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Lake Eutrophication: Types and Effects
Eutrophication is the process by which lakes are enriched naturally or, more usually, by human activity.
As a result, an oligotrophic lake (“underfed” or “nutrient poor”) may become eutrophic (“well-fed” or “nutrient rich”) in due course of time.
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The key nutrients responsible for eutrophication are nitrogen and phosphorus. Excess nutrients come from a variety of sources of which sewage effluents and agricultural runoff are more important.
Natural Eutrophication:
The process of lake aging characterized by nutrient enrichment is known as natural eutrophication. This process gradually converts the original oligotrophic lake into a eutrophic lake. Natural fertility of lakes is increased by addition of nutrient rich loads of sediment and organic matter from the catchment area. This permits the production of larger quantities of phytoplankton and algal blooms, and other aquatic vegetation, including aquatic weeds, water hyacinth, water fern, and water lettuce, which in turn provide more ample food for herbivorous zooplankton and fish.
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When these plants and animals die, their bodies settle at the lake bottom and accumulate along with the sediment brought by tributary streams. The plant and animal remains then gradually decompose, thus releasing nutrients that could be channeled into the living bodies of future generations of organisms. As the natural process of lake ageing continues over the years, the original oligotrophic lake is gradually converted into an eutrophic lake.
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Cultural Eutrophication:
When die process of eutrophication is speeded up by human activity, it is called cultural eutrophication. About 80 per cent of the nitrogen and 75 per cent of the phosphorus added to lakes and streams has its source in human activities. This human-generated nutrient input is derived from several sources, including domestic sewage, agricultural fertilizers, detergents, livestock wastes and industrial wastes.
When the average concentration of soluble inorganic nitrogen exceeds 0.30 parts per million (ppm) and the soluble inorganic phosphorus exceeds 0.01 ppm, algal blooms may appear. During the summer the algal bloom problem usually becomes more intense with adverse effects on the whole biota of the lake.
Such cultural entrophication has caused lake Erie to age 15000 years in only 25 years between 1950 and 1975 (Owen, 1985). Lake Mendota and lake Washington have also undergone rapid eutrophication as a result of human activities. In India, the ever going process of cultural eutrophication has already reduced the recreational value of the Kashmir lakes and the Nainital Lake is also undergoing rapid eutrophication, especially as a result of sewage, domestic waste and detergent discharges.
Effects of Eutrophication:
The adverse effects of nutrient pollution on a lake are many and may be summarised as follows:
(i) Eutrophication destroys the aesthetics of the lake, rendering it repulsive to swimmers and sport enthusiasts. In other words, the recreational values of the lake are reduced.
(ii) The algal blooms impair water quality by giving it a bad taste and odour. If the lake is a source of drinking water, considerable expense may be involved in improving its quality.
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(ii) Toxic gas hydrogen sulphide that emanates from rotting algae has foul smell. Some of the blue-green algae release chemicals that are poisonous to other organisms, including fish and humans.
(iv) Dense algal blooms at the lake surface reduce penetration of sunlight to the lake bottom. As a result, the deeper waters contain less amount of dissolved oxygen, which is further reduced by decomposing algae and organic matter accumulated at the lake bottom. The oxygen depletion often causes winterkill of fish in northern lakes.
(v) The rooted weeds gradually reduce the lake size and also interfere with fishing, navigation, irrigation, and the production of hydro-electric power.
(vi) As the clean water becomes turbid and dirty, the game fish such as the Trout and the Mahseer are replaced with trash fish.
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(vii) The lake basin is gradually filled in by sediments (sediment pollution) rendering the lake shallow, and in due course of time the lake becomes “extinct” as a result of eutrophication.
