Transport of Hypoxic Waters: An Estuary-Subestuary Exchange

Hypoxic or anoxic conditions in the subpycnocline water of Chesapeake Bay persist throughout the summer. The effect on the dissolved oxygen concentration in the deep basin of the lower Rappahannock River, a subestuary on the western side of the bay, was studied with an observational program. The data indicate that in the lower portion of the water column the subtidal (or residual) current was directed into the subestuary most of the time. The mass fluxes of salt and dissolved oxygen into the subestuary through a point near the estuarine bottom at the river mouth were calculated for tidal and subtidal components respectively. From the analyses, we conclude that the mass exchange owing to the tidal component is at least an order of magnitude smaller than that resulting from the subtidal component. Characteristic differences in the properties of water transported into the subestuary were observed. On five occasions, each lasting about two days, during the one month period of field measurements, the imported water was characterized by low dissolved oxygen and high salinity, typically 3 to 4 mg 1−1 lower dissolved oxygen and 2 to 3 psu higher salinity than at other times. The low dissolved oxygen, high salinity water masses were all accompanied by strong subtidal current and southwest wind. During periods of strong wind from the southwest quadrant, the density-driven current near the bottom was enhanced by the wind-driven circulation. Furthermore, the surface set-up favors the transport of the water from the deep portion of the bay resulting from (1) tilting of the pycnocline in the bay and (2) shoreline and bathymétrie configurations around the estuary-subestuary junction.https://scholarworks.wm.edu/vimsbooks/1169/thumbnail.jp