Freshwater lake, Canada

Physical Environment: Freshwater habitats are traditionally divided into lentic, or standing-water, habitats (from the Latin lenis, calm), and lotic, or running-water, habitats (from the Latin lotus, washed). The lentic habitat consists of still, often deep water. Its physical characteristics depend greatly on the surrounding land, which dictates what nutrients collect in the lake. Young lakes often start off clear and with little plant life. Such lakes are called oligotrophic. With age, the phytoplankton bloom and algae spread, reducing the water clarity. Such lakes are termed eutrophic. The process of eutrophication is discussed more thoroughly in Chapter 57.  Location: Throughout all the continents of the world. The Great Lakes of Canada represent the largest series of freshwater habitats on Earth.  Plant Life: In addition to free-floating phytoplankton and algae, lentic habitats may have rooted vegetation, which often emerges above the water surface (emergent vegetation), such as cattails, plus deeper-dwelling aquatic plants and algae.  Animal Life: Animals include fish, frogs, turtles, crayfish, and many species of insects (including aquatic insect larvae). In tropical and subtropical lakes, alligators and crocodiles commonly are seen.  Effects of Humans: Agricultural runoff, including fertilizers and sewage, can greatly increase lake nutrient levels and speed up the process of eutrophication. This may result in algal blooms and fish kills. In some areas, exotic species of invertebrates and fish are outcompeting native species.