Oxygen is a critical necessity for life. In aquatic systems oxygen is found in the form of dissolved oxygen (DO) and without it, the fish, crabs, and oysters cannot survive. Low dissolved oxygen concentrations can lead to reduced growth and reproduction rates, change the distribution and behavior patterns of the aquatic organisms, and can lead to death.

Dissolved oxygen is most commonly measured in milligrams per liter (mg/l) and can enter the water through photosynthesis from aquatic grasses, phytoplankton, or algae as well as by the physical process of wind mixing. Dissolved oxygen levels are usually better on those days, or seasons, when there is greater wind mixing, however, the ecosystem quickly regresses back to being dominated by low dissolved oxygen in the absence of wind if nutrient levels are otherwise excessive. Wind, rain, water temperature, and tide are constantly affecting dissolved oxygen readings. Understanding the trends in dissolved oxygen requires many readings, in all seasons, all weather, and in both rainy and dry years to start to see a pattern.

Why We Care about DO:

Low dissolved oxygen levels can be caused by algae blooms which are fuelled by excessive nutrients (usually from fertilizer or sediment—The South River is surrounded by sediment that naturally has Phosphorus bonded to it). Algae is short lived and the bacteria that decompose the algae use up immense amounts of dissolved oxygen. These pockets of no, or very low dissolved oxygen levels are known as Dead Zones, due to their lethal nature to aquatic life. The South River regularly has dead zones in the spring and late summer, which is very disruptive to the river’s ecosystem.

How we measure Dissolved Oxygen (DO)

In tidal areas, dissolved oxygen (DO) is measured with a water quality sonde, an electronic device that Intern Rachel Denby with Hydrolabprovides immediate readings for various physiochemical parameters. The Federation uses a Hydrolab sonde that is lowered into the water from a boat. The Hydrolab takes readings every ¼ or ½ meter to paint a picture of the water column environment. Confusingly, surface readings are really taken 1 meter down to mitigate the affect of wind on the reading. For our interactive web map we created in 2018, we use bottom dissolved oxygen readings which are actualy taken from the bottom of the water column where DO is commonly the lowest (and harshest). Understanding the environment at the bottom of the South River and its tributaries allows us to protect bottom dwellers, like oysters, clams, and worms that are a vital part of the river’s food chain.

YSIsondeIn non-tidal streams, or upstream areas not influenced by tides, dissolved oxygen is measured with a smaller water quality device. The Federation uses a YSI Pro, which is lowered into the flowing stream to take physiochemical readings. It provides immediate readings similar to the sonde described above. Adequate DO levels are critical for the survival of freshwater fish as well as migratory fish using these upstream environments as spawning grounds. Both water quality devices are calibrated in the lab before and after a day’s use to ensure accurate readings in the field.

For our Church Creek Restoration Projects, we use continuous monitoring sondes that takes measurements of several different water quality parameters every 15 minutes, including dissolved oxygen. We are excited to work with the Smithsonian Environmental Research Center to study the cumulative impact of our many projects installed in Church Creek has on water quality. We are looking forward to the day when Church Creek is no longer our most polluted creek on the South River.

DO chart3

Impact on Aquatic Life

Dissolved oxygen concentrations of 5.0 mg/L or greater will allow marine creatures to live and thrive. 2mg/l or less is considered Hypoxic, and 0.2mg/l or less is considered anoxic. The oxygen requirements vary from species to species, the complexity of the species and where the animal resides in the South River. Worms and small clams living in the South River’s muddy bottom, where oxygen levels are naturally low, only need dissolved oxygen 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 or greater, and spawning migratory fish, their eggs and larvae need up to 6 mg/l.