CAUSES OF POLLUTION
Many causes of pollution including sewage and fertilizers contain nutrients such asnitrates and phosphates. In excess levels, nutrients over stimulate the growth of aquatic plants and algae. Excessive growth of these types of organisms consequentlyclogs our waterways, use up dissolved oxygen as they decompose, and block light todeeper waters.This, in turn, proves very harmful to aquatic organisms as it affects the respirationability or fish and other invertebrates that reside in water.Pollution is also caused when silt and other suspended solids, such as soil, washoff plowed fields, construction and logging sites, urban areas, and eroded river bankswhen it rains. Under natural conditions, lakes, rivers, and other water bodies undergoEutrophication, an aging process that slowly fills in the water body with sediment andorganic matter. When these sediments enter various bodies of water, fishrespirationbecomes impaired, plant productivity and water depth become reduced, andaquatic organisms and their environments become suffocated. Pollution in the form of organicmaterial enters waterways in many different forms as sewage, as leaves and grassclippings, or as runoff from livestock feedlots and pastures. When natural bacteriaand protozoan in the water break down this organic material, they begin to use up theoxygen dissolved in the water. Many types of fish and bottom-dwelling animalscannot survive when levels of dissolved oxygen drop below two to five parts per million. When this occurs, it kills aquatic organisms in large numbers which leads todisruptions in the food chain.