Freshwater biomes

Key points

• A biome is a community of plants and animals living in, and adapted to, a certain climate.
• Many of Earth’s biomes are aquatic, or water-based. The characteristics of aquatic biomes are heavily influenced by the concentration of dissolved oxygen and nutrients in the water.
• Some aquatic biomes are freshwater biomes, where the water contains little or no salt. Freshwater biomes include ponds, lakes, streams, and rivers, and are a vital source of drinking water.
• Ponds and lakes are natural bodies of standing water. Ponds are smaller and may be seasonal, whereas lakes are larger and more permanent. Large lakes have four main zones.
  • The littoral zone is the top, near-shore layer of a lake. This zone is shallow with plentiful sunlight and nutrient inputs from the surrounding land. As a result, this zone supports a wide variety of plants and animals.
  • The limnetic zone (or open-water zone) is a lake’s sunlit surface layer where most photosynthesis takes place. It starts away from shore, just past the littoral zone, and extends to the depth penetrated by sunlight. This zone supports phytoplankton and zooplankton, along with freshwater fish.
  • The profundal zone (or deep-water zone) is a lake’s deep, open-water layer, which is too dark for photosynthesis. The water in this zone is cooler and contains less dissolved oxygen than the shallower zones. This zone supports fish adapted to these cooler waters.
  • The benthic zone includes the soil and soil organisms that live at the bottom of a lake. This zone is inhabited mostly by decomposers and other organisms that feed on dead and decaying material, and can withstand a low-oxygen environment.

• Streams and rivers are flowing bodies of water that drain the landscape, and are important agents of erosion. The flow of a typical stream or river can be characterized by three zones.
  • Streams and rivers start at the source zone, or headwaters, where water at high elevations collects from precipitation and snowmelt. This water is cold, fast moving, has lots of dissolved oxygen, and a low nutrient content. There are usually only a few plant species that grow near a river's source.
  • Headwaters merge together to form the transition zone of a river. Here, the river is wider, slower moving, has less dissolved oxygen, and carries more sediment than the source zone. The water is warmer and more nutrient-rich, so this zone tends to support a larger variety of plants and animals.
  • From the transition zone, a river meanders toward its mouth through a flat, wide floodplain zone. This zone forms because, occasionally, the bulk of water in the river becomes greater than the river channel itself, and water spills out onto the land forming wetlands and temporary lakes. The water in this zone usually contains more sediment and less dissolved oxygen than the transition zone. This zone's warm, nutrient-rich waters support the greatest variety of plants and animals.