Eutrophication of Lakes
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The Eutrophication of lakes.
Eutrophication is defined as ÐŽ§the aging of a lake by the biological enrichment of its waterÐŽÐ (encyclopedia.com). This biological enrichment is caused by the addition of nutrients. There is natural eutrophication and cultural eutrophication. Cultural eutrophication is usually defined as the over-enrichment of lakes and rivers with nutrients, usually phosphorous, leading to excessive growth of algae and other aquatic plants. Cultural eutrophication is becoming more common with increased agricultural activity and development of cities around lakes.
At first, when a lake is young and just has been formed, the water is usually cold and clear. These conditions support little life but with time drainage of streams into the lake add new compounds, including phosphorous and nitrogen compounds.
Natural eutrophication can take many years. On the other hand, cultural eutrophication is a relatively fast process. Human settlement near lakes causes a great increase in nutrient input to the lake. In addition, sewage and wastes are usually thrown into nearby streams that drain into the lake and further increase the nutrient supply. The main nutrients that are present in cultural eutrophication are P and N containing compounds coming from sewage. Some algae and blue-green bacteria can live off of these compounds and rapidly grow in number. This phenomenon is called algal blooming. Eventually, the rate in growth is so great that it becomes unsustainable in natural systems.
Since eutrophication is a worldwide phenomenon and is of great importance economically, much effort and money has been put into creating models to experiment with lake manipulation. Hoping eventually to come up with an answer to stop or slow the process eutrophication.
In 1968, Richard A. Vollenweider developed a practical model to compare European and North American lakes. His model consists of the trophic status of a lake, which he says is ÐŽ§a function of the rate of areal nutrient supply (g/m2/yr).ÐŽÐ His model formed the basis for action in many jurisdictions for stopping cultural eutrophication. Another model compelled by P.J Dillon and F.H. Rigler focused more on the total phosphorus concentration (mg/L). Their equation is the following:
єЛTPєР= (Lp/qs) * (1-R)
The Lp is the annual areal phosphorus loading, qs is the annual areal water loading (mean depth/hydraulic residence time) and R is the P retention coefficient (1 – (annual outflow P load/annual inflow P load)). Their model allows to set a P concentration target, and then determine the loadings necessary to achieve the target. Both of these models were used in Ontario to stop and reverse the process of cultural eutrophication.
The Great Lakes in Northern America have been suffering of cultural eutrophication since the early settlements. Sewage and household waste were dumped into streams that connected to the lakes. After World War II, urban population increased near the Great Lakes basin, which lead to large-scale eutrophication problem. In 1972 Great Lakes Water Quality Agreement was signed by the United States and Canada. The primary emphasis was to reduce phosphorus inputs to Great Lakes, particularly Lake Erie. The International Joint