Dissolved oxygen (DO) is the concentration of oxygen gas dissolved in water, typically measured in milligrams per liter (mg/L). Oxygen enters the water through diffusion from the atmosphere and as a byproduct of photosynthesis by aquatic plants and algae. Adequate DO is required for the respiration of nearly all aquatic life, including fish, invertebrates, and aerobic bacteria. When DO levels decrease, the health and function of the aquatic ecosystem are compromised.
How Temperature Affects Oxygen Levels
One direct and predictable cause of decreased dissolved oxygen is an increase in water temperature. The relationship between temperature and gas solubility is inverse, meaning that warmer water physically holds less dissolved gas than colder water. As water warms, the kinetic energy of the water molecules increases.
The faster movement of water molecules makes it easier for dissolved oxygen to escape the liquid phase and move back into the atmosphere. This shift reduces the overall saturation level. For example, when water temperatures rise above 20°C, the saturation capacity for oxygen declines significantly compared to cooler conditions. Consequently, aquatic habitats often experience their lowest DO concentrations during the summer months when temperatures are highest.
Oxygen Depletion Through Microbial Activity
The largest biological drain on dissolved oxygen comes from decomposition. When organic matter—such as dead algae, leaves, animal waste, or sewage—enters a water body, it becomes food for microscopic organisms, primarily aerobic bacteria. These microorganisms consume oxygen through respiration as they break down complex organic compounds into simpler, stable inorganic substances.
This consumption is measured by Biological Oxygen Demand (BOD), which quantifies the oxygen microbes require to degrade organic material. A high concentration of organic waste leads to a high BOD, indicating that the bacteria are rapidly depleting the available DO. If the input of organic material is substantial, microbial consumption can quickly outpace the rate at which oxygen is replenished from the atmosphere or photosynthesis. This intense biological activity can drive oxygen levels down to concentrations that are harmful to other aquatic organisms.
Nutrient Overload and Algal Blooms
The excessive input of nutrients, known as eutrophication, often fuels microbial oxygen consumption. This condition is typically caused by runoff containing high concentrations of nitrogen and phosphorus from sources like agricultural fertilizers, urban stormwater, and wastewater discharge. These nutrients act as a fertilizer for the aquatic ecosystem, triggering a proliferation of phytoplankton and algae known as an algal bloom.
While algae produce oxygen during daytime photosynthesis, the bloom ultimately causes a sharp net decrease in DO through two mechanisms. First, the dense layer of algae blocks sunlight, reducing photosynthesis and causing organisms below the surface to die. Second, when the algae die and sink, the decay of this dead organic material provides a massive food source for bacteria. This dramatically increases the BOD and strips oxygen from the water, especially in deeper layers where mixing is limited. Furthermore, surviving algae respire at night, consuming dissolved oxygen and exacerbating the drop in DO levels.
Impact on Aquatic Ecosystems
The decrease in dissolved oxygen negatively affects the health of aquatic ecosystems. When DO concentrations fall below a certain threshold, the water body enters a state of hypoxia, typically defined as levels below 2 to 4 mg/L. If oxygen is completely exhausted, the condition is anoxia, meaning zero dissolved oxygen.
Hypoxia causes severe stress in aquatic organisms. Mobile animals like fish may migrate out of the affected area or move to the surface to “gulp” air. Less mobile creatures, such as shellfish, bottom-dwelling invertebrates, and developing fish eggs, cannot escape and often perish during these low-oxygen events. Prolonged hypoxic conditions create “dead zones,” areas where life requiring oxygen cannot survive, leading to a loss of biodiversity. The loss of sensitive species, such as trout or salmon, disrupts the food web, leaving behind only the most tolerant organisms.