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 forested and protected watershed. Over great time sediments accumulated and the lake depth
decreased. As the lake became shallower, plants begin to grow along the littoral zone where
light could penetrate to the shallow sediments. This then tended to allow materials to accumulate
more efficiently in those areas and the sediments become richer in nutrients. With a great amount
of additional time the lake became shallow enough to be considered a wetland with aquatic
plants emerging in the middle of the old lake bed. Finally trees and other plants colonized the
drier sediments. This `death' or "eutrophication" of a natural lake was later applied to the rapid
processes associated with the artificial nutrient pollution during recent times that contributed to
some of these aging processes. Man-induced nutrient enrichment is considered "cultural
eutrophication" because it is largely due to the characteristics of our culture that we contribute
to those nutrient additions.
Earlier in the 20th century there were developed a number of indices for comparing the
status of different lakes and other surface waters. The most recent example of such an index is
the trophic state index or TSI (. This is based on the relationship between a number of related
lake characteristics such as water clarity (secchi depth), nutrient concentration, chlorophyll
concentration, and others. This index is a useful tool for initial comparisons but its applicability
must be carefully judged for each situation.
In reservoirs, eutrophication is usually viewed in terms of nutrient enrichment and the
outcome of nutrient enrichment because there is no historical perspective for comparison. Most
reservoirs (nearly all the large ones) were built during the 20th century and despite that, some
have already substantially filled with sediments. This is an outcome of some of the differences
between natural lakes and reservoirs that were outlined earlier in this course.
In northern natural lakes, cores of old accumulated sediments clearly show the shift of
terrestrial plant pollen types from forests dominated by oaks and other trees to grasses and the
definitive early successional plant, Ambrosia (ragweed), that became common after the forests
were cut by the first settlers. This pollen record not only tells the story of ecosystem change in
the landscape, it also parallels the change in sedimentation rate, and the changes in the types of
organisms living in the lake. Algal fossils (diatoms, mostly) tell of sudden changes of species
associated with cultural eutrophication the result of increased nutrient and sediment loading to
the aquatic environment. Similar trends have also been observed in the patterns of zooplankton
fossils in old lake sediments. These changes have continued for hundreds of years in North
America, thousands of years in Europe and Asia, and in the 1950's and 60's the changes
accelerated rapidly with increased use of phosphate detergents, pesticides, development,
agriculture, and other impacts to the watershed.
Ecological Growth Control. Nuisance growth of algae (and other plants) is related to
eutrophication (cultural eutrophication), nutrient loading, sedimentation, land use, and many
other factors. The specific causes of nuisance growth relate to most of those factors. Ecologists
are divided in their assessment of the factors controlling algal growth. While it is acknowledged
that all of the factors can affect growth, these factors are divided into three categories related to
1.2.47
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