Untreated sewage effluent and agricultural run-off carrying fertilizers are examples of human-caused eutrophication. Eutrophication generally promotes excessive plant growth and decay, favouring simple algae and plankton over other more complicated plants, and causes a lack of oxygen needed for fish and shellfish to survive.
Fish kills in bayous, streams, and other freshwater habitats are often caused by oxygen depletion associated with excessive levels of fertility and algae. Like most plants, many algae produce oxygen during the daylight as a by-product of photosynthesis. At night these algae consume oxygen, but usually much less than was produced during the daylight. Many common situations, however, can reduce the amount of oxygen a bloom produces without reducing its nighttime oxygen demand. Extremely calm or cloudy days may reduce photosynthesis and oxygen production. This type of oxygen depletion may kill fish directly or weaken their immune systems through prolonged stress.