Pollution introduces significant stress into aquatic environments by severely limiting the oxygen available for living organisms. Dissolved oxygen (DO) is the oxygen gas physically mixed into water, which is necessary for nearly all aquatic life to survive. When pollution enters a water body, it initiates biological and chemical reactions that ultimately deplete this oxygen supply, fundamentally altering the water’s ability to support a healthy ecosystem. The health of a water body is often directly judged by its DO concentration, making the impact of pollution a primary concern for environmental scientists.
Understanding Dissolved Oxygen
Dissolved oxygen is the amount of gaseous oxygen physically dissolved in water, measured in milligrams per liter (mg/L) or parts per million (ppm). This oxygen enters the water primarily through diffusion from the atmosphere and as a byproduct of photosynthesis by aquatic plants and algae. Moving water, like fast-flowing streams or wind-agitated surfaces, increases the rate of atmospheric oxygen absorption, which helps maintain healthy levels. The ability of water to hold oxygen is directly tied to its temperature and pressure, a concept known as saturation. Colder water naturally holds more dissolved gas than warmer water, meaning the maximum possible DO concentration is always higher in winter than in summer.
Oxygen Consumption by Organic Pollutants
The most direct way pollution depletes oxygen is through the introduction of organic matter, such as sewage, decaying plant material, or food processing waste. These organic pollutants serve as food for vast populations of aerobic bacteria. To break down this complex waste, these bacteria must consume dissolved oxygen from the water, a process called aerobic respiration. The amount of oxygen consumed during this microbial decomposition is measured as Biochemical Oxygen Demand (BOD). A high BOD value indicates a large quantity of biodegradable pollution is present, exerting a heavy demand on the oxygen supply.
Secondary Factors: Nutrients and Temperature
Pollution affects dissolved oxygen through nutrient loading and thermal changes. Excess nutrients (nitrogen and phosphorus) from agricultural runoff and wastewater trigger eutrophication, causing explosive algal blooms. While algae produce oxygen during the day, dense populations block sunlight, stopping oxygen production by deeper plants. When the bloom dies, the organic material is consumed by bacteria, which consumes huge quantities of DO, leading to severe depletion, a condition known as hypoxia. Separately, thermal pollution occurs when industries discharge heated cooling water, immediately lowering the maximum possible DO concentration because warmer water holds less dissolved oxygen.
Consequences for Aquatic Ecosystems
The severe reduction of dissolved oxygen, particularly below 2 to 3 mg/L, results in hypoxia, creating dangerous conditions for aquatic organisms. Fish are especially sensitive to these drops because they rely on gills for efficient gas exchange, and they begin to experience stress when DO levels fall below 5.0 mg/L. More sensitive species, such as trout and salmon, require higher DO concentrations (often between 6 and 7 mg/L). When DO concentrations remain extremely low (typically below 1 to 2 mg/L), mass mortality events known as fish kills occur. This oxygen depletion leads to a significant loss of biodiversity and can result in the formation of “dead zones” where the lack of oxygen makes it impossible for most complex marine life to survive.