Their metabolic activity increases with water temperature also. This growth increases dissolved oxygen use even more. Remember, as water warms it can hold less dissolved oxygen. Dissolved oxygen gets into the water by diffusion from the air. It also comes from photosynthesis in the water.
Diffusion requires exposing water to the air.
However, diffusion is possible only at the water surface. It is most efficient when wind mixes the water. Diffusion is the process used by mechanical aerators. Green plants require sunlight, water, carbon dioxide and minerals to make sugars and oxygen. Algae are small, green plants.