Aquatic Pollution and Coastal Management

Unit 3 : Agricultural wastes and eutrophication

3.8.Significance of dissolved oxygen
Most aquatic organisms require oxygen in specified concentration ranges for respiration and efficient metabolism, and DO concentration changes above or below this range can have adverse physiological effects. Even short-lived anoxic & hypoxic events can cause major "kills" of aquatic organisms. Exposure to low oxygen concentrations can have an immune suppression effect on fish which can elevate their susceptibility to diseases for several years. Moreover, the toxicity of many toxicants (lead, zinc, copper, cyanide, ammonia, hydrogen sulphide and pentachlorophenol) can double when DO is reduced from 10 to 5 mg/l. The death of immobile organisms and avoidance of low-oxygen conditions by mobile organisms can also cause changes in the structure and diversity of aquatic communities. In addition, if dissolved oxygen becomes depleted in bottom waters (or sediment), nitrification, and therefore denitrification, may be terminated, and bioavailable orthophosphate and ammonium may be released from the sediment to the water column. These recycled nutrients can give rise to or reinforce algal blooms. Ammonia and hydrogen sulphide gas, also the result of anaerobic respiration, can be toxic to benthic organisms and fish assemblages in high concentrations.