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Cultural Eutrophication is defined as: the accelerated aging, or rate of eutrophication, of a lake as a result of human activities (Minnesota Water 2000).
Eutrophication
Eutrophication is the process of nutrient enrichment where a body of water becomes more biologically productive. The trophic state of a lake is basically a measure of the lake's fertility. Trophic states are broken into four categories.
- Oligotrophic
- Mesotrophic
- Eutrophic
- Hypereutrophic
An oligotrophic lake is one of low biological productivity. These lakes are nutrient poor and usually contain small amounts of algae and aquatic plants. One expects an oligotrophic lake to have clear water. A eutrophic lake is one with high biological productivity. With large amounts of nutrients available eutrophic lakes often have large amounts of algae and plankton or many aquatic plants. A mesotrophic lake is simply a lake whose fertility level falls between the fertility levels that are considered oligotrophic and eutrophic. A hypereutrophic lake is a lake whose fertility is much higher than the average eutrophic lake for the area.
Remember that the trophic state of a lake is relative depending on the region where the lake is located. What is considered eutrophic in some areas may only be considered mesotrophic in other regions. The trophic level of a lake is usually determined by either the phosphorus levels in the water column or through secchi disk readings.
Are high trophic states "bad?"
From the recreational and aesthetic view a eutrophic or hypereutrophic lake most likely isn't desireable. The high fertility in these lakes allows for the algal blooms that can form floating mats of scum on the water surface and foul odors. Highly eutrophic lakes can also suffer from fish kills. The large amount of algae and aquatic plants supported by eutrophic lakes can cause the water to become anoxic. After the algae and plants die, they sink to the bottom and decompose. However, the process of decomposition requires oxygen. In shallow lakes it is possible for the large amounts of decomposing organic matter to use up enough oxygen that the water can no longer support fish life, resulting in a fish kill.
At the same time, it is possible that a lake is naturally eutrophic. Fish kills and algae blooms can be the natural and historical norm for a specific lake. Which brings us back to cultural eutrophication. How do human activities influence the trophic states of surface water? The main human impact is from phosphorus loading to the bodies of water as determined by the land use in the surrounding watershed (Fritz 1989).
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