Lakes
Suppose you suddenly found yourself on a sandy beach. Waves break on the shore. The water stretches as far as your eye can see. Gulls screech overhead. Where are you? Although you might think you're in the ocean, this immense body of water could actually be a lake! you could be on a beach in Indiana, on the shore of Lake Michigan. Although most lakes are not as large as Lake Michigan, they are generally bigger and more profound than ponds. Most lakes are deep enough that sunlight does not reach all the way to the bottom. A lake bottom may consist of sand, pebbles, or rock. The bottom of a pond is usually covered with mud and algae.
Lake Formation
Lakes from in many ways. As you read in the Before part, a cut-off river meander may become an oxbow lake. Ice sheets that melted at the end of the Ice Age created depressions that became lakes. Some lakes were created by the movements of the Earth's crust. Such movements created the deep valleys in central Africa below Lake Tanganyika and Lake Victoria. Other lakes are the result of volcanoes. An erupting volcano can cause a flow of lava or mud that blocks a river and forms a lake.
People can also create a lake by building a dam across a river. The lake may be used for supplying drinking water, irrigating fields, and boating and fishing. A lake that stores water for human use is called a reservoir. One of the largest reservoirs in the United States is Lake Mead in Nevada, behind Hoover Dam on the Colorado River.
Lake Habitats
Like a pond, a lake provides habitats for many organisms. In the shallow water near shore, the wildlife is similar to that in a pond. Water beetles scurry over the slippery, moss-covered rocks. Loons and kingfishers pluck fish from the open water. But unlike a pond, Sunlight does not reach the bottom at the center of a lake. Without sunlight, plants cannot live in the deep water. As a result, fewer other organisms live in the chilly, dark depths of the lake. A few worms and mollusks do live on the bottom. They feed on food particles that drift down from the surface. The deep waters of lakes are also the home of large, bony fish such as pike and sturgeon. These fish eat the tiny bottom dwellers. They also Swim to the surface to feed on fish and even small birds.
Changes in a Lake
Particularly in cool, northern areas of North America, many lakes undergo changes with the seasons. In the summer, the sun warms the upper layer of water in the lake. The warm water floats on top of the cooler, denser lower layer. But in the fall, the top layer cools off, too. As the water cools, it becomes denser and sinks. This causes the lake waters to mix together. As the water mixes, minerals, plant matter, and other nutrients rise from the lake bottom to the surface. Called lake turnover, this seasonal change refreshes the supply of nutrients throughout the lake.
A second type of change that occurs in a lake happens over a long period. The organisms in a lake happen over some time. The organisms in a lake constantly release waste products into the water. The wastes and the remains of dead organisms contain nutrients such as nitrates and phosphates. Algae feed on these nutrients. Over many years, the nutrients build up in the lake in a process called eutrophication(you troh fish Kay shun). As eutrophication causes more algae to grow, a thick, green scum form on the water's surface. Have you ever forgotten to clean a fish tank for a few weeks? You may have observed the process of eutrophication as algae began to grow on the sides of the tank.
When the algae layer becomes so thick that it begins to block out the sunlight, plants in the lake cannot carry out photosynthesis. They stop producing food and oxygen and die. As dead organisms in the lake decay, the amount of oxygen in the water decreases. The lake environment changes. Many of the fish and other animals no longer have enough oxygen to live. Material from decaying plants and animals piles up on the bottom, and the lake beomes more shallow. The sun warms the water to a higher temperature. Now many plants take root in the rich mud on the lake bottom. Eventually, the lake becomes completely filled with plants. The remaining water evaporates, and a grassy meadow takes the place of the former lake.
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