Particulate phosphorus (PP) in the water column is an essential component of phosphorus (P) cycling in the Chesapeake Bay because P often limits primary productivity, yet its composition and transformation remain undercharacterized. To understand the mobilization of PP and P sequestration in the water column, we studied seasonal variations in particulate organic and inorganic P species at three sites in the Chesapeake Bay, using chemical extractions, 1-D (31P) and 2-D (1H-31P) NMR spectroscopies, and electron microprobe analyses. Our results suggest that an average of 9% and 50% of water column PP was recycled in shallow and deep sites, respectively, primarily through remineralization of organic P, which was 3 times higher than Fe-bound P remobilization. P recycling efficiency was highest in the warm and anoxic seasons. Organic P compositions and concentrations responded strongly to seasonal and redox variations: orthophosphate monoesters and diesters, and diester-to-monoester ratios (D/M) decreased with depth; both esters and D/M ratios were lower in the anoxic waters in July and September. In contrast, pyrophosphate concentration increased with depth and polyphosphate concentration was high in anoxic seasons. Our analyses suggest the presence of Ca-phosphate minerals (Ca-P) in the water column but with concentrations comparable to sediment Ca-P. It is unclear, however, whether authigenic precipitation occurred in the water column or resuspended from sediments. Overall, these results reveal the dominance of internal P cycling particularly via organic P remineralization and controlling P availability in the water column of the Chesapeake Bay.
Bibliographical noteFunding Information:
We thank Debbie McKay, Kristen Heyer, Laura Fabian, and others at the Maryland Department of Natural Resources and Captain Rick Younger and the crew of R/V Kerhin for help with sample acquisition. We acknowledge Lisa Stout, Ha Vu, Nirman Dhakal, Mingjing Sun, Dengjun Wang, Hui Li, Qiang Li, and Balakrishna Avula for help with collection and preprocessing water samples. Nancy Washton is acknowledged for discussions on NMR methods. The work was supported by grants from the U.S. Department of Agriculture (2015-67020 and 2016-08499) and the National Science Foundation (1301765 and 1654642). A portion of the research was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research located at Pacific Northwest National Laboratory (EMSL proposal 48437). We are thankful to the Chesapeake Bay Program for sharing the water column geochemistry data, which can be obtained freely from the CBP Data Hub (http://www.chesa-peakebay.net/data). Supporting data are included as tables in the supporting information; any additional data can be obtained from J.L. (e-mail: lijiying@udel. edu). We thank reviewers for their constructive comments that helped improve the quality of the manuscript.
- Authigenic precipitation of calcium phosphate is speculated in the P-rich hypoxic water column
- Particulate organic P remineralization is a significant source of dissolved P in the water column of the chesapeake bay
- The composition and transformation of particulate organic P responds strongly to seasonal and redox conditions