REMINERALIZATION OF ORGANIC PHOSPHORUS AND PRECIPITATION OF AUTHIGENIC PHOSPHATE IN THE WATER COLUMN IN THE MID-CHESAPEAKE BAY

Phosphorus (P) cycling plays important roles in controlling trophic states of aquatic environments. However understanding the pathways and mechanisms of P cycling is challenging in the Chesapeake Bay where strong temporal and spatial variability exists both in P concentrations and rates of P cycling. To understand internal P cycling in the mid-Bay, the most eutrophic region in the Bay, we characterized organic and inorganic P speciation in suspended particles in the water column using chemical extraction and 1D (³¹P) and 2D (¹H-³¹P) NMR spectroscopies, and traced P transformation using phosphate oxygen isotope ratios (d¹⁸O_P). Our results show strong temporal and spatial variability in organic P composition in the suspended particles in the surface waters, with monoesters being the dominant organic P (7.9%–36.5%), followed by diesters (1.5–19.9%). Both esters decreased with depth, suggesting active remineralization of settling organic debris in the water column, and this was found to be most pronounced in July. Interestingly, intense remineralization coincided with high concentrations of authigenic P in suspended particles both in temporal (peak in July) and spatial (increasing towards the bottom waters) scales, thus indicated potential precipitation of authigenic apatite P in the water column largely from remineralized P. Furthermore, d¹⁸O_P values of dissolved phosphate in the water column are indicative of P released from remineralization, which is subsequently cycled, as expected during algal bloom on the surface waters. In summary, these results show important roles of organic P remineralization in internal P cycling in the water column by supplying orthophosphate for microbial uptake and cycling as well as likely precipitation of authigenic apatite in the water column.