Dissolved oxygen (DO) is the amount of oxygen that is present in the water. It is measured in milligrams per liter (mg/L), the number of milligrams of oxygen dissolved in a liter of water.
Why is dissolved oxygen important?
Humans use their lungs to inhale oxygen from the air. But worms, fish, crabs and other underwater animals use gills to get oxygen from the water. As water moves across an animal’s gills, oxygen is removed and passed into the blood.
Gills work better when there is more oxygen in the surrounding water. As dissolved oxygen levels decrease, it becomes harder for animals to get the oxygen they need to survive.
How much dissolved oxygen do animals need?
Scientists generally agree that the Bay’s critters need dissolved oxygen concentrations of 5.0 mg/L or more to live and thrive. However, the amount of oxygen an animal needs varies depending on how large or complex the animal is and where it lives.
- Worms and clams that live in the Bay's muddy bottom—where oxygen levels are naturally low—need dissolved oxygen concentrations of at least 1 mg/L.
- Fish, crabs and oysters that live or feed along the bottom require dissolved oxygen concentrations of 3 mg/L or more.
- Spawning migratory fish and their eggs and larvae need up to 6 mg/L during these sensitive life stages.
How does oxygen get into the water?
Oxygen can get into the water in several ways:
- Oxygen from the atmosphere dissolves and mixes into the water’s surface.
- Algae and underwater grasses release oxygen during photosynthesis.
- Water flows into the Bay from streams, rivers and the ocean. Ocean waters generally have more oxygen. River waters are fast-moving, which helps oxygen from the air mix in.
How do low-oxygen areas form?
Hypoxic, or low-oxygen, areas are regions with less than 2 mg/L of dissolved oxygen. Anoxic, or no-oxygen, areas are regions with less than 0.2 mg/L of dissolved oxygen. These areas are often called “dead zones,” because most animals cannot survive there. Areas in the Bay that have low dissolved oxygen levels are the result of a complex interaction of several natural and man-made factors, including temperature, nutrient pollution, water flows and the shape of the Bay's bottom.
Temperature limits the amount of oxygen that can dissolve in water: water can hold more oxygen during winter than during the hot summer months. However, even at the warmest temperatures seen in the Bay (around 91 degrees Fahrenheit), water is capable of having dissolved oxygen concentrations of 6 to 7 mg/L. So, although high temperatures can influence dissolved oxygen levels, temperature is not the only cause of low-oxygen areas found in the Bay each summer.
Excess nutrients in the water (known as eutrophication) can fuel the growth of algae blooms. Oysters, menhaden and other filter feeders eat a portion of the excess algae, but much of it does not end up being consumed. The leftover algae die and sink to the Bay’s bottom, where they are decomposed by bacteria. During this process, bacteria consume oxygen until there is little or none left in these bottom waters.
Flow of water
The division between water flowing from the ocean and from the Bay’s freshwater rivers and streams can also influence dissolved oxygen levels. 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 floats on top of it—although wind and other strong mixing forces may change this pattern.
The boundary where the fresh water layer meets the salt water layer below is called the pycnocline. The pycnocline acts as a physical barrier that prevents the two layers from mixing together. During the summer, when algae-consuming bacteria are most active, the pycnocline cuts off oxygen-deprived bottom waters from oxygen-rich surface waters. This can create large areas of low- or no-oxygen at the bottom of the Bay.
Shape of the Bay's bottom
The bottom of the Bay is not flat—rather, it has varying shallow and deep areas. In certain bowl-shaped areas at the bottom of the Bay, the pycnocline can act like a “lid” that cuts off bottom waters from receiving any oxygen. This phenomenon often takes place each summer in:
- 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, and
- The lower part of Eastern Bay, near Kent Island.