Dissolved oxygen (DO) refers to the amount of oxygen gas that is dissolved within water bodies, rather than being part of the water molecule itself. This dissolved oxygen is fundamental for the survival of most aquatic organisms, much like oxygen in the air is for terrestrial life. Sufficient dissolved oxygen indicates healthy water quality and supports diverse aquatic ecosystems. Without adequate dissolved oxygen, aquatic life cannot sustain metabolic processes, grow, or reproduce effectively.
Temperature’s Influence on Dissolved Oxygen
Water temperature directly affects how much oxygen can dissolve in water. There is an inverse relationship between water temperature and dissolved oxygen solubility: as water temperature increases, the amount of oxygen it can hold decreases. Warmer water causes gas and water molecules to gain energy and move more rapidly, which weakens the molecular interactions that allow oxygen to remain dissolved, causing oxygen to escape into the atmosphere.
This physical principle explains why dissolved oxygen concentrations are higher in colder seasons like winter and early spring, and lower in warmer periods such as summer and autumn. Water bodies can reach oxygen saturation, the maximum amount of oxygen that can be dissolved at a particular temperature and pressure. When water warms, its capacity to hold oxygen diminishes, leading to a natural reduction in dissolved oxygen levels.
Biological Consumption of Oxygen
Biological processes are major causes of reduced dissolved oxygen in aquatic environments. All aquatic organisms, including fish, invertebrates, plants, and microorganisms, consume oxygen through respiration. They absorb free oxygen from the water into their systems for survival. The metabolic rates of aquatic organisms also increase with warmer temperatures, further intensifying their oxygen demand while the supply simultaneously decreases.
Decomposition of organic matter by bacteria and other microorganisms is a major factor in oxygen depletion. When organic materials like dead plants, animal waste, or sewage enter a water body, decomposers consume dissolved oxygen to break down this material. This consumption of oxygen is measured as biochemical oxygen demand (BOD). The more organic matter present, the greater the bacterial activity and oxygen demand, leading to a rapid decline in dissolved oxygen.
Eutrophication, caused by excess nutrients like nitrogen and phosphorus, impacts dissolved oxygen levels. Nutrient runoff from agricultural land or wastewater can stimulate algae and aquatic plant growth, leading to algal blooms. While these plants produce oxygen during the day through photosynthesis, dense blooms can block sunlight from reaching submerged plants, reducing their oxygen production. When these algal blooms eventually die, their decomposition by bacteria consumes much dissolved oxygen, often resulting in hypoxic (low oxygen) or even anoxic (no oxygen) conditions, creating “dead zones” that cannot support most aquatic life.
Chemical Reactions and Physical Barriers
Beyond biological consumption, chemical reactions and physical conditions also deplete dissolved oxygen. Chemical pollutants can directly consume oxygen through oxidation processes. For example, the oxidation of substances like iron, sulfides, or ammonia uses up dissolved oxygen. Industrial discharges or chemical runoff can contribute to this direct chemical oxygen demand, impacting the overall oxygen balance.
Physical conditions influence dissolved oxygen levels by limiting oxygen replenishment or trapping low-oxygen water. Stagnation, or reduced water movement, limits the water’s contact with the atmosphere, the primary source of oxygen replenishment. Fast-moving water, like streams, naturally incorporates more oxygen through aeration compared to still water.
Thermal stratification, where water forms layers of different temperatures, can create a barrier to oxygen mixing. Warmer, less dense surface water (epilimnion) floats above colder, denser bottom water (hypolimnion), preventing oxygen-rich surface water from reaching deeper layers. The hypolimnion, isolated from atmospheric re-aeration and light for photosynthesis, experiences oxygen depletion due to ongoing decomposition, sometimes leading to anoxic conditions.
Turbidity, caused by suspended particles like soil or organic matter, also contributes to decreased dissolved oxygen. These suspended solids reduce light penetration, limiting the ability of submerged aquatic plants to perform photosynthesis and produce oxygen. If the suspended particles contain organic material, their decomposition further consumes dissolved oxygen, exacerbating the problem.