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Eutrophication is the natural aging of a lake by biological enrichment of its water.

Lakes are classified according to their nutrient content and primary productivity.

  • Oligotrophic lakes: These are poorly nourished lakes because they have a small supply of plant nutrients.These lakes are deep and steep and have crystal-clear water and are well oxygenated as these are new lakes. They have small population of phytoplankton and fishes. Also, lakes have low net primary productivity (NPP), i.e., low biological production, as nutrient content is low.

With time sediments, organic material, and inorganic nutrients such as nitrogen and phosphorus wash into most oligotrophic lakes, which supports plant and animal growth in them. Death and decomposition of organic matter starts to settle in bottom sediments, converting oligotrophic lake into eutrophic lakes.

  • Eutrophic lakes: These are well-nourished lakes because of their high levels of nutrients and may be depleted of oxygen.These lakes are shallow and have light brown or green water. These lakes have high NPP, i.e., high biological production due to continuous supply of nutrients needed by producers.
  • Inputs of pollutants (nutrients) by human activities through urban (home and sewage waste) , agricultural areas and industrial effluents can accelerate the aging process of lakes which is called as Cultural or Accelerated EutrophicationThis process provides lakes with excess load of nutrients which are then described as hypereutrophic lakes. Many lakes fall somewhere between the Oligotrophic and Eutrophic extremes of nutrient enrichment and are called mesotrophic lakes.
  • Prime contaminants of lakes are nitrates and phosphates, which act as plant nutrients. These nutrients overstimulate the growth of algae (algal blooms) which makes an algal layer on top of water creating unpleasant odors. This excessive algal growth in lakes due to eutrophication is finally responsible for deficiency of dissolved oxygen in water. Non-availability of oxygen in water leads to killing of fishes and other aquatic organisms.
  • At the same time, other pollutants flowing into a lake may poison whole populations of fish, whose decomposing remains further deplete the water’s dissolved oxygen content (as decomposers also need oxygen).
  • In this way, a lake can lead to death.
  • Heated (thermal) wastewaters flowing out of thermal power plants, constitute another important category of pollutants. Thermal wastewater eliminates or reduces the number of organisms sensitive to high temperature, and may enhance the growth of plants and fish in extremely cold areas but, only after causing damage to the indigenous/ natural flora and fauna.
  • So, Eutrophication is nutrient enrichment of an ecosystem, which results in increased primary production and reduced biodiversity. In lakes, eutrophication leads to algal blooms, reduced water clarity, and fish mortality as a consequence of oxygen depletion.
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