOP in the nutrition of microbial communities. Our results support recent hypotheses concerning the environmental controls of marine nitrogen fixation identifying the key role of DOP utilization.