<P> Two important subclasses of lakes are ponds, which typically are small lakes that intergrade with wetlands, and water reservoirs . Over long periods of time, lakes, or bays within them, may gradually become enriched by nutrients and slowly fill in with organic sediments, a process called succession . When humans use the watershed, the volumes of sediment entering the lake can accelerate this process . The addition of sediments and nutrients to a lake is known as eutrophication . </P> <P> 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 . </P> <P> The major zones in river ecosystems are determined by the river bed's gradient or by the velocity of the current . Faster moving turbulent water typically contains greater concentrations of dissolved oxygen, which supports greater biodiversity than the slow moving water of pools . These distinctions form the basis for the division of rivers into upland and lowland rivers . The food base of streams within riparian forests is mostly derived from the trees, but wider streams and those that lack a canopy derive the majority of their food base from algae . Anadromous fish are also an important source of nutrients . Environmental threats to rivers include loss of water, dams, chemical pollution and introduced species . A dam produces negative effects that continue down the watershed . The most important negative effects are the reduction of spring flooding, which damages wetlands, and the retention of sediment, which leads to loss of deltaic wetlands . </P> <P> Wetlands are dominated by vascular plants that have adapted to saturated soil . There are four main types of wetlands: swamp, marsh, fen and bog (both fens and bogs are types of mire). Wetlands are the most productive natural ecosystems in the world because of the proximity of water and soil . Hence they support large numbers of plant and animal species . Due to their productivity, wetlands are often converted into dry land with dykes and drains and used for agricultural purposes . The construction of dykes, and dams, has negative consequences for individual wetlands and entire watersheds . Their closeness to lakes and rivers means that they are often developed for human settlement . Once settlements are constructed and protected by dykes, the settlements then become vulnerable to land subsidence and ever increasing risk of flooding . The Louisiana coast around New Orleans is a well - known example; the Danube Delta in Europe is another . </P>

Plant like organisms that are the producers in aquatic ecosystems