Full context:   Limnology is the study of inland bodies of water and related ecosystems. Limnology divides lakes into three zones: the littoral zone, a sloped area close to land; the photic or open-water zone, where sunlight is abundant; and the deep-water profundal or benthic zone, where little sunlight can reach. The depth to which light can penetrate depends on the turbidity of the water, which is determined by the density and size of suspended particles. A particle will be in suspension if its weight is less than the random turbidity forces acting upon it. These particles can be sedimentary or biological in origin (including algae and detritus) and are responsible for the color of the water. Decaying plant matter, for instance, may account for a yellow or brown color, while algae may cause a greenish coloration. In very shallow water bodies, iron oxides make the water reddish brown. Bottom-dwelling detritivorous fish stir the mud in search of food and can be the cause of turbid waters. Piscivorous fish contribute to turbidity by eating plant-eating (planktonivorous) fish, thus increasing the amount of algae (see aquatic trophic cascade).