Lakes and Freshwater Wetlands

As the amount of water changes and the amount of sediments increases, lakes can be transformed into wetlands and eventually into dry land.

Real-World Reading Link Have you ever felt the bottom of a lake with your feet? It was probably soft and squishy from deposits of fine sediments. Lakes and ponds receive materials that are carried by rivers from upland areas. Over time, accumulation of these sediments changes the characteristics of the lake.

Origins of Lakes

Natural lakes, bodies of water surrounded by land, form in different ways in surface depressions and in low areas. As you learned in Section 9.2, oxbow lakes form when streams cut off meanders and leave isolated channels of water. Lakes also form when stream flow becomes blocked by sediment from landslides or other sources. Still other lakes have glacial origins, as you learned in Chapter 8. The basins of these lakes formed as glaciers gouged out the land during the ice ages. Most of the lakes in Europe and North America are in recently glaciated areas. Glacial moraines originally dammed some of these depressions and restricted the outward flow of water. The lakes that formed as a result are known as moraine-dammed lakes. In another process, cirques carved high in the mountains by valley glaciers filled with water to form cirque lakes. Other lakes formed as blocks of ice left on the outwash plain ahead of melting glaciers eventually melted, leaving depressions called kettles. When these depressions filled with water, they formed kettle lakes such as those shown in Figure 9.19.

Lakes And Freshwater Wetlands

238 Chapter 9 • Surface Water

Phil Schermeister/CORBIS

238 Chapter 9 • Surface Water

Phil Schermeister/CORBIS

| Lakes Undergo Change

1 Water from precipitation, runoff, and underground

| sources can maintain a lake's water supply. Some lakes

| contain water only during times of heavy rain or exces-

J sive runoff from spring thaws. A depression that

J receives more water than it loses to evaporation or use

| by humans will exist as a lake for a long period of time.

f However, most lakes are temporary water-holding areas;

J over hundreds of thousands of years, lakes usually fill in s with sediment and become part of a new landscape.

Eutrophication Through the process of photosynthesis, plants such as green algae add oxygen to lake water. Animals that live in a lake need oxygen in the water. Throughout their life cycle, the animals add waste products to the water. Oxygen is also consumed during the decay process that occurs after plants and animals living in the body of water die. Scientist use the amount of dissolved oxygen present in a body of water to assess the overall quality of the water. Dissolved oxygen is one quality a body of water must have to support life.

The process by which bodies of water become rich in nutrients from the surrounding watershed that stimulate excessive plant growth is called eutrophication. Although eutrophication is a natural process, it can be sped up with the addition of nutrients, such as fertilizers, that contain nitrogen and phosphorus. Other major sources of nutrients that concentrate in lakes are animal wastes and phosphate detergents.

When eutrophication occurs, the animal and plant communities in the lake can change rapidly. Algae growing at the surface of the water can suddenly multiply very quickly. The excessive algae growth in a lake or pond appears as a green blanket, as shown in Figure 9.20. Other organisms that eat the algae can multiply in numbers as well. In addition, the population of algae on the surface can block sunlight from penetrating to the bottom of the lake, causing sunlight-dependent plants and other organisms below the surface to die. The resulting overpopulation and, later, the decay of a large number of plants and animals depletes the water's oxygen supply. Fish and other sensitive organisms might die as a result of the lack of oxygen in the water. In some cases, the algae can also release toxins into the water that are harmful to the other organisms. ^

^p Reading Check Identify the effects of eutrophication on the aquatic animals in an affected lake system.

Figure 9.20 Eutrophication is a natural process that can be accelerated with the addition of nitrogen and phosphorus to a body of water. Once the process begins, it can cause rapid changes in the plant and ani mal communities in the affected body of water.

Figure 9.20 Eutrophication is a natural process that can be accelerated with the addition of nitrogen and phosphorus to a body of water. Once the process begins, it can cause rapid changes in the plant and ani mal communities in the affected body of water.

Chemistry Eutrophication Pictures

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    Is a major source of algae supporting nutrients that concentrate in lakes phosphate detergents?
    8 years ago

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