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.

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.