Dissolved oxygen (DO) is the amount of oxygen that is present in the water. It is measured in milligrams per liter (mg/L), or the number of milligrams of oxygen dissolved in a liter of water.
Just like humans, all of the Chesapeake Bay's living creatures – from the fish and crabs that swim through its waters to the worms that bury themselves in its muddy bottom – need oxygen to survive.
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.
Scientists generally agree that the Bay’s creatures 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.
Areas with less than 0.2 mg/L of dissolved oxygen are called anoxic. Most animals cannot live in these areas, which is why they are often called “dead zones.”
Oxygen gets into the water when:
Most areas in the Bay that have low dissolved oxygen levels are the result of a complex interaction of several natural and man-made factors. These include temperature, nutrient pollution, how water flows in the Bay, and the shape of the Bay's bottom.
Temperature limits the amount of oxygen that can dissolve in water. The Bay’s waters 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 6-7 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.
Too many nutrients in the water (called 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.
Another factor influencing dissolved oxygen levels is the division between water flowing in from the ocean and out of the Bay’s freshwater 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 actually floats on top of it. (Wind and other strong mixing forces may change this pattern.)
The boundary where the fresh water layer meets the saltier 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.
The Bay's bottom is not flat; rather, it has varying shallow and deep areas. In certain bowl-shaped areas of the Bay's bottom, the pycnocline can act like a “lid” that cuts off bottom waters from receiving any oxygen.
This phenomenon often takes place each summer in: