Dissolved Oxygen (DO) is found in microscopic bubbles ofoxygen that are mixed in the water and occur between water molecules. DO isa very important indicator of a water body's ability to support aquatic life.Fish "breathe" by absorbing dissolved oxygen through their gills. Oxygenenters the water by absorption directly from the atmosphere or by aquatic plant and algae photosynthesis. Oxygen is removed from the water byrespiration and decomposition of organic matter.
Dissolved Oxygen can be measured with an electrode and meter or with field test kits. The electronic meter does not measure oxygen directly; rather, it uses electrodes to measure the partial pressure of oxygen in the water, which is converted to oxygen mass weight concentration. The field test kits (such as a drop bottle, a microburet, or a digital titrator)involve adding a solution of known strength to a treated sample of water from the stream. The amount of solution required to change the color of the sample reflects the concentration of DO in the sample. The amount of oxygendissolved in water is expressed as a concentration, in milligrams per liter(mg/l) of water. Dissolved oxygen levels are also often reported in percentsaturation. Temperature affects DO concentrations, and calculating thepercent saturation will factor out the effect of temperature. The"saturation level" is the maximum concentration of dissolvedoxygen that would be present in water at a specific temperature, in theabsence of other factors. Scientists have determined the saturation DO levelfor various temperatures. Saturation levels also vary with elevation.Percent saturation is calculated by dividing the measured dissolved oxygenconcentration by the saturation level and multiplying by 100.
This equation is shown as:
% Saturation = (DO / Saturation Level) x 100
Factors Affecting DO
Volume and velocity of water flowing in the water body
In fast-moving streams, rushing water is aerated by bubbles as itchurns over rocks and falls down hundreds of tiny waterfalls. These streams,if unpolluted, are usually saturated with oxygen. In slow, stagnant waters,oxygen only enters the top layer of water, and deeper water is often low inDO concentration due to decomposition of organic matter by bacteria thatlive on or near the bottom of the reservoir.
Dams slowwater down, and therefore can affect the DO concentration of water downstream. If water is released from the top of the reservoir, it can bewarmer because the dam has slowed the water, giving it more time to warm upand lose oxygen. If dams release water from the bottom of a reservoir, thiswater will be cooler, but may be low in DO due to decomposition of organicmatter by bacteria.
Climate/Season
Thecolder the water, the more oxygen can be dissolved in the water. Therefore, DO concentrations at one location are usually higher in the winter than inthe summer.
During dry seasons, water levels decrease and the flowrate of a river slows down. As the water moves slower, it mixes less withthe air, and the DO concentration decreases. During rainy seasons, oxygenconcentrations tend to be higher because the rain interacts with oxygen inthe air as it falls.
More sunlight and warmer temperaturesalso bring increased activity levels in plant and animal life; depending onwhat organisms are present, this may increase or decrease the DOconcentration.
The type and number oforganisms in the water body
During photosynthesis, plantsrelease oxygen into the water. During respiration, plants remove oxygen fromthe water. Bacteria and fungi use oxygen as they decompose dead organicmatter in the stream. The type of organisms present (plant, bacteria, fungi)affect the DO concentration in a water body. If many plants are present, thewater can be supersaturated with DO during the day, as photosynthesis occurs. Concentrations of oxygen can decrease significantly during the night, due to respiration. DO concentrations are usually highest in the late afternoon,because photosynthesis has been occurring all day. For an example of how DOcan vary from day to night, select here.
Altitude
Oxygen is more easily dissolved into water atlow altitudes than at high altitudes, because of higher atmosphericpressure.
Dissolved or suspended solids
Oxygen is more easily dissolved into water with low levels of dissolved or suspended solids. Waters with high amounts of salt, such as the ocean (which contains about 35 grams of salt for each 1000 grams of water) have lowconcentrations of DO. Freshwater lakes, streams, and tap water generallycontain much less salt, so DO concentrations are higher. As the amount ofsalt in any body of water increases, the amount of dissolved oxygendecreases. An increase in salt concentration due to evaporation of waterfrom an ecosystem tends to reduce the dissolved oxygen available to theecosystems inhabitants.
Runoff from roads and other paved surfacescan bring salts and sediments into stream water, increasing the dissolvedand suspended solids in the water.
Amount of nutrients in the water
Nutrients are food for algae, and water with highamounts of nutrients can produce algae in large quantities. When these algaedie, bacteria decompose them, and use up oxygen. This process is called eutrophication. DO concentrations can drop too low for fish to breathe,leading to fish kills. However, nutrients can also lead to increased plantgrowth. This can lead to high DO concentrations during the day asphotosynthesis occurs, and low DO concentrations during the night whenphotosynthesis stops and plants and animals use the oxygen duringrespiration. For an example of how DO can vary from day to night, select here.
Nitrate and phosphateare nutrients. Nitrate is found in sewage discharge, fertilizer runoff, andleakage from septic systems. Phosphate is found in fertilizer and somedetergents. 
Organic Wastes
Organic wastesare the remains of any living or once-living organism. Organic wastes thatcan enter a body of water include leaves, grass clippings, dead plants oranimals, animal droppings, and sewage. Organic waste is decomposed bybacteria; these bacteria remove dissolved oxygen from the water when they breathe. If more food (organic waste) is available for the bacteria, morebacteria will grow and use oxygen, and the DO concentration will drop.
Directly downstream from where sewageeffluent is discharged to a river, DO content often decreases, because ofthe increase in growth rate of bacteria that consume the organic mattercontained in the effluent. The degree and extent of the DO "sag"depends on the Biological Oxygen Demand (BOD) of the effluent (how muchoxygen the effluent can consume) (Giller and Malmqvist, 1998).
Riparian Vegetation
Shading tends to lower average summer temperature and reducethe daily duration of higher temperature. Removing trees reduces shade onthe creek, allowing the sun to warm the water. This can affect DOconcentrations in different ways. As mentioned above, in general, as watertemperature increases, DO drops. Also, the bare soil exposed from removingthe tree can erode, increasing the amount of dissolved and suspended solidsin the water. This also leads to a decrease in DO concentrations. However,direct sunlight, along with increased nutrients can increase the growth rateof aquatic plants. These plants release oxygen to the water during the day,but then remove oxygen from the water at night. This can cause DOconcentrations to become very high during the day, then very low during thenight. For an example of how DO can vary from day to night, select here .
Groundwater Inflow
Theamount of groundwater entering a river or stream can influence oxygen levels. Groundwater usually has low concentrations of DO, but it is also oftencolder than stream water. Therefore, groundwater may at first lower the DOconcentration, but as groundwater cools the stream or river, the ability ofthe water to hold oxygen improves.
Water Quality Standards and Other Criteria Regarding DO
Colorado Department of Public Healthand Environment Water Quality Control Division (CDPHE-WQCD) regulations (5CCR 1002-31) state that waters to be used for domestic water supply shouldnot have DO concentrations below 3 milligrams per liter (mg/l) (Reg. 31 - BasicStandards and Methodologies for Surface Water).
CDPHE-WQCD regulations state that waters used for recreation (both primary and secondary contact) should not have DO concentrations below 3 milligrams per liter(mg/l).
CDPHE-WQCD regulations state that waters classifed as
"Class 1 Cold Water Aquatic Life" should not have DOconcentrations below 6 mg/l, and not below 7 mg/l during spawning. Watersclassified as "Class 1 Warm Water Aquatic Life" should not have DOconcentrations below 5 mg/l. (These are 1-day minima)
Very high DO concentrations can also be harmful to aquatic life. Fish inwaters containing excessive dissolved gases may suffer a condition in whichbubbles of oxygen block the flow of blood through blood vessels, causingdeath. Abrupt changes in dissolved oxygen induce stress and subsequentlymake fish more susceptible to disease.
The ideal dissolved oxygenconcentration for many fish is between 7 and 9 mg/l; the optimal DO foradult brown trout is 9-12 mg/l. Most fish cannot survive at concentrationsbelow 3 mg/l of dissolved oxygen.
Other Information aboutDO
When dissolved oxygen concentrations drop, major changes in thetypes and amounts of aquatic organisms found living in the water can occur.Species that need high concentrations of dissolved oxygen, such as mayflynymphs, stonefly nymphs, caddisfly larvae, pike, trout, and bass will moveout or die. They will be replaced by organisms such as sludge worms,blackfly larvae, and leeches which can tolerate lower dissolved oxygenconcentrations. Waters that have low dissolved oxygen sometimes smell badbecause of waste products produced by organisms that live in low oxygenenvironments.
Because of the relationship between temperature, rate ofphotosynthesis, and DO, fish kills usually occur in late summer just before dawn.
Very low DO concentrations can result in mobilization of tracemetals.
A fish that is under stress caused by low oxygen levels in thewater is more susceptible to poisoning by insecticides or heavy metals(Caduto, 1990).
