Changes in Phosphorus Biogeochemistry across the Transition from Freshwater to Saltwater

Abstract of a paper presented at the 8th International Estuarine Biogeochemistry Symposium Sponsored by the National Science Foundation, Chesapeake Biological Laboratory, Solomons, Maryland, May 2004.

T. E. Jordan, J. C. Cornwell, and W. R. Boynton

More than 90% of the phosphorus (P) carried by rivers to estuaries is associated with suspended solids. We used serial extractions to quantify the different forms of particulate P (PP) in suspended sediments entering the Patuxent River estuary and in suspended and deposited sediments along the salinity gradient. About 50-90% of the PP was extracted by a citrate, dithionite, bicarbonate solution, which removes mostly phosphate bound to iron oxides. Lesser amounts of PP were released by extractions designed to quantify organic P, loosely sorbed phosphate, and authigenic and detrital apatite P. Deposited sediments in areas where salinity was <1 ppt had higher concentrations of total PP and more iron oxide bound phosphate than did sediments where salinity varied from 1-6 ppt seasonally. The decline in iron oxide bound phosphate with increasing salinity coincides with a marked increase in the efflux of dissolved phosphate from sediments, peak dissolved phosphate concentrations in the water column, and a shift in the interstitial dissolved inorganic N:P ratio from 40 to 5. Sulfide production supported by sulfate from sea salts may enhance the mobilization of iron oxide bound phosphate from sediments. This salinity induced release of dissolved phosphate may contribute to a shift from P limitation in freshwater to N limitation in saltwater.