What is Eutrophication?
Eutrophicaton is a natural phenomenon in which the water becomes green and degraded. It is a natural process which became a problem because of human activities. Human activities increases the rate of eutrophication in water bodies.
Within the past 50 years, eutrophication—the over-enrichment of water by nutrients such as nitrogen phosphorus—has emerged as one of the leading causes of water quality impairment. The two most acute symptoms of eutrophication are oxygen depletion and harmful algal blooms, which can destroy aquatic life in affected areas.
The pictures above show how much nice, clean, blue water can change over time a look like this. This is what eutrophication does.