One of the most important characteristics of the tidal area is the wide range of salinity from the fall line to the Bay's mouth at the Atlantic Ocean. Salinity also affects the physical processes in the Bay. As freshwater from the tributaries flows toward the Bay, it mixes with saltwater. That generally means that the closer water is to the Bay's mouth and the Atlantic Ocean, the saltier it will be. Saltwater also is denser than freshwater.
Flows tend to higher in the spring and lower in the summer.
In deeper areas of the Bay, this difference in density can lead to the formation of a pycnocline, which is a layer in the water column separating two areas of different density. The pycnocline can be a physical barrier to exchange between surface and bottom waters and is a contributing factor in the depletion of dissolved oxygen from bottom waters. This natural process has been occurring in the Bay for thousands of years. Nutrients are a necessary component of the Bay's food web. Excess phytoplankton dies, consuming oxygen needed by other living resources.