ponds are small bodies of freshwater with shallow and still water, marsh, and aquatic plants. they can be further divided into four zones: vegetation zone, open water, bottom mud and surface film. the size and depth of ponds often varies greatly with the time of year; many ponds are produced by spring flooding from rivers. food webs are based both on free-floating algae and upon aquatic plants. there is usually a diverse array of aquatic life, with a few examples including algae, snails, fish, beetles, water bugs, frogs, turtles, otters and muskrats. top predators may include large fish, herons, or alligators. since fish are a major predator upon amphibian larvae, ponds that dry up each year, thereby killing resident fish, provide important refugia for amphibian breeding. ponds that dry up completely each year are often known as vernal pools. some ponds are produced by animal activity, including alligator holes and beaver ponds, and these add important diversity to landscapes.