Freshwater Zones

Low levels of dissolved salts characterize freshwater ecosystems. The salt content of fresh water is about 0.005 percent. Examples of freshwater ecosystems include lakes, ponds, freshwater wetlands, clear mountain streams, and slow, sediment-rich rivers.

Lakes and Ponds

Ecologists divide lakes and ponds into two categories. Eutrophic (yoo-TRAHF-ik) lakes are rich in organic matter and vegetation, so the waters are relatively murky. As the number of plants and algae in a lake grows, the number of bacteria feeding on decaying organisms also grows. These bacteria use the oxygen dissolved in the lake's waters. Eventually, the reduced amount of oxygen kills organisms that need oxygen to survive. Lakes naturally become eutrophic over a long period of time.

In contrast, oligotrophic (AHL-i-goh-TRAHF-ik) lakes contain little organic matter. The water is much clearer, and the bottom is usually sandy or rocky. Fishes inhabit both eutrophic and oligotrophic lakes. Freshwater lakes and ponds support a variety of other animals, including mammals such as the otter and muskrat, birds such as ducks and loons, reptiles such as turtles and snakes, and amphibians such as salamanders and frogs.

Rivers and Streams

A river or stream is a body of freshwater that flows down a gradient, or slope, toward its mouth, as shown in Figure 21-13. Water flows swiftly down steep gradients, and organisms are adapted to withstand powerful currents. For example, the larvae of caddis flies and the nymphs of mayflies cling to the rocky bottom, and brook trout and other fishes have evolved the strength to swim upstream. Slow-moving rivers and their backwaters are richer in nutrients and therefore support a greater diversity of life. Rooted plants and the fishes that feed on them are adapted to the weaker currents of slow-moving rivers.

Comparing Organisms in Terrestrial and Aquatic Biomes

Materials paper, scissors, magazines, glue

Procedure Cut out and collect images of plants, animals, and other organisms from magazines. Sort out the images based on which major biome each organism would live in. Develop a collage of each major biome.

Analysis Of the images you collected, which organisms live in terrestrial biomes (specify the type of terrestrial biome)? in aquatic biomes (specify the type)? What types of adaptations do each of the organisms have that allow them to live in the biome?

figure 21-13

Streams in mountain areas flow rapidly down steep slopes. These streams usually have rocky bottoms and clear water (a). Rivers that flow slower carry more fine sediment and commonly have muddy bottoms (b).

figure 21-14

The American alligator, Alligator mississippiensis, is the largest reptile in North America. It lives in marshes and swamps in Florida, Louisiana, South Carolina, Georgia, Alabama, and Texas. The alligator is protected by law because of its similarity to the very rare American crocodile.

Freshwater Wetlands figure 21-14

The American alligator, Alligator mississippiensis, is the largest reptile in North America. It lives in marshes and swamps in Florida, Louisiana, South Carolina, Georgia, Alabama, and Texas. The alligator is protected by law because of its similarity to the very rare American crocodile.

www.scilinks.org Topic: Aquatic

Ecosystems Keyword: HM60087

SCltNKS.

Maintained by the National Science Teachers Association

Freshwater wetlands are areas of land that are covered with fresh water for at least part of each year. The two main types of freshwater wetlands are marshes and swamps. Marshes contain nonwoody plants, such as cattails, and swamps are dominated by woody plants, such as trees and shrubs. Another type of freshwater wetland is a bog, which is dominated by sphagnum mosses.

Freshwater wetlands are the most productive freshwater ecosystem. They contain a wide variety of birds, fishes, mammals, amphibians, invertebrates, and reptiles. Wetlands in the United States are home to rare, large predators such as the whooping crane, the Florida panther, the American crocodile, and the American alligator, shown in Figure 21-14. Many wetlands are important as stopovers for migratory birds. Wetlands, like estuaries, provide protection for spawning organisms, such as fishes. Wetlands are also important to people economically and environmentally. Wetlands act as filters to clean pollutants out of the water flowing through them and also act as flood control when they absorb large quantities of water that could otherwise flood homes, farms, and businesses.

For many decades, wetlands have been diverted, drained, and filled in. But now, the vital importance of wetlands to the economic and environmental health of many areas is being recognized. In some cases, government agencies and private organizations are working together to protect wetlands and to restore areas that have been damaged. One of the most important wetlands in the world is the Everglades National Park in southern Florida. There is currently a major effort to restore the flow of water through the Everglades, upon which the entire ecosystem depends. The Everglades has been designated a World Heritage Site, an International Biosphere Reserve, and a Wetlands of International Importance.

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