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Natural Eutrophication
The small amount of natural eutrophication that occurs in a fast-moving stream usually gets transported downstream to a larger lake or pond. Therefore the stream is little affected, but the pond is not. Over time, algae and other organisms indicative of abundant nutrients will collect in lakes and ponds. Gradually, the accumulation of nutrients affects the whole pond and can back up into the mouth of the stream as it enters the pond. Over very large periods of time, the whole stream might be affected by this process.
Man-made Eutrophication
Introduction of nitrogen and phosphorus into the ecosystem can easily upset the natural pattern of eutrophication. This most common situation occurs during farming activities, when nutrient-rich fertilizer is applied to a farmer's field. The result is a water runoff that is heavy with compounds of nitrogen and phosphorus. The extra nutrients will result in an increase in certain types of algae, but oxygen depletion and the blockage of sunlight will mainly occur downstream or in deep pools.
Limiting Factors
Not only do phosphorus and nitrogen enhance the biological productivity of a water system, but beyond a certain point they can have a negative impact on the ecological system. The turning point occurs when these organisms utilize all available oxygen, which then adversely affects other plants and animals. The abundance of algae and other green plants then depletes the oxygen supply in the water, thus killing other aquatic organisms. Also, the rapid plant growth prevents sunlight from penetrating to the bottom. This happens first in areas of slow-moving or stagnant water.