Water exchange and its relationships with external forcings and residence time in Chesapeake Bay

Water exchange, featured by bottom inflow and surface outflow in a typical estuary, determines the transport and redistribution of salt, nutrients, pollutants, and suspended sediments and organisms. Water exchange in Chesapeake Bay, the largest estuary in the US, has been extensively studied, yet its long-term interannual variability and its relationship with the external forcings are not fully understood. Based on a long-term (1980-2011) numerical model simulation, this study examines the water exchanges between Chesapeake Bay and the adjacent coastal shelf, between different regions within the bay, as well as their relationships with river discharge, wind, and residence time. Through an EOF analysis of the bottom inflow and surface outflow at seven selected cross-bay sections, we found that over 90% of the spatiotemporal variations of water exchange can be explained by the first two EOF modes, which are highly correlated with the freshwater discharge and northwesterly wind, respectively. Unlike the outflow that increases linearly with river discharge as commonly expected, the inflow responds non-monotonically to river discharge. The relationship between the river discharge and inflow can be described by a combination of the Michaelis-Menten/Monod equation and a linearly decreasing function, i.e., the inflow initially increases with river discharge due to enhanced gravitational circulation and then levels, and gradually declines due to overwhelming seaward barotropic current. We found a locally enhanced water exchange in the lower-middle bay, which can be attributed to the persistent reflux of surface outflow due to the irregular geometry and rapid shoaling in the channel bathymetry. The water exchange and the mean residence time can be connected reciprocally through the bay volume, yet the validation of this relationship depends on the timescale to be considered since the residence time at a given time is controlled by the future hydrodynamics. A delay effect should be considered when using the relationship to estimate outflow interchangeably with the direct computation of the outflow.

Automated summary

Water exchange in Chesapeake Bay is influenced by external forcings such as river discharge and wind, with different regions within the bay exhibiting varying control mechanisms on the exchange. The relationship between inflow and outflow is complex, with both increasing and decreasing processes involved, and there is also locally enhanced water exchange due to irregular geometry and shoaling in the channel bathymetry.