Phosphorus availability and turnover in the Chesapeake Bay: Insights from nutrient stoichiometry and phosphate oxygen isotope ratios

Understanding phosphorus (P) availability and its control on eutrophication in the Chesapeake Bay is complicated by variable sources and biogeochemical transformations. 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 3 sites along the salinity gradient. We demonstrate that particulate nutrient ratios can be used as indicators of nutrient limitations 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 equilibration under P limiting conditions, suggesting rapid biological P turnover. δ18OP values suggest multiple phosphate sources, including remobilization of terrestrial inorganic P phases, and remineralization of organic P. P from both sources is sufficiently cycled by organisms, 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.