Dissolved Oxygen

What is Dissolved Oxygen?

Dissolved Oxygen is the measurement of how much oxygen is present in the water and available to aquatic organisms. Dissolved Oxygen is measured as a concentration in milligrams per liter (mg/L) and as a percent-saturation, which indicates how much oxygen the water is capable of holding at any given temperature. Dissolved Oxygen enters non tidal freshwater streams and rivers through physical mixing at shallow stream riffles and man-made structures and as a by-product of photosynthesis by algae and aquatic plants.

Why is Dissolved Oxygen important?

Dissolved Oxygen is an essential element for the survival of most aquatic animal life, such as the fin-fish and stream macroinvertebrate insects that breathe underwater. These aquatic animals may avoid areas of water with very low levels of Dissolved Oxygen. Sometimes, the aquatic animals will become trapped in areas with low-levels of Dissolved Oxygen, which means that they may suffocate and die. Oftentimes, very low levels of Dissolved Oxygen result when algae blooms are decomposed by bacteria, which rapidly consume Dissolved Oxygen in the water, or when chemical pollutants, such as sulfur, react to rapidly deplete levels of Dissolved Oxygen.

How do we measure Dissolved Oxygen?

Dissolved Oxygen is measured with a probe that is lowered by staff or volunteers into the water at each station location. Dissolved Oxygen readings are recorded half-way between the water’s surface and the river bottom. We assess the Dissolved Oxygen data using the State of Maryland’s instantaneous numeric threshold of 5.0 mg/L for protecting the ecological health of fish populations in the State’s freshwater streams and rivers.