Understanding phosphorus (P) availability and its control on eutrophication in the Chesapeake Bay is complicated by variable sources and biogeochemical reactions transforming P forms. We investigated seasonal and spatial variability in P limitation and biological utilization in the Bay using nutrient stoichiometry (of both dissolved and particulate forms), phosphate oxygen isotope ratios, and alkaline phosphatase activity at three sites along the salinity gradient. We demonstrate that particulate nutrient ratios can be used as indicators of nutrient limitation in the Bay and suggest strong seasonal and spatial variability in P availability: the surface water is P limiting in spring, but this condition is alleviated in summer and in the deeper waters. Variability in P limitation is well reflected in the trends of phosphate oxygen isotope composition (δ18OP), with values approaching isotopic equilibrium under P limiting conditions, suggesting rapid biological P turnover. Furthermore δ18OP values suggest multiple phosphate sources including remobilization of terrestrial inorganic P phases and remineralization of organic P and P from both sources is sufficiently cycled by microorganisms, suggested by the extensive equilibrium oxygen isotope exchange. Our results further suggest high P utilization in the deeper euphotic zone where nutrients are abundant, raising caution on studying nutrient availability and limitation only in the surface water.
Bibliographical noteFunding Information:
We thank Debbie McKay, Kristen Heyer, Laura Fabian, and others at the Maryland Department of Natural Resources, Rick Younger, and the crew of R/V Kerhin for help with sample acquisition and sharing the shipboard data. We are very thankful to the Chesapeake Bay program for sharing the water column geochemistry data. We acknowledge Lisa Stout, Ha Vu, Nirman Dhakal, Mingjing Sun, Dengjun Wang, Hui Li, Qiang Li, and Balakrishna Avula for help processing unusually large volume of water samples. The work has been supported by grants from the U.S. Department of Agriculture (2015-67020 and 2016-08499) and National Science Foundation (1301765 and 1654642). All CBP data were obtained freely from the CBP Data Hub (http://www.chesapeakebay.net/data). Supporting data are included as three tables in a supporting information file; any additional data may be obtained from J.L. (e-mail: firstname.lastname@example.org). We thank the two anonymous reviewers for comments that helped improved the manuscript.
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- Chesapeake Bay
- microbial turnover