Salt and Nutrient Fluxes in the Chester River Estuary

The residence time of water in estuarine tributaries strongly affects estuarine biogeochemical and ecological processes. These processes are not well understood in shallow estuaries where strong benthic-pelagic coupling exists. As part of an effort to advance coupled hydrodynamic-biogeochemical models of shallow estuaries, circulation in the Chester River estuary, a branched tributary with several fresh water sources in Chesapeake Bay, is modeled using the Regional Ocean Modeling System (ROMS). Model output is being used to force a biogeochemical model of chlorophyll and diel-cycling hypoxia, an increasingly observed phenomena in eutrophied estuaries worldwide. In this work, we investigate how the estuarine salinity field dynamically drives water mass residence time in tributaries. Steady and tidal oscillatory components of the downgradient salt flux in the along-estuary salt balance are estimated and passive tracers are used to quantify exchange between the channel, flanking shoals and tributaries under different estuarine forcings. Under normal river flow conditions, simulations show that tidal trapping is a significant mechanism that retains water in side embayments over the course of a tide and cross-channel asymmetry exists in up-estuary salt flux between the channel and shoals, with the main channel responsible for the maximum flux. While the influence of hydrodynamic processes on seasonally stratified systems experiencing hypoxia are relatively well studied, hydrodynamic influences on diel-cycling hypoxia are necessary to understand the influence of hypoxia on estuarine nursery habitats.