Each aquatic organism has a different level of oxygen that it must have to survive. For instance, trout need more oxygen and colder waters than a carp, a warm water fish with a tolerance for low oxygen levels. In extreme circumstances, oxygen depletion can cause all oxygen dependent species to move elsewhere or perish. Along with limiting biodiversity and affecting the health of organisms, lack of oxygen makes water taste pour and smell bad. Sewage treatment plants remove anaerobic conditions during treatment by aerating the wastewater.
Extra fertilizers promote excessive plant growth, which at first adds plenty of oxygen to the water.
This process is known as eutrophication. These organic wastes also need to be broken down by bacteria, using up oxygen. High levels of BOD are bad for a stream. Humans can also add too much oxygen to the water, which can be toxic to aquatic organisms. One common example is turbulent water released from a dam.