Nutrients released to open waters may cause deterioration of water quality and impact aquatic ecosystems. Although a significant fraction of phosphorus (P) entering brackish tidal waters may be retained in the riverbed, such storage could be temporary because a fraction of retained P can be remobilized during the fluctuation of physiochemical and biological conditions in the river water. This study aimed to identify the roles of environmental parameters in remobilizing P from selected sections of East Creek on the Delmarva Peninsula in the Chesapeake Bay watershed. It included a series of field measurements and controlled laboratory experiments using sediment and water samples from the creek. Environmental parameters chosen to identify P release included biologically mediated redox change, pH, salinity, and temperature. Our results show that enhanced microbial activity and pH fluctuation play major roles in P release from all sites, and the extent of P release from headwater sediments (agriculture‐intensive area) was three to four times higher than that in the lower reaches of the creek (wetlands). Increases in salinity and temperature promoted release of P, albeit in insignificant amounts. Calculation based on regression equations developed from statistical analysis of P release and retention data indicated that a significant amount of P could be remobilized from the creek bed sediments under current conditions. Furthermore, the relationship of water O isotope ratios with tide and water column P concentrations indicates that the tides play a major role in diluting and removing P from the upstream region of the creek. These results highlight the role of biogeochemical processes in remobilization of legacy P from river bed sediment and export from a watershed.

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This biochemical transformation can be supported by nitrate diffusion and infiltration into sediment and soils from overlying water (i.e., direct denitrification NO 3 − →N 2 ) or by NO 3 − generated from nitrification within these substrates (coupled nitrification-denitrification: NH 4 + → NO 3 − →N 2 ). Because these N transformations can potentially ameliorate eutrophication (i.e., N enrichment) in inland and coastal waters (All dred & Baines, 2016;Mitsch et al., 2005), denitrification is considered a key ecosystem service in coastal zones that are often subjected to high N loads as a result of human impacts (

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