Salinity, temperature and circulation are three important physical characteristics of water in the Chesapeake Bay. Each of these physical characteristics affects and is affected by the others.
Salinity is a measure of the amount of dissolved salts in the water. It is usually expressed in parts per thousand (ppt), or the number of grams of dissolved salts present in 1,000 grams of water.
In general, the lower Chesapeake Bay is salty and the upper Bay is fresh. Salinity gradually decreases as you move north and increases as you move south.
The Chesapeake Bay’s salinity varies widely from season to season and from year to year, depending on the amount of fresh water flowing from its rivers. The Bay tends to be fresher in spring, when snow melts and heavy rainstorms frequently fall. During drier months, the Bay is usually saltier.
Salinity also increases with depth. Fresh water remains at the surface because it is less dense than salt water.
The water on the Bay’s eastern shore tends to be saltier than water on the western side. This is due to two factors:
Because the Chesapeake Bay is so shallow, it cannot store heat over time. Water temperatures in the Bay fluctuate widely throughout the year, ranging from 34 degrees in winter to 84 degrees in summer. Changes in water temperature influence where bay grasses can grow and when fish and crabs feed, reproduce and migrate.
During spring and summer, the Bay's surface and shallow waters are warmer than deeper waters. This creates two distinct temperature layers that physically separate deeper waters from surface waters. These temperature layers have a major influence on the Bay’s summer dissolved oxygen levels.
Just as circulation moves blood throughout the human body, water transports important materials such as plankton, oxygen, minerals and larvae throughout the Chesapeake Bay.
Circulation is primarily driven by the movement of fresh water from the north and salt water from the south. Nutrients and other important materials are mixed and resuspended in the area where fresh and salt water meet. This area is called the zone of maximum turbidity. Many organisms use this zone as a nursery area because of the amount of available nutrients.
Weather – particularly wind – can disrupt or reinforce the two-layered flow of fresh and salt water. Wind can mix the Bay's waters and occasionally reverse the direction of the flows. Wind can also raise or lower surface water levels.
Together, salinity, temperature and circulation dictate the physical characteristics of water.
Warmer, lighter fresh water flows down toward the ocean. Below it, a layer of saltier and denser water flows up the Bay. The two water layers are separated by the pycnocline: a zone of intensive mixing and rapid increases in salinity.
Division of the two layers – called stratification – varies depending on the season and rainfall.