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Lake Tips: EUTROPHICATION
What is it?
Eutrophication is the process by which lakes age.
What happens?
When a lake is first formed, it has very low nutrient levels, appears
clear and has few aquatic plants. This is the oligatrophic stage. As the lake
matures, it gains nutrients, plants and other organisms that help achieve the
most productive, eutrophic stage. In the middle of these two extremes is the
mesotrophic stage. These differing stages are typical of most lakes, but there
are many exceptions. Some lakes never appear clear (most often human-made
lakes), and others continue to remain clear, accumulating very few nutrients
over time (usually the case for high-altitude lakes). Naturally, eutrophication
takes hundreds to thousands of years to occur, but with human influence, it can happen in
only decades of years. Due to the nature of this process it often becomes difficult
to determine what may be influencing the lake. Continuous monitoring
of lakes can show what impact humans are having and what stage the
lake is in.
Symptoms of human-induced (cultural) eutrophication are:
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Increased algal growth (stimulated by increased supply of
nutrients);
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Increased rooted aquatic plant growth (stimulated by increase in
nutrient supply, as well as additional shallow growing areas due to accumulation
of sediments, silt and organic matter);
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Lower dissolved oxygen concentrations in all or parts of the
lake ( resulting from increases in plant respiration and organic decomposition,
which can kill fish and other aquatic life).
In addition to eutrophication, there are other cycles that
dictate what and how many organisms are able to survive. One such cycle is
stratification. During the warmer months, the water temperature becomes
layered, with the warm water remaining on top and the more dense, cooler water
sinking to the bottom. In between the extremes is the thermocline layer that
consists of rapidly changing temperatures. Stratification affects oxygen
content, light penetration and photosynthesis, which then determines what
organisms are able to survive in the different temperature regions.
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