What is dissolved oxygen?
Dissolved oxygen is the measurement of the amount of oxygen freely available in water.

What factors influence the amount of dissolved oxygen in water?
1. Temperature has a big effect on amount of dissolved oxygen water can hold. Warmer water can hold less dissolved oxygen than colder water. Consequently, this may also vary depending on the season, weather, the time of day, and the amount of thermal pollution.
2. When there is an overabundance of organic matter like dead algae, aquatic aerobic bacteria can grow rapidly. These bacteria consume oxygen during respiration and as a result, the amount of dissolved oxygen in the watershed is decreased.
3. Low atmospheric pressure found at higher altitudes slightly decreases the solubility of dissolved oxygen.
4. The various minerals dissolved in water can lower the water’s capacity to hold oxygen. Consequently, a lower salinity equals a higher potential for dissolved oxygen concentration.
5. Mixing of air and water caused by swiftly flowing water over rocks, by wind, or thermal upwelling, increases dissolved oxygen concentrations. This process is called aeration. A body of water that is very stagnant may result in very low dissolved oxygen concentration.

What is the minimal amount of dissolved oxygen needed for most aquatic plants and animals to survive?
1. Dissolved oxygen can range form 0-18 mg/L, but most natural water systems require 5-6 mg/L to support a diverse aquatic population.
2. A dissolved oxygen level of 9-10 mg/L is considered very good. Generally, a higher dissolved oxygen reading indicates a better water quality.
3. A dissolved oxygen reading of below 3 mg/L is very stressful to most aquatic organisms and may result in death through suffocation.

What factors are affected by the amount of dissolved oxygen in your watershed?
1. Increases in water temperature can cause changes in aquatic plants. For example, as the temperature increases, the rate of photosynthesis increases. As photosynthesis increases, the number of aquatic plants increases. This can lead to a number of plants or an algal bloom. The faster plants grow, the faster they die. When they die they are decomposed be aerobic bacteria, which consume oxygen in this process. This event may also decrease the amount of dissolved oxygen in the water, which organisms need to survive.
2. Decreases in dissolved oxygen can cause changes in the types and numbers of aquatic macroinvertebrates and vertebrates that live in the water ecosystem. Species that cannot tolerate decreases in dissolved oxygen levels include mayfly nymphs, stonefly nymphs, caddisly larvae, and beetle larvae, pike, bass, and trout. As the dissolved oxygen decreases, these pollutant-intolerant organisms are replaced by pollutant-tolerant organisms such as sludge worms, leeches, and black fly larvae.