Sometimes the Bay’s deeper waters have little or no oxygen while surface waters have more oxygen. This usually happens in summer because of a combination of factors, including temperature, nutrient pollution, the way water flows in the Bay, and the shape of the Bay’s bottom.

Temperature, nutrient pollution, the way water flows in the Bay, and the shape of the Bay’s bottom all interact with each other to affect the amount of oxygen in the water.

Extreme temperatures can have a negative effect on some bay grasses. For example, eelgrass cannot grow in water that is too warm. In 2005, very high temperatures caused large areas of eelgrass in the lower Chesapeake Bay to die. Other species of bay grasses react positively to high temperatures and grow more quickly when the water is warmer.