https://www.yosemitesensors.com/dosensor/Y600-A-Y507-A-Portable-Dissolved-Oxygen-Meter.html

Temperature: The solubility of oxygen in water decreases as the temperature increases. This means warmer water holds less dissolved oxygen compared to cooler water.

Pressure: Atmospheric pressure affects dissolved oxygen levels. Higher pressure increases the amount of oxygen that can dissolve in water. Conversely, at higher altitudes where pressure is lower, dissolved oxygen levels tend to be lower.

Salinity: Higher salinity reduces the solubility of oxygen in water. Therefore, freshwater holds more dissolved oxygen compared to seawater with higher salt content.

Water Movement: The rate of water movement can influence oxygen levels. Faster-moving water can increase oxygen levels by promoting more mixing with the air.

Biological Activity: The presence of microorganisms and plant life affects DO levels. Photosynthesis by aquatic plants increases DO during daylight, while respiration by plants and animals decreases it, especially at night.

Organic Matter: The decomposition of organic matter by microbes consumes oxygen, reducing DO levels. High concentrations of organic pollutants can lead to significant oxygen depletion.

Chemical Reactions: Certain chemical substances in water can react with oxygen and affect its concentration.