Inorganic and organic sinking particulate phosphorus fluxes across the oxic/anoxic water column of Cariaco Basin, Venezuela

Phosphorus (P) is a vital nutrient that is essential for all organisms and may limit growth on both modern and geologic timescales. The major removal mechanism of P from marine systems is via the transformation of dissolved P into sinking particulate P pools. Although most particulate P is remineralized before it reaches the seafloor, little is known about the processes that control its breakdown into dissolved phases. In this study, the P composition of sinking particles captured by five sediment traps distributed through the oxic and anoxic water column of the Cariaco Basin, Venezuela is examined. Samples were collected from January 1996 to December 2004. Total particulate P (TPP), particulate inorganic P (PIP), and particulate organic P (POP) fluxes varied considerably over the course of the nine year study, yet there were no significant seasonal differences in the overall flux of POP. In contrast, PIP, which comprises a major portion of TPP (averaging 52 ± 19% across all depths) had fluxes that were 30% higher during non-upwelling periods. Poor relationships between PIP and biologically derived constituents, e.g. particulate organic carbon (POC), suggest that most of this material was derived from non-biological sources, namely terrestrial runoff from rivers. Rapid remineralization of POP occurred relative to POC and PIP in oxic surface waters, whereas PIP was quickly remineralized relative to POC and POP in subsurface anoxic waters. This suggests a significant and alternating source of particulate P to the dissolved P pool that depends on oxygen availability. Thus, particulate P release to the dissolved phase and the upwelling of dissolved P into the euphotic zone is a potentially important positive feedback mechanism for enhanced primary production and carbon sequestration in continental margin sediments regardless of oxic versus anoxic conditions.