Dissolved Oxygen Information Sheet

Background

An essential gas for healthy maintenance of lakes and rivers, dissolved oxygen (DO) is soluble microscopic molecules that are in the air spaces between water molecules (H2O). Dissolved oxygen can range from 0 mg/L to 20 g/L with higher amounts of DO corresponding to better water quality.

Oxygen from the atmosphere is naturally incorporated into water through surface diffusion and when water tumbles over rocks in rapids and riffles, over waterfalls, and from the waves caused by wind. Higher water flows and turbulence (after storms, snow melts) allow for higher rates of oxygen diffusion.

In addition, photosynthetic aquatic life such as phytoplankton (algae, some protists, and cyanobacteria), and macrophytes (flowering, leafy plants and mosses) produce oxygen during photosynthesis. Sunlight is needed for photosynthesis, so oxygen levels are greatest during the afternoon when there is the most intense light. At night, plants do not photosynthesize and produce oxygen. Instead they respire (respiration), which consumes oxygen. Therefore, dissolved oxygen levels are at their lowest right before dawn.

There is a strong connection between temperature and dissolved oxygen. Any activity that changes the temperature of water is also affecting the dissolved oxygen. Colder water has a higher capacity (can "hold" more) for dissolved oxygen because the gas molecules in water are moving slower and are more compact. At higher temperatures, dissolved oxygen and other gases in water move faster and spread farther apart, including out of the water. Also at higher temperatures, the water molecules may move faster and bump out oxygen. Since water has a high specific heat (it takes a long time to heat up), water temperatures are often highest in the late summer and early fall, after the warmest months of the year.

Higher temperatures affect the dissolved oxygen levels in other ways. With warmth, the metabolic rates (body processes) of fish and aquatic creatures increase, resulting in greater consumption of dissolved oxygen in the water. Higher temperatures also increase the rate of decomposition (another process that is the opposite of photosynthesis) which consumes oxygen, lowering the concentration of dissolved oxygen.

After determining the temperature and dissolved oxygen level (mg/L) of water, the percent saturation of oxygen can be determined. This gives a percentage for the amount of oxygen available in the water. For instance, if the temperature is 5°C and a test kit determined there to be 13.0 mg/L of DO, the stream is at 100% saturation, which is the exact, balanced amount expected to be in the stream. At 5°C, if the DO was only 9 mg/L, the stream is only at 70% percent saturation - something (possibly a pollutant) is keeping 30% of the oxygen from being in the water. Rivers and streams below 90% saturation may have an overabundance of oxygen demanding materials and organisms. See the Percent Saturation Chart on the following pages for more information.

Unlike surface waters, groundwater is nearly devoid of oxygen because there is no interaction with the atmosphere and no plant life. When groundwater flows up to the surface from a spring, it may not have DO right away. Because groundwater is usually cold during warm weather months, it is capable of obtaining dissolved oxygen quickly.

Human Impact

Fish and other aquatic organisms need oxygen to survive. When levels are reduced, they must alter their breathing patterns or lower their level of activity. It retards (slows) their development, causes reproduction problems (increased egg mortality and defects), or deforms them. Each aquatic organism has a different level of oxygen that it must have to survive. For instance, trout need more oxygen and colder waters than a carp, a warm water fish with a tolerance for low oxygen levels. If the oxygen levels in a stream are reduced long term, an ecological shift can occur involving the replacement of species not tolerant of low oxygen (mayflies nymphs, stonefly nymphs, caddisfly larva, trout) with more tolerant species (worms, midge larva, carp, largemouth bass, and bluegills). In extreme circumstances, oxygen depletion can cause all oxygen dependent species to move elsewhere or perish.

Along with limiting biodiversity and affecting the health of organisms, lack of oxygen makes water taste pour and smell bad. Bacterial breakdown of organic matter under anaerobic (non oxygenated) conditions forms sulfide, which can have a musty odor at low concentrations, and a "rotten eggs" smell at higher concentrations. Sewage treatment plants remove anaerobic conditions during treatment by aerating the wastewater.

So how can humans affect the levels of dissolved oxygen? Any actions that affect the temperature of the stream (see Temperature Information Sheet) will affect the DO levels. Humans can also disrupt the natural balance of photosynthesis and respiration/decomposition by accidentally adding fertilizers (nutrients) to a stream from agricultural fields and lawns. Extra fertilizers promote excessive plant growth, which at first adds plenty of oxygen to the water. But eventually those extra plants die, and aerobic (oxygen demanding) bacteria decompose them, consuming oxygen in the process. This process is known as eutrophication.

In addition to extra dead plants, other organic wastes (anything once part of a living plant or animal) can be added to waterways by humans: sewage, animal waste from farms, organic materials from soil erosion, waste from industry, paper mills, and food processing plants. These organic wastes also need to be broken down by bacteria, using up oxygen. Biochemical Oxygen Demand (BOD) is another water test that measures the quantity of oxygen used by microorganisms in the break down of organic matter. High levels of BOD are bad for a stream.

Humans can also add too much oxygen to the water, which can be toxic to aquatic organisms. One common example is turbulent water released from a dam.

Water Quality Criteria

For aesthetic purposes, the EPA requires only enough oxygen needed to maintain aerobic conditions. Specifically, the minimum amount of DO required to prevent negative effects on organisms is at least 5 mg/L in most places. Cold water fish (for example, trout) require at least 6 mg/L; warm water fish, 5 mg/L.

Example Data for Dissolved Oxygen - French Creek

The Pennsylvania Department of Environmental Protection (DEP) has systematically collected water quality data from French Creek, at Meadville since about 1973. A summary of these data is below to provide an understanding of past values of dissolved oxygen and demonstrate relationships of past dissolved oxygen levels.

Average: 10.05 mg/L

Maximum: 13.80 mg/L

Minimum: 6.2 mg/L

The graph shows a change in the concentration of dissolved oxygen from February through December 1991. The highest DO concentration is in the winter months while the lowest concentrations are in the late summer months of August and September. This change in dissolved oxygen concentration shows an inverse relationship to changes in temperature. Dissolved oxygen concentrations can be affected by natural as well as human sources. Temperature is a primary factor to the amount of dissolved oxygen contained in the waters of French Creek - water at lower temperatures contains higher concentrations of DO. In addition to the natural variation attributed to temperature, artificial influences of French Creek can cause lower DO concentrations. Inputs of nutrients (phosphates and nitrates) from sewage and fertilizers can cause algae to grow and eventually die. The death and decomposition of the algae use oxygen and lowers DO levels in streams in the watershed.