One of the central issues regarding water quality in Chesapeake Bay concerns restoration of dissolved oxygen (DO) to reasonable levels in deeper waters during the warmer portions of the year. In the past few decades DO in deeper waters has been depleted and this situation effectively removes this portion of the bay as useful habitat for higher organisms. In addition to food web and habitat considerations, DO status plays a central role in regulation of some aspects of sediment-water nutrient exchanges which, in turn, impacts water quality conditions. Data is examined for influences of DO conditions on phosphorus fluxes.
Sediment phosphorus fluxes appear to be controlled largely by one of two mechanisms. In cases where DO levels are adequate and sediments support a macrofaunal community, phosphorus fluxes generally follow the annual temperature pattern. When temperatures are low, fluxes are low and fluxes are correspondingly higher when temperatures are higher during summer. The most likely mechanism is that flux is maintained by the burrowing and irrigation activities of the macrofaunal community. In cases where bottom sediments are exposed to oxygen, but the macrofaunal community is depleted or absent, phosphorus tends to be sequestered in sediments resulting in small fluxes to the water column.
The second mechanism controlling phosphorus fluxes involves oxygen conditions in waters proximal to the sediment surface. Typically, much of the phosphorus in oxidized estuarine sediments is complexed as oxyhydroxides and is quite insoluble. However, under hypoxic conditions phosphorus becomes soluble and can diffuse from sediments to overlying waters. This pattern has been evident at stations where water quality conditions vary between oxic and hypoxic and which typically have small macrofaunal communities. Conversely, at higher dissolved oxygen (DO) concentrations (> 2 mg l-1) large phosphorus (PO4-) fluxes are not observed. A pattern of high release of phosphorus from sediments associated with a drop of bottom water dissolved oxygen concentration below about 2.0 mg l-1 is repeated throughout our data record. Once released this phosphorus is again available for phytoplanktonic uptake, algal growth, deposition and decomposition, reinforcing the downward water quality trajectory.