“Scientific” oriented discussion on Dissolved Oxygen (DO)

From: Tony Caputo

Water quality in the Inland Bays (IB) has dramatically deteriorated over recent years as development and other land disturbing activities have taken place, and pollutants have accumulated.  Unfortunately, some of the waters in the IB are polluted and others can be vastly improved.   One of the major factors in the water deterioration and pollution is low Dissolved Oxygen (DO).

DO is the oxygen molecules dissolved in water, and it’s a major indicator of water quality.

In water DO is usually is expressed in milligrams per liter (mg/L), or parts per million (ppm), or percent of saturation.  At sea level, typical 100% DO concentrations in fresh water will range from 7.56 mg/L (7.56 parts oxygen / 1,000,000 parts water) at 86 degrees Fahrenheit to 14.62 mg/L at 32 degrees Fahrenheit.

DO is directly affected by water temperature, atmospheric pressure and salinity. Cold water can accept and hold more oxygen than warm water. As temperature rises water releases some of its oxygen into the air.  Water also holds less DO at higher elevations because there is lower atmospheric pressure.  Solubility of DO also decreases as salinity increases.  The waters in the IB are brackish, that is a mixture of seawater with fresh water known as salinity.  Salinity in the IB varies by location and water movement.

Many factors degrade IB waters and their concentrations can affect the severity of pollution.  For example, some “potential” pollutants like nitrogen are good at certain levels, but very harmful at other levels.  It is relatively easy to measure some potential “pollutants” like nitrogen, and the “results of pollution” like low Dissolved Oxygen (DO).  However, to be useful they need to be measured over time, in certain locations, and under certain conditions.  Conditions effecting DO can also evolve over time.   For example, a new housing development can reduce permeable surfaces, increase the use of fertilization and rain water runoff.

The IB aquatic life, such as fish, crabs, zooplankton, oysters and certain plants require a sufficient level of DO in order to survive.  Obvious “Fish kills” have occurred in the bays due to low DO.  Water bodies can also “die” due to extremely low DO in a process called “eutrophication”.  When DO concentrations drop below 4 mg/l, the IB’s more sensitive organisms become stressed, especially if exposed to these conditions for prolonged periods.  Fish tend to be more easily stressed, while some bottom-dwelling organisms such as certain worms and can survive at levels down to 1 mg/l.  Most of the Bays’ more visible living resources will not survive exposure to waters of less than 1 mg/l DO for more than a few hours.

There are electronic meters that can measure DO right at the water site, while measuring potential pollutants like bacteria requires sending refrigerated samples to laboratories.  In addition to direct variables of temperature, atmospheric pressure and salinity, there are indirect variables that affect DO measurements such as vegetation, seasonality, sunlight, pH, water movement, time of day and rain volumes.   Accordingly DO levels can vary significantly by location.

The IB’s greatest contributors to surface and groundwater low DO levels are the two nutrients of nitrogen and phosphorus.  They enter our waters directly and indirectly and produce “highly enriched” nutrient environments in some locations.  Nitrogen and phosphorus are essential for plant and animal growth; however excess amounts have been entering our bays.   They can enter from fertilization run off from farms and lawns, discharges from municipal and private wastewater treatment facilities, and urban storm water runoff that contains nitrogen from sources far off.

Water quality deteriorates as aquatic plant growth accelerates due to the highly enriched nutrient environment, and accordingly DO levels drop.  As algae dies and decomposes it consumes DO. This process of decomposition is called Carbonaceous Biochemical Oxygen Demand (CBOD).   Excess bacteria, harmful in and of itself, can also consume oxygen as organic matter decays.

DENRC algae harvester hard at work in the Brandywine Canal in South Bethany
DENRC algae harvester hard at work in the Brandywine Canal in South Bethany
In the spring algae becomes detached and begin to build up along the bays’ shorelines and in canals, these buildups are both environmental and health hazards.  To combat the buildups the DNREC’s Macro-algae Harvesting Team, under the direction of the Division of Soil and Water Conservation harvests the algae.  The Harvesting Program started 1997 and generally operates a water harvester from early April through August. DNREC conducts weekly inspections and takes calls from residents throughout the Inland Bays to locate potential or known problem areas for harvesting. This Program is a valuable asset in DNREC’s efforts to protect, preserve, and restore the natural resources of the bays.  The harvesting has a considerable positive effect on water quality as it collects and removes substantial amounts of algae before it begins to decompose and drive DO levels low.  The harvesting also reduces potential water turbidity.


Measuring DO and other water quality factors in the IB is performed by individuals using hand held meters and meters stationed semi-permanently.  In the Inland Bays there is a “Citizens Monitoring Program” that works in close cooperation with the University of Delaware Sea Grant Program.  Since 1991 the Citizen Monitoring Program’s many volunteers have been taking water samples on a regular basis throughout Delaware’s coastal watershed to measure a broad range of important water quality characteristics. The data they gather provides UD Sea Grant Program scientists and resource managers with a clearer picture of the estuary’s health and the trend information needed to understand and manage the ecosystem.  You can find many reports on DO in the Inland Bays at the University of Delaware Citizen Monitoring Program website at: http://citizen-monitoring.udel.edu/reports.shtml.

The Inland Bays Foundation is always looking for new members. We are an all-volunteer, non-profit environmental organization dedicated to improving water quality in our community. If you are interested in cleaning up Delaware’s Inland Bays and tributaries–we need you! Membership is only $25 and you can sign up on line or make a personal donation.  Help us move forward with any level of financial help. The state of our Inland Bays depends on your generosity.  http://www.inlandbaysfoundation.org