BENTHIC IRON AND PHOSPHORUS FLUXES FROM PERMEABLE SEDIMENTS UNDER VARIABLE OVERLYING WATER COLUMN OXYGEN CONDITIONS

Coastal ecosystems are becoming increasingly hypoxic due to anthropogenically induced eutrophication and global warming. Hypoxia affects sedimentary biogeochemical processes, particularly the cycling of redox sensitive metals such as iron (Fe) and associated nutrients like phosphorus (P). The effects of hypoxia on benthic Fe and P fluxes remains poorly investigated, especially in sandy sediments in which physical and biological advection are major drivers of porewater transport. To address this, we performed sediment incubation experiments using enclosed chambers with individual recirculating systems and included bioirrigation mimics programed to match observed natural patterns. Overlying water oxygen concentrations were manipulated to establish repetitive oxic-hypoxic cycles, like those observed during diel-cycling hypoxia. Fe and P fluxes were estimated for normoxia, oxygen decline, sustained hypoxia, oxygen recovery using newly developed in-line accumulators. We found that Fe and P fluxes were higher in chambers with irrigation mimics, and highest during the sustained hypoxia and oxygen recovery phases with estimated fluxes exceeding 1 mmol m^-2 d^-1 and 100 µmol m^-2 d^-1 for Fe and P, respectively. Fe and P fluxes increased during subsequent oxic-hypoxic cycles, reaching fluxes 100x larger than initial values in the final hypoxic cycle. This pattern could be the result of advective forcing from bioirrigation which transports dissolved Fe and P mobilized at depth closer to the sediment-water interface. Upon reoxygenation of the water column, the upper centimeter of sediment becomes increasingly enriched in Fe-oxides, forming a large pool of available Fe and Fe-oxide bound P that can be more easily released into the overlying water during subsequent hypoxic conditions. These results have important implications for coastal areas where diel-cycling hypoxia is becoming more pronounced. Repetitive exposure to hypoxic overlying water could enhance the benthic metal and nutrient fluxes, potentially exacerbating the problem of eutrophication. We demonstrate significant Fe fluxes out of permeable sediments, particularly during times of hypoxia, suggesting that sandy deposits should be considered in global Fe flux estimates and that current benthic Fe cycling fluxes are underestimated.