The North American continent was discovered by Western Europeans very recently. Following discovery, there was a period of very rapid development by exploiting the trees in the forest and turning forests and grasslands into agricultural areas, the controlling of rivers, using them for the transportation of logs and to generate power and hydroelectricity. Dams were built and wetlands drained. Interruption of natural channels has made many stretches of river impassable to migratory fish. Rivers were used to dispose of all types of waste - these wastes in combination with excessive nutrient runoff from improved farmland led to significant eutrophication of many lakes.
Eutrophication is a common consequence of intensified use of our environment. However, in lakes in the modern world, eutrophication does not normally happen in isolation: there can be many more factors involved. The North American Great Lakes are one of the best examples of this as they have been greatly changed in many ways over the last 300 yrs.
Increased nutrient input into a water body will have a large impact on primary production and the fish biomass derived from it. Partly, increased nutrient supply increases the amount of primary production that can feed through to fish production, and partly it changes environmental conditions that can interfere with the ecology of particular species, in some cases causing their complete loss. The general changes resulting from eutrophication of lakes can be summarised as taking the fish community from one dominated by fish typical of cool, clear well oxygenated waters such as salmonids through percids to a community dominated by fish tolerant of warm de-oxygenated water such as the cyprinids. This happens to also result in a shift from commercially valuable species to those of less value. From the narrow point of view of fisheries, in water bodies where the initial conditions are very oligotrophic, then the first modest increase in nutrients may increase fish production and can be considered to be beneficial. This can also be the case in eutrophic waters containing species already adapted to eutrophic conditions. This phenomenon is taken to its extreme in some types of aquaculture where large amounts of fertilisers are used to support intensive fish production. However, in the Great Lakes excessive algal production caused deoxygenation of deeper waters especially in Lake Erie, ironically causing massive die-offs of Alewives - fish that had been partly responsible for enhancing the symptoms of eutrophication by changing zooplankton populations.
Page Author: Dr Rick T Leah, Univ of L'pool - Contents last reviewed 05/01/2007
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