How Lakes Function

Lakes are essential elements of the landscape for several reasons.

They provide important habitat for wildlife including fish and other aquatic species, many species of birds, and a multitude of mammals. People enjoy lakes for their beautiful scenery and use them for recreational activities such as fishing, hunting and boating.

Lakes also provide important ecosystem services. They act as natural regulators of river flow, trapping sediments and nutrients from rivers and streams that flow into them. The riparian plants that grow along the shorelines help to stabilize the sediments and provide complex habitats for terrestrial animals.

Lakes can be divided into two basic habitats: deep, open water (pelagic zone) and bottom areas (benthic zone). The deep, open water zone is where we find free-floating organisms like microscopic phytoplankton (algae) and zooplankton (animals); and larger organisms like our many different fish species. In contrast, the benthic zone is where we find attached algae (periphyton) on the surface of rocks and other substrates, larger macroalgae, macroinvertebrates (e.g. insect larvae, snails, amphipods, clams, etc.), and young fish.

The benthic zone can be further subdivided into illuminated areas (littoral zone) where the algae are able to grow, and dark areas generally lacking vegetation (profundal zone). The profundal zone is characterized by a lack of photosynthetic organisms, but that does not imply a complete lack of life. Life in this zone is dependent on the contribution of organic carbon from other areas of the lake or surrounding watershed, for example, dead leaves, macroalgae, or phytoplankton. As decomposers break down this organic material, they use oxygen from the surrounding water. The depletion of oxygen levels in some lakes can disrupt the lake’s community of fish and other animals that depend on well-oxygenated water. Increased nutrient inputs from human activity can also cause greater than normal decreases in oxygen in this zone of the lake.

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