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March 1, 2019

Why are Clarity and Dissolved Oxygen Important?

Water Quality

Water Clarity is the measurement of how far sunlight can pass through the water column. Sunlight is one of the key elements needed for underwater grasses to grow. When the water column’s clarity is reduced, the underwater grasses do not receive the sunlight necessary for growth. Excess sediment is the leading factor for the South River’s poor health.

Because of their small size, the particles of sand, silt and clay that we call “sediment” often float through the water rather than settling to the bottom, and can be carried long distances during rainstorms. When there are too many sediment particles suspended in the water, the water becomes cloudy and muddy-looking. Cloudy water does not allow sunlight to reach the plants that grow on the bottom of the Bay’s shallows. Without sunlight, these plants—including underwater grasses—die, which affects the young fish and shellfish that depend on them for shelter.

Impacts of Excessive Sediment

  • Nutrients and chemical contaminants can bind with sediment, spreading through the Bay and its waterways with particles of sand, silt and clay. Fish and shellfish that live and feed on or near contaminated sediment can become contaminated themselves, triggering fish consumption advisories in various portions of the watershed.
  • Excess sediment can smother oysters and other bottom-dwelling species.
  • Accumulating sediment can clog ports and channels, affecting commercial shipping and recreational boating.

How Do We Measure Water Clarity?

Despite advances in science, researchers still use a technique for tidal waters that is hundreds of years old to measure clarity: a secchi disk. This black and white sectioned disk is lowered into the water until the user can no longer see the division between the colors. The disk is then slowly raised until the user begins to make out the distinct sections again. This depth is used as a measureof the transparency of the water. The penetration of sunlight is crucial for the growth of underwater grasses, which acts as a filter, dissolved oxygen producer, and a vital nursery habitat for countless aquatic species. Scientists predict that underwater grasses can survive when the water clarity averages 1.0 meters during the growing season.

For freshwater streams, measuring clarity is more complicated. The Federation traditionally used a conductivity probe on a hand held YSI sonde that is placed in the stream. However, conductivity doesn’t directly measure clarity, but rather the electrical conductivity in a solution to monitor the amount of nutrients, salts or impurities in the water. Thus, if there is road salt dissolved in the stream, the stream may appear clear, but still have a high conductivity reading. Recently, the Federation has invested in a turbidity probe, which does measure the amount of suspended particles by measuring how the light is refracted, but the particles could be sediment, algae, bacteria or other. Both of these methods only provide the percentage of contaminates in the water. Without knowing the volume of water going past the probe, you cannot get the total amount of contaminates or load. With all our modern science, we do not know the amount or sediment pouring into the South River within an order of magnitude. We can only tell whether it is improving or worsening.  Yet sediment remains our largest source of pollution for the South River, both by volume and impact.

Dissolved Oxygen


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 provides 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.

In 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.

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.