Internal Cycling of Phosphorus in the Chesapeake Bay: Comparison of P Cycling in Water and Sediment Columns.

Understanding phosphorus (P) cycling in aquatic environments and its roles in controlling trophic states requires integrated knowledge of P cycling in both water and sediment columns. To understand the internal cycling of P in the Chesapeake Bay, we investigated P speciation in dissolved and suspended particles in the water column and shallow sediments at three sites located at upper, middle, and lower sections in the Bay. We characterized organic and inorganic P speciation using chemical extractions and ³¹P NMR spectroscopies, and traced P transformation using phosphate oxygen isotope ratios (d¹⁸O_P). Our results show active remineralization of settling organic debris, suggested by strong decrease of organic P concentrations in suspended particles with water depths. The P released from remineralization may have been subsequently cycled to support primary productivity, as suggested by d¹⁸O_P values of dissolved phosphate in the water column. Intense remineralization also coincided with the high concentration of the authigenic P pool in the suspended particles, thus indicating the potential precipitation of authigenic apatite in the water column from remineralized-P. These results are similar to those in the sediments where remineralization of organic matter was evidenced from the decrease in organic P concentrations with depth and corresponding precipitation of authigenic apatite and vivianite. These two minerals were found to be important sinks for sedimentary P. Overall, our results suggest the importance of internal P cycling, particularly organic matter remineralization, in supplying P in the water column as well as precipitating minerals in the water and sediment columns.