Dissolved oxygen (DO) is the amount of oxygen present in water, measured as a concentration or percent saturation. The level of DO in water is affected by several factors, including temperature, salinity, and algae growth.

Fish, aquatic invertebrates, and air-breathing insects require different amounts of DO for normal respiration and growth. Prolonged exposure to low DO levels for any length of time can kill fish and other organisms, and significantly reduce the health and reproduction of aquatic invertebrates and air-breathing insects.

DO is also an important indicator of a water body's ecological status, as higher DO concentrations indicate a more productive environment and healthier water quality. DO levels are also influenced by the physical state of a water body, as microbial decomposition and the absence of photosynthesis lower DO concentrations.

Temperature is a major factor in oxygen concentration, as warm temperatures decrease DO levels. DO is also affected by the presence of vegetation, as plants release oxygen into water during photosynthesis.

pH is another important factor in dissolved oxygen concentrations, as water that has a high pH will contain less DO than water with a low pH. A pH of 5 is very acidic, and a pH of 7 is very alkaline.

Other important factors that affect DO in water are biological oxygen demand, aeration, and temperature. Biological oxygen demand is the consumption of dissolved oxygen by animals and bacteria, while aeration and temperature affect DO levels through diffusion into the surrounding water.