Lentic ecosystems, such as lakes and ponds, occur in basins containing standing water. Depending on the rates of input and output of water, the flushing time of these ecosystems can range from days, in the case of small pools, to centuries, in the case of the largest lakes. The biological character of lentic ecosystems is strongly influenced by water chemistry, especially nutrient concentration and water transparency. Waters with a large nutrient supply are highly productive, or eutrophic, while infertile waters are unproductive, or oligotrophic. Commonly, shallow water bodies are much more productive than deeper water bodies of the same surface area. However, plant productivity is also influenced by how far light can penetrate into the water column. This factor is restricted in waters with large concentrations of turbidity associated with silt, or with a brown color caused by dissolved organic matter. In such cases, primary productivity is smaller than might be expected on the basis of nutrient supply. The ecosystems of lentic waters are usually highly zonal in two dimensions. Horizontal zonation is associated with variations of water depth, usually related to slope and length of the shoreline. Vertical zonation of deeper waters is determined by light availability, water temperature, and nutrient and oxygen concentrations. There are also distinct, benthic ecosystems in the sediment of lentic ecosystems.
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