COUPLED FE-P OR C-P PATHWAYS OF P CYCLING IN THE CHESAPEAKE BAY SEDIMENTS: COMPARISON OF THREE SITES WITH CONTRASTING BOTTOM WATER HYPOXIA

Excessive input of phosphorus (P) in the Chesapeake Bay has severely degraded the water quality and caused environmental problems such as hypoxia. Hypoxia-related biogeochemical changes in the bottom water may affect sediment P speciation and pathways of P cycling. The objectives of the current study are to investigate the dominance of coupled Fe-P or C-P pathways in sediment in response to bottom water hypoxia and identify corresponding P efflux at the sediment-water interface. We extracted sediment P pools and measured phosphate oxygen isotope ratios (d¹⁸O_P) of major P pools and identified the composition of organic P from the North, middle, and South sites of the Chesapeake Bay. Our result suggests ferric Fe-bound and authigenic P are the dominant P pools in the sediments in all sites with highest P content of both pools in the mid-Bay sediments. Similarly, solution ³¹P NMR data show the highest organic P content in the mid-Bay sediments compared to other sites, consistent with most eutrophic surface water in this site. Authigenic phosphate isotope data suggest that the regeneration of inorganic P from organic matter degradation (remineralization) is the predominant, if not sole, pathway for authigenic P precipitation in the mid-Bay sediments. It means P cycling is dominated by C-P pathways in sediment and excess P generated from this pathway is effluxed to bottom water in this site. Comparison of these results among three sites provides differences in P cycling impacted by bottom water conditions which are also compounded by the differences in physico-chemical properties of sediments in these sites. Overall, combination of these advanced research tools allowed better understanding of the impact of hypoxia on biogeochemical cycling of P in the Chesapeake Bay sediments.