What is dissolved oxygen and why is it important?

Dissolved oxygen is required by all aquatic animals. Low dissolved oxygen levels (hypoxia) can impair animal growth or reproduction, and the complete lack of oxygen (anoxia) will kill animals. Animals with limited mobility such as oysters and clams are particularly vulnerable to hypoxic or anoxic conditions. Even fast swimmers such as fish can become trapped in areas with low oxygen or no oxygen.

Dissolved oxygen levels are affected by a number of factors, including:

• Water temperature

• How well mixed the water is (for example, the presence or absence of a pycnocline)

• How much oxygen is being produced by biological processes (such as photosynthesis by plants), and

• How much oxygen is being used up by abiotic and biological processes (for example, respiration or the decomposition of organic matter such as dead phytoplankton) in the water column or in the sediments and, at the sediment-water interface.

The pycnocline is the place in the water column where there is an abrupt change in density due to a change in salinity and/or temperature. The lower layer is saltier, colder, and denser than the surface layer and forms a "salt wedge," bringing salty water up the Bay and its tributaries from the ocean. If the pycnocline is very pronounced, it serves as a partial barrier between the upper layer and the lower layer, and little mixing occurs. This means that oxygen from the surface does not get down to the bottom layer, allowing the bottom lay to become depleted of oxygen.

High levels of algae produce oxygen during photosynthesis while the cells are alive and in the presence of sunlight. However, during the night photosynthesis shuts down, and these same cells use up the available oxygen in the process of respiration. After the algal cells die, they sink and bacteria decompose them; the decomposition process also uses up oxygen from the water column or sediment layer. If a pycnocline is present, dead algal cells fall to the lower layer (below the pycnocline) where they decompose. Thus the bottom waters can become rapidly depleted of oxygen, and oxygen from the upper layer is not well mixed into the lower layer because the pycnocline serves as a partial barrier.

In the Chesapeake Bay, dissolved oxygen levels are usually lowest in bottom waters during the summer months. Oxygen availability can drop to low levels that are likely to harm aquatic animals. Dissolved oxygen levels in the summer bottom waters below 2 mg/L are considered poor, and oxygen levels above 5 mg/L are considered good. 

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