Dissolved Oxygen

Dissolved oxygen (DO) refers to the amount of oxygen that is present in the water. It is measured in units of milligrams per liter (mg/L), or the milligrams of oxygen dissolved in a liter of water.

Why is dissolved oxygen important?

Just like humans, all of the Bay's living creatures—from the worms that inhabit its muddy bottom to the fish and crabs found in its waters—need oxygen to survive. While humans use their lungs to extract oxygen from the air, worms, fish and crabs use some form of gills to get oxygen from the water.

As water moves across these gills, DO is removed and passed into the animal's blood. Gills operate more efficiently as the oxygen concentration of the surrounding waters increases. Conversely, as oxygen decreases, it becomes harder for animals to get the oxygen they need to survive.

How much dissolved oxygen do the Bay's animals need?

Bay scientists generally agree that DO concentrations of 5.0 mg/L or greater will allow the Bay's aquatic creatures to live and thrive. However, DO requirements vary from species to species based on habitat and evolutionary history.

• Worms and small clams living in the Bay's muddy bottom, where oxygen levels are naturally low, only need DO concentrations of at least 1 mg/L.
• Fish, crabs and oysters that live or feed along the bottom require oxygen concentrations of 3 mg/L and greater.
• Spawning migratory fish and their eggs and larvae need up to 6 mg/L during these sensitive life stages.

Areas with less than 0.2 mg/L of DO are called anoxic and are unable to support most forms of life.

How does oxygen get into the Bay's waters?

Sources of DO to the Bay include:

• Oxygen from the atmosphere that dissolves and mixes into surface waters.
• Algae and underwater bay grasses, which release oxygen during photosynthesis.
• Water flowing into the Bay from streams, rivers and the ocean. Ocean waters generally have higher DO levels, while fast-moving waters from rivers flowing into the Bay help mix in oxygen from the air.

Why do some areas of the Bay have low dissolved oxygen levels?

Most areas of the Bay with low DO are the result of a complex interaction of several natural and man-made factors, including temperature, nutrient pollution, how water flows in the Bay and the shape of the Bay's bottom.

Temperature

The amount of oxygen that can dissolve in water is strongly limited by the temperature of the water. The colder the water, the more oxygen it can hold. Therefore, the Bay waters have a greater capacity to hold DO during the cold winter months than they do during the summer.

However, even at the warmest temperatures seen in the Bay (around 91 degrees), water is capable of having DO concentrations of 6 to 7 mg/L. So while high water temperatures can impact DO levels, they are not solely responsible for the areas of low oxygen seen in the Bay each summer.

Nutrient Pollution

High amounts of nutrients, or nutrient pollution, in the water can fuel the growth of excessive amount of algae, called algae blooms. A portion of algae is consumed by filter feeders like oysters and menhaden. Unconsumed or “leftover” algae die and sink to the bottom of the Bay, where they are decomposed by bacteria. During this process, bacteria consume oxygen until there is little or no DO left in these bottom waters.

Flow of Water

Another factor influencing DO in the Bay is the division between water flowing in from the ocean and out of the watershed's many rivers and streams.

• Water flowing from the ocean is generally salty and cooler, while river water is fresh and warmer. Because of these differences, river water weighs less than ocean water and will actually float on top of ocean water. (Winds and other strong mixing forces may change this pattern.)
• The zone or boundary where the fresher water layer on the surface meets the saltier water layer below is called the pycnocline. The pycnocline can be a physical barrier that prevents mixing or exchange between the two layers.
• During warm summer months when algae-consuming bacteria are most active, the pycnocline cuts off oxygen-deprived bottom waters from oxygen-rich surface waters. This can cause large areas of low or no DO in the Bay.

Shape of the Bay's Bottom

The Bay's bottom is not flat; rather, it has varying shallow and deep depths, collectively referred to as the Bay's bathymetry. The pycnocline can interact with the Bay's bathymetry to isolate bottom waters from surface waters. In certain “bowl-shaped” areas of bathymetry, the pycnocline can act as a “lid” that isolates bottom waters from any source of oxygen. Some areas where this takes place regularly each summer are:

• The middle of the Bay's mainstem, from the Bay Bridge south to the mouth of the Potomac River.
• The lower Chester, Potomac and Rappahannock rivers.
• The lower part of Eastern Bay, near Kent Island.