LAUSR

Abstract Phosphorus (P) cycling in the coastal ocean along deltaic coastlines is predominantly controlled by river discharge and biogeochemical properties of the sediments. In the Louisiana shelf, sediment biogeochemistry is strongly influenced by seasonally-fluctuating bottom water O2, which, can potentially transition the shelf sediments from being a sink to source of P. Sediment P fluxes determined using persistent anaerobic and aerobic intact core incubations were 11.46 ± 3.2 mg m−2 d−1 and 0.67 ± 0.16 mg m−2 d−1 respectively, indicating a 17 times greater P efflux from O2-deprived sediments. During the transition from aerobic to anaerobic conditions, the high sedimentary O2 consumption rate of 889 ± 33.6 mg m−2 d−1 resulted in a progressive decrease in the dissolved O2 in the overlying water, leading to a sediment P flux of 7.2 ± 5.5 mg m−2 d−1, which was mostly driven by P release towards the end of the incubation when the overlying water became near anoxic. This P flux led to a 34.6% loss in sedimentary Fe-bound P during the two-day incubation period which underscores the importance of coupled Fe P biogeochemistry and O2 penetration depth in a coastal hypoxia setting. The high DIN: DIP ratio of river water (∼50:1; P limited) coupled with high potential release of P from the shelf sediments can further increase primary production, further exacerbating hypoxic conditions though increased organic matter (OM) deposition and subsequent decomposition.