How micro organisms decrease the level of oxygen which grow under water bodies

When algae die, they decompose and the nutrients contained in that organic matter are converted into inorganic form by microorganisms. This decomposition process consumes oxygen, which reduces the concentration of dissolved oxygen.

Inside the water bodies when dead and decaying matters are present then it starts decomposing.This decomposition is carried out by microorganism like bacteria and fungi.This decomposition process takes place either in aerobic or in anaerobic medium.But in this case decomposition takes place in aerobic medium i.e require oxygen.So the level of oxygen decreases inside water bodies.