INSIGHTS INTO PHOSPHORUS CYCLING ACROSS OXIC-ANOXIC BOUNDARIES IN MARINE SYSTEMS

Phosphorus (P) availability plays a critical role in marine phytoplankton cellular composition, community structure, growth and production. As natural and anthropogenically-induced climatic changes occur, biological production has been hypothesized to become increasingly P limited. At the same time, oxygen minimum zones are expanding in both the open ocean (due to declines in ocean circulation, vertical mixing, and reduced subduction and advection of deep waters) and along the coast (due to increased fertilizer runoff from rivers and atmospheric nitrogen deposition from fossil fuel burning). Yet our understanding of phosphorus biogeochemistry within and across oxic – anoxic boundaries remains limited. Here, we show that the composition and magnitude of suspended and sinking particulate P pools are significantly influenced by surface water food web structure and biotic and abiotic processes that occur across a redox boundary in the Cariaco Basin, a continental margin ecosystem. While surface water inventories of suspended P pools have declined over the past decade, the sinking flux of particulate P has remained relatively constant. At the same time, organic P phases are preferentially remineralized over other nutrients in oxic waters, while inorganic P phases are preferentially removed in deep anoxic waters. Within the redoxcline, abiotic reactions associated with a manganese and iron redox shuttle and biotic reactions associated with a large and diverse chemoautotrophic prokaryotic community may further contribute to sinking particulate P fluxes providing another unexpected twist in phosphorus dynamics relative to oxygen availability.