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.
Chesapeake Bay salinity
In general, the lower Chesapeake Bay is salty and the upper Bay is fresh. Salinity gradually decreases as you move north, farther away from the ocean, and increases as you move south.
- Salinity is highest at the mouth of the Bay—averaging 25 to 30 ppt—where water from the Atlantic Ocean enters.
- The head of the Bay and its tidal rivers are fresh, with a salinity of less than 0.5 ppt.
- The middle portion of the Bay and its tidal rivers are brackish: a mixture of salt and fresh water. Brackish water has a salinity of greater than 0.5 ppt but less than 25 ppt. Most of the water in the Bay is brackish.
Changes in salinity
Salinity varies widely from season to season and from year to year, depending on the amount of fresh water flowing from the Bay’ rivers. The Bay tends to be fresher in spring, when snow melts and heavy rainstorms are frequent. 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, due to two factors:
- Most fresh water enters the Bay from its northern and western tributaries, including the Susquehanna and Potomac rivers.
- The Coriolis Force, a phenomenon caused by the earth’s rotation, pushes flowing water in the Northern Hemisphere to the right. So saltier water moving up the Bay veers toward the eastern shore.
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 underwater grasses can grow and when fish and crabs feed, reproduce and migrate.
During spring and summer, 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 dissolved oxygen levels in the summer.
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 water and occasionally reverse the direction of flow. Wind can also raise or lower surface water levels.
- Strong northwest winds associated with high pressure areas push water away from the Atlantic Coast and create exceptionally low tides.
- Conversely, strong northeast winds associated with low pressure areas produce exceptionally high tides.
- Strong winds can also pile surface water against one shore of the Bay.
How do salinity, temperature and circulation interact?
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.
- In spring, stratification is usually highest. Melting snow and frequent rain increase the amount of fresh water in the Bay.
- Throughout the summer, these stratified layers are maintained as surface waters warm.
- In autumn, fresher surface waters cool faster than deeper waters. The fresh water layer sinks and the two layers mix rapidly, usually overnight. This vertical mixing does two important things: it moves nutrients up from the bottom, making them available to phytoplankton and other organisms closer to the water’s surface, and it distributes oxygen to deeper waters.
- During the winter, water temperature and salinity are relatively constant from surface to bottom.