Water bodies often experience a reduction in dissolved oxygen levels when the amount of waste within them increases. This is a direct result of biological and chemical processes as pollutants enter aquatic environments. This article explores the scientific explanations behind this phenomenon.
What is Dissolved Oxygen and Why it Matters
Dissolved oxygen (DO) is the amount of oxygen gas physically dissolved in water. This dissolved gas is fundamental for the survival of most aquatic organisms, including fish, invertebrates, and many microorganisms, as they rely on DO for respiration, a biological process that converts nutrients into energy.
Healthy aquatic ecosystems depend on sufficient dissolved oxygen. For instance, cold-water fish like trout generally require DO concentrations of 6 milligrams per liter (mg/L) or higher to thrive. Warmer water holds less dissolved oxygen, but even so, most aquatic life needs adequate levels to prevent stress or mortality.
The Nature of Organic Waste in Water
In the context of water quality, “waste” primarily refers to organic matter introduced into aquatic systems. This can originate from various sources, including untreated sewage, agricultural runoff containing animal waste and plant debris, and industrial discharges. Dead plant material, animal carcasses, and discarded food scraps also contribute to this organic load.
This organic waste is composed of complex carbon-based molecules that serve as an energy and nutrient source for microorganisms. These include carbohydrates, proteins, and fats, which are biodegradable. The presence of these substances impacts the water’s oxygen balance through biological processes.
How Microbes Consume Oxygen During Decomposition
When organic waste enters a water body, it becomes a food source for naturally occurring aerobic microorganisms, predominantly bacteria and some fungi. These microbes break down the complex organic molecules into simpler compounds through a process called cellular respiration. This biological process requires oxygen, which the microorganisms draw directly from the dissolved oxygen.
A greater amount of organic waste in the water supports a larger population of these aerobic microbes. As their numbers increase, so does their collective demand for dissolved oxygen. This demand is often quantified as Biochemical Oxygen Demand (BOD), which measures the amount of oxygen consumed by microorganisms to decompose organic matter over a specific period, typically five days at 20 degrees Celsius. The decomposition process is not instantaneous; it typically takes several days to a week for a significant microbial population to develop and for substantial oxygen depletion to become evident.
Consequences of Depleted Oxygen Levels
A sustained reduction in dissolved oxygen levels can lead to conditions known as hypoxia, where oxygen concentrations fall below levels necessary to support most aquatic life, generally considered less than 2-3 mg/L. If oxygen is completely absent, the condition is termed anoxia. Both hypoxia and anoxia stress aquatic organisms, often leading to widespread fish kills and a significant loss of biodiversity.
Species that cannot tolerate low oxygen levels either die or migrate away, altering the entire ecosystem structure. Under anaerobic conditions, different types of microorganisms thrive, producing harmful gases such as hydrogen sulfide, which smells like rotten eggs, and methane. These gases can further degrade water quality and create an environment unsuitable for many forms of life.