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Online exhibits : The world's biomes
The freshwater biome
Freshwater is defined as having a low salt concentration usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration (i.e., ocean). There are different types of freshwater regions:
Ponds and lakes
The topmost zone near the shore of a lake or pond is the littoral zone. This zone is the warmest since it is shallow and can absorb more of the Sun's heat. It sustains a fairly diverse community, which can include several species of algae (like diatoms), rooted and floating aquatic plants, grazing snails, clams, insects, crustaceans, fishes, and amphibians. In the case of the insects, such as dragonflies and midges, only the egg and larvae stages are found in this zone. The vegetation and animals living in the littoral zone are food for other creatures such as turtles, snakes, and ducks.
The near-surface open water surrounded by the littoral zone is the limnetic zone. The limnetic zone is well-lighted (like the littoral zone) and is dominated by plankton, both phytoplankton and zooplankton. Plankton are small organisms that play a crucial role in the food chain. Without aquatic plankton, there would be few living organisms in the world, and certainly no humans. A variety of freshwater fish also occupy this zone.
Plankton have short life spans when they die, they fall into the deep-water part of the lake/pond, the profundal zone. This zone is much colder and denser than the other two. Little light penetrates all the way through the limnetic zone into the profundal zone. The fauna are heterotrophs, meaning that they eat dead organisms and use oxygen for cellular respiration.
Temperature varies in ponds and lakes seasonally. During the summer, the temperature can range from 4° C near the bottom to 22° C at the top. During the winter, the temperature at the bottom can be 4° C while the top is 0° C (ice). In between the two layers, there is a narrow zone called the thermocline where the temperature of the water changes rapidly. During the spring and fall seasons, there is a mixing of the top and bottom layers, usually due to winds, which results in a uniform water temperature of around 4° C. This mixing also circulates oxygen throughout the lake. Of course there are many lakes and ponds that do not freeze during the winter, thus the top layer would be a little warmer.
Streams and rivers
Visit our gallery of wetlands images, which illustrate the amazing diversity of wetland ecosystems.
Top photo by David K. Smith, UCMP. Ponds & lakes photos, from left: Glenn and Martha Vargas © 2004 California Academy of Sciences; David K. Smith, UCMP; Gerald and Buff Corsi © 2001 California Academy of Sciences; Glenn and Martha Vargas © 2004 California Academy of Sciences. Streams & rivers photos, from left: Dr. G Dallas and Margaret Hanna © 1999 California Academy of Sciences; Susan Middleton © 2003 California Academy of Sciences; Lorraine Elrod © 2000 California Academy of Sciences; Dr. Robert Thomas and Margaret Orr © 2004 California Academy of Sciences. Wetlands photos, from left: Dr. Robert Thomas and Margaret Orr © 1999 California Academy of Sciences; David K. Smith, UCMP; Gerald and Buff Corsi © 2005 California Academy of Sciences.
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