Just as circulation moves much-needed blood throughout the human body, circulation of water transports plankton, fish eggs, shellfish larvae, sediments, dissolved oxygen, minerals and nutrients throughout the Bay. Circulation is driven primarily by the movements of freshwater from the north and saltwater from the south. Circulation causes nutrients and sediments to be mixed and resuspended. This mixing creates a zone of maximum turbidity that, due to the amount of available nutrients, is often used as a nursery area for fish and other organisms. Weather can disrupt or reinforce this two-layered flow.
Wind plays a role in the mixing of the Bay's waters.
Wind also can raise or lower the level of surface waters and occasionally reverse the direction of flow. Strong northwest winds, associated with high pressure areas, push water away from the Atlantic Coast, creating exceptionally low tides. Strong northeast winds, associated with low pressure areas, produce exceptionally high tides. Together, salinity, temperature and circulation dictate the physical characteristics of water. The warmer, lighter freshwater flows seaward over a layer of saltier and denser water flowing upstream.