Hypoxia is a drastic reduction in the amount of oxygen dissolved in water—a state that can be fatal to fish and other gill-breathing animals. Hypoxia is most often caused by pollution from nitrogen and phosphorus compounds derived from fertilizers, animal waste, sewage, or atmospheric contaminants. The pollutants stimulate an excessive growth of plant material. When these plants—typically algae—die and decay, they support large populations of bacteria, which take oxygen from the water. Pollution with sewage solids has a similar effect. Prevention involves controlling the sources of pollution by improved agricultural practices, treatment of sewage, and, to a lesser extent, reduction of emissions from the burning of fossil fuels.