Weather

Rain, wind and temperature can have wide-ranging effects on the Bay's habitat, water quality and fish and shellfish populations. All plants and animals can adapt to periodic changes in environmental conditions. However, scientists cannot predict with certainty how the diverse Bay ecosystem will react to prolonged periods of extreme weather conditions.

Rainfall

The amount of rainfall the Bay region receives affects river flow, or the volume of fresh water that flows into the Bay from its many tributaries. Under normal weather conditions, fresh water flowing from rivers and streams makes up about half the Bay's entire water volume. But extremes in rainfall—either too much or too little—can have varying effects on the Bay ecosystem.

During large rain events:

• Stormwater carries more pollution from the land into streams and rivers, and eventually the Bay. Excess nutrients can cause algae blooms, and increased suspended sediments reduce the amount of sunlight that reaches underwater grasses growing in the Bay's shallow waters.
• River flow increases, pouring more fresh water into the Bay and decreasing salinity. This can directly affect some Bay species, such as blue crab migration, oyster spat sets and spring spawning of striped bass.

Too little rain can also impact the Bay.

• Drought conditions reduce the amount of water flowing from tributaries into the Bay. This means that fewer pollutants are washed into the Bay. However, nutrients and sediment instead become trapped in tributaries, where they may increase algae growth and reduce water clarity.
• Prolonged periods of low fresh water flow allow salty ocean water to move further up into the Bay. This can have varying effects on some Bay species.
  • Oysters have higher spat sets in saltier waters; however, MSX and Dermo, two diseases that have ravaged Bay oyster populations, thrive in higher salinities.
  • Many species of bay grasses can only grow in fresh water. Since bay grasses cannot migrate when salinity increases, they ultimately die. Fewer bay grasses mean less food and habitat for crabs, waterfowl and juvenile fish.
  • Sea nettles, which are restricted to higher salinities, tend to move further up into the Bay during dry summers.
• A lack of precipitation can disrupt groundwater supplies. Many local communities throughout the Bay watershed use groundwater as a source of public water for drinking and irrigation.

Temperature

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

Temperature plays a critical role in determining the amount of dissolved oxygen in the Bay's waters. The colder the water, the more oxygen it can hold. Therefore, Bay waters have a greater ability to hold oxygen during the winter than they do during the summer.

High water temperatures may also affect underwater grass beds.

• In 2005, warmer than average water temperatures caused a large-scale loss of eelgrass in Tangier Sound. The Chesapeake is the southernmost part of eelgrass's Atlantic growing range, so higher-than-average water temperatures can greatly impact this vital underwater grass species.
• Algae flourish during the Chesapeake's hot summer months, soaking up nutrients and the sun's rays, and often multiplying to unhealthy proportions. Algae blooms block out sunlight needed by bay grasses, and can produce toxins that cause fish kills and impact recreational activities.

Wind

Wind can disrupt or reinforce the Bay's two-layered flow of fresh and salt water by mixing the Bay's waters and occasionally reversing 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.