Abstract
The oceanic phosphorus (P) cycle is often overlooked in biogeochemical studies because it is dominated by geological processes. However, recent reports indicate that some oceanic regimes are P limited; thus an evaluation of the P pools and the biological cycling of P is warranted in these locations. The subtropical and tropical North Atlantic is one area proposed to be P deficient. A cruise across a wide swath of the tropical North Atlantic (∼25–60°W and 0–15°N) allowed us to investigate spatial differences in these parameters, including in waters influenced by the Amazon River plume. The majority of the total P pool was present as dissolved organic phosphorus (DOP, ∼80%), with soluble reactive phosphorus (SRP) and particulate organic phosphorus (POP) comprising much smaller fractions. Concentrations of both SRP and POP were elevated in areas influenced by the Amazon River, while DOP was not. The turnover time of the PO43− pool was more rapid on the western side of the basin (<10 h) and slower to the east (>100 h). Fast turnover times are indicative of P deficiency, and the observed trend suggests an east to west increase in P deficiency in the tropical North Atlantic. The maximal PO43− uptake rate (Vmax) of Trichodesmium spp., a well‐studied nitrogen‐fixing cyanobacterium, also indicates higher P deficiency in the western compared to eastern basin. These data support the hypothesis that P could be an important nutrient limiting certain biological processes in the North Atlantic, although it may be spatially (as well as temporally) variable in this basin.
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