Processes Affecting the Behavior of Redox-Sensitive Elements in Chesapeake Bay

Several processes can affect variations in the chemical composition of an estuary, including (1) biogeochemical reactions in the water column, (2) advective transport, and (3) diffusion from underlying sediments. The purpose of the study was to establish whether estuarine redox reactions are controlled by a complex interaction of processes 1 to 3. Short-term and long-term studies were conducted in Chesapeake Bay. Redox species involved in redox cycling include oxygen, iron, manganese, hydrogen sulfide, nitrogen species, arsenic and antimony species, while phosphate is indirectly involved in redox cycles. Silicate was used as a non-redox tracer. For the short-term study, a Lagrangian drifter was used to track a parcel of bottom water, and thus allow monitoring of in situ water column processes and sediment-water exchange. In addition, a fixed (Eulerian) station was established to integrate processes 1 to 3. Seasonal changes in porewater and water column profiles were used to calculate fluxes and establish the coupling between benthic inputs and the inventory of redox sensitive chemical species in bottom waters of the Bay. Estuarine modeling was used on the long-term data sets from the upper Bay to estimate riverine and estuarine fluxes. During summer, there was estuarine flux (internal production) for ammonia, phosphate, iron, manganese, and silicate. A comparison of these estuarine fluxes and benthic fluxes from cores taken in the upper Bay showed that the total flux of materials produced within this portion of the estuary is strongly influenced by sediment-water exchange, particularly for ammonia and phosphate. Benthic flux of silicate was a more important contribution to the estuarine flux of silicate in summer. The mechanistic details of the coupling of the transport and in situ biogeochemical processes were closely examined by the short-term study. At the fixed station, large hourly variations in the concentration of redox species indicate that advection controls Eulerian behavior. From the Lagrangian study, ...