Dissolved oxygen

This refers to oxygen (O2) dissolved in an aqueous solution. Oxygen gets into water by diffusion from the surrounding air, by aeration, and as a waste product of photosynthesis, from wetland plants in particular. Aerobic microbial activity uses up the oxygen in a waterway, so it is vital that air is added to nutrient-rich water to keep it healthy and prevent monocultures such as algae taking over. Cold water holds more oxygen than warm water.