Oxygen is the most important constituent of water body health. Oxygen is an essential element for all aquatic organisms that breathe. Therefore, there is a direct relationship between the oxygen concentrations and exchanges occurring in a water body, and the physiological status of aquatic organisms.
Without oxygen at the bottom of the water body, anaerobic bacteria (those that live without oxygen) produce acids. These acids not only increase acidity but also cause a massive release of phosphorus and nitrogen, two major fertilizers, from the organic sediment and into the water column. These same anaerobic bacteria put toxic gases in the water including hydrogen sulfide (that rotten egg smell), ammonia, carbon dioxide, and methane. These gases are all toxic to fish, beneficial bacteria and insects.