Just as there is oxygen in the air, there is oxygen in the water. The oxygen in the water is a dissolved gas. A familiar example of a gas dissolved in a liquid is carbon dioxide dissolved in soda.

Oxygen is essential for all life in Greenwich Bay. It enters the water from two sources:

1. Photosynthesis by plants in the water
2. From the air

In order for a body of water to be a healthy habitat, it should have oxygen levels of 6 or more milligrams per liter (mg/l). Prolonged exposure to low dissolved oxygen levels (less than 5 mg/l ) may not directly kill an animal, but may make it weak, so that can be harmed by other stress factors in the environment. When oxygen drops to about 4 mg/l, fish will begin to feel stressed and move away from the area. Below 3 mg/l, fish kills may be observed and shellfish begin to shut down. At about 2 mg/l or lower, animals living in the sediments will start to die. Exposure to less than 2 mg/l oxygen for one to four days may kill most organisms, leaving only air-breathing insects and anaerobic bacteria.

Several things may affect oxygen levels in Greenwich Bay:

Temperature has an important effect. The warmer the water is, the less ability it has to hold oxygen. Think about what happens to the bubbles in soda when the soda gets warm. And, because Greenwich Bay is shallow, it warms up easily.

Automatic monitoring stations regularly measure temperature and dissolved oxygen at three locations in Greenwich Bay.

Nutrients may also be important. If large amounts of nutrients enter the bay, plankton blooms can result. When all these plants die, the bacteria that break them down use up large quantities of oxygen. Sources of these nutrients include septic systems and runoff.

When a body of water experiences low levels of oxygen, the condition is known as hypoxia. When oxygen levels drop to virtually none, the condition is called anoxia.

Anoxia and hypoxia are most likely to occur in Greenwich Bay in the summer when the water is warm.