Physical Characteristics

The distribution and stability of the Bay's ecosystem depends on three important physical characteristics of the water: salinity, temperature and circulation. Each affects and is affected by the others.

Salinity

Salinity is the number of grams of dissolved salts present in 1,000 grams of water. It is usually expressed in parts per thousand (ppt). Salinity levels within the Bay vary widely, both seasonally and from year to year, depending on the volume of fresh water flowing into the Bay from its tributaries.

• The Bay's salinity is highest at its mouth, where sea water from the Atlantic Ocean enters, and gradually decreases northward.
• The water found at the head of the Bay and in each of the Bay's tidal tributaries is fresh. Fresh water contains few salts (less than 0.5 ppt) and is less dense than sea water in the ocean, which averages 35 ppt. This difference in density causes salinity to increase with depth, as fresh water remains at the surface.
• Brackish waters in the middle of the Bay and each tidal tributary range from 1 ppt to 10 ppt. The lower sections of the Bay and each tidal tributary are moderately salty, from 10 ppt to 18 ppt.

Temperature

Because the Bay is so shallow, it has a relatively small capacity to store heat over time. As a result, water temperature fluctuates widely throughout the year, ranging from 34 to 84 degrees Fahrenheit. Changes in water temperature influence where underwater bay grasses can grow and when fish, crabs and oysters feed, reproduce or migrate to other areas.

During spring and summer, the Bay's surface and shallow waters are warmer than deeper waters, creating two distinct temperature layers. This difference in water temperature physically separates deeper waters from surface waters, influencing summer dissolved oxygen levels.

Circulation

Just as circulation moves blood throughout the human body, water transports much-needed materials such as plankton, oxygen, minerals and shellfish larvae throughout the Bay. Circulation is driven primarily by the movements of fresh water from the north and salt water from the south. In the area where fresh and salt water meet, nutrients and sediments are mixed and resuspended. Fish and other organisms often use this zone as a nursery area because of the amount of nutrients.

Weather 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. It can also raise or lower surface water levels.

• Strong northwest winds, which are associated with high pressure areas, push water away from the Atlantic Coast and create exceptionally low tides in the Bay.
• Conversely, strong northeast winds, which are associated with low pressure areas, produce exceptionally high tides.
• Strong winds can pile surface water against one shore of the Bay.

Interaction of Physical Characteristics

Together, salinity, temperature and circulation dictate the physical characteristics of water. Warmer, lighter fresh water flows southward toward the ocean over a layer of saltier and denser water flowing up the Bay. The two layers are divided by the pycnocline, a zone of intensive mixing and rapid increases in salinity.

Stratification of the two layers varies within any season depending on rainfall.

• Stratification is usually highest in the spring as the amount of fresh water in the Bay increases due to melting snow and frequent rain.
• These stratified layers are maintained as surface waters warm throughout the summer.
• In autumn, fresher surface waters cool faster than deeper waters. This causes the fresh water layer to sink, and the two layers mix rapidly (usually overnight). The vertical mixing moves nutrients up from the bottom, making them available to phytoplankton and other organisms living in higher water levels. This turnover also distributes much-needed dissolved oxygen to deeper waters.

During the winter, water temperature and salinity are relatively constant from surface to bottom.