Fisheries - The need for AerationWater can only hold a limited amount of oxygen which is vital for both sustained fish and plant life. The amount of oxygen which can be held is determined by atmospheric pressure, temperature and salinity. In a natural setting, oxygen is added to water by atmospheric diffusion at the surface, by wind circulation (augmented surface diffusion) and by photosynthesis (oxygen produced by phytoplankton or algae). Photosynthesis accounts for most of the oxygen in water, highlighting the need for aquatic plants. The oxygen content of water increases with increasing atmospheric pressure and decreasing temperature and salinity. The amount of oxygen in water is measured as milligrams per liter (mg/l) dissolved oxygen (DO).
Dissolved oxygen is one of the key parameters when assessing water quality, fish and aquatic animals cannot produce oxygen from water (H2O) or other oxygen-containing compounds. Only green plants and some bacteria can do this through photosynthesis and similar processes.
As water temperature increases so its capacity to hold dissolved oxygen decreases. So if water is too warm, there may not be enough oxygen in it for the organisms that rely upon it. When there are too many bacteria and/or aquatic animals in a defined enclosed area of water, they may overpopulate it (exceeding the waters carrying capacity), using DO in great amounts to the extent sometimes of depleting it. Depending on how low the dissolved oxygen concentration is and how long it remains low, fish may consume less feed, grow more slowly, convert feed less efficiently, be more susceptible to infectious diseases, or suffocate and die.
Oxygen levels can also be indirectly reduced through over fertilisation of water plants by run-off from farm fields containing phosphates and nitrates (the ingredients in fertilizers).
It is worth noting that dissolved oxygen contents in water vary greatly both seasonally and daily. Dissolved oxygen contents are at their lowest just before sunrise because oxygen is being taken up by fish and plants during the night but none is being produced.