1. Paleoecology/paleolimnology
Paleoecology, the scientific study of ancient environments
and the interrelationships of their plants and animals, and
paleolimnology, the scientific study of evidence of ancient
inland waterways, including lakes, ponds, freshwater
marshes, and streams, were both established at the end of
the nineteenth century. The roots of both disciplines can be traced to early nineteenth-century
botanical and chemical studies, and later, geological studies of the consolidated sediments of
ancient lake beds, in which their organisms and environments were examined in relationship to
both the lakes and the surrounding uplands.
By the early 1920s, limnologists began to collect sediment cores from lakes and to interpret
stratigraphic data from plant and animal fossils as a record of a lake's history. This provided
clues to how lakes had changed over time as a result of either natural events or human
activities. Data collected from lakes and wetlands provide baselines to compare the impacts
of a activities such as land clearance, drainage , water pollution (/science-and-
technology/biology-and-genetics/environmental-studies/water-pollution) , and air pollution
(/science-and-technology/biology-and-genetics/environmental-studies/air-pollution) . They
can also be used to assess the rate of recovery from the activities once they have ended.
Paleoecology/Paleolimnology
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2. Lake and wetland sediments contain detailed archeological records of how the overlying
ecosystems have been affected over time. It is in many ways equivalent to archeological
reconstruction of past civilizations by examining their stratified remains. Lake sediments
(/earth-and-environment/geology-and-oceanography/geology-and-oceanography/lake-
sediments) and wetland peat deposits form a selective trap for a variety of plant and animal
remains and elements such as carbon , phosphorus , sulfur, iron, and manganese that are
stored at varying concentrations depending on the activities that were occurring at the time
the sediment layer was formed. Changes in this profile record not only the history of the lake or
peatland but also what happened in the surrounding watershed . Initially, limnologists
collected core samples from lakes and interpreted them on the basis of plant and animal
fossils. The limnologists observed that in some lakes there were thin laminated sediments.
These were shown to result from an annual deposition cycle which involves summer
deposition of calcium carbonate (/science-and-technology/chemistry/compounds-and-
elements/calcium-carbonate) and deposition of organic matter (/earth-and-
environment/ecology-and-environmentalism/environmental-studies/organic-matter) during the
rest of the year. These alternating light and dark layers represent a yearly cycle. Using these
bands, or laminae, scientists can count back in time and date individual strata of sediment
cores.
Studying plant and animal microfossil remains (such as pollen, diatom shells, and remains of
zooplankton bodies) in the sediment cores and knowing the environments in which these
organisms occur today allow a limnologist to reconstruct historical conditions in a lake as well
as in its drainage basin. Such wetland studies were limited, however, until the development of
radioisotope dating methods in the 1950s and 1960s. Today radioisotopes such as carbon-14
and lead-210 can be used to date the time a sediment was deposited.
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3. Other methods limnologists use include the pollen, which indicates what types of land
vegetation were present. They also examine plant and animal fossil remains, including cell
fragments and molecules such as plant pigments, all of which provide clues about vegetation
and aquatic life. Limnologists also search for organic pollutants and certain trace elements,
whose presence indicates the result of human activities.
Analysis of the layers in sediment cores from lakes and wetlands has provided information
about regional variations in past climatic conditions and watershed vegetation patterns which
indicates the causes of environmental change. Studies on recently deposited lake sediments
have provided evidence for the timing and causes of lake pollution , including the effects of
excess nutrients on lake ecology and information about atmospheric transport of various
pollutants.
These techniques have also been applied to address basic plant ecology questions regarding
plant succession , on which there are two major schools of thought. The first is the community
concept which has three basic attributes: vegetation occurs in recognizable and characteristic
communities; community change through time occurs because of the vegetation; and the
changes occur in a sequence that leads to a mature stable climax ecosystem . The community
explanation can be contrasted with the continuum concept. Here, the distribution of vegetation
is determined by the environment . Since each plant species adapts differently, no two occupy
the exact same location. Additionally, the observed replacement sequence is influenced by the
chance occurrence of viable seeds that allows a certain plant to grow on the site. This results ×
4. in a continuum of overlapping sets of species. In this scenario although ecosystems change, it
is not directed toward a particular climax community (/science-and-technology/biology-and-
genetics/environmental-studies/climax-community).
One of the areas where these two concepts directly collide is in the study of wetlands and
peatlands . The classical community view of succession is that wetlands are a transitional
stage in the progression from shallow lake to terrestrial forested climax community (/science-
and-technology/biology-and-genetics/environmental-studies/climax-community). This view
requires that lakes gradually fill in as organic material from dying plants accumulates and
minerals are carried down from upslope into the water. At first change is slow, but it
accelerates when the lake becomes shallow enough to support rooted aquatic plants. When
the water becomes even more shallow, it supports the development of a peat mat, allowing
trees and shrubs to grow which further modifies the site by not only adding organic matter
(/earth-and-environment/ecology-and-environmentalism/environmental-studies/organic-
matter) but also by drying the site through evapotranspiration . According to the community
version, in the final stages a climax forest occupies the site. This sequence implies that most
of the change is caused by plants and not external environmental changes.
Paleoecological analyses of peat beds have provided information refuting the validity of the
community concept's explanation. Fossil records including analysis of pollen in northern peat
lands provide two generalizations: in some sites the present vegetation has existed for several
thousand years and climatic change and glaciation had a large impact on plant species and
distribution. Generally, bogs expanded during warm, wet periods and contracted during cool,
drier periods although the influence of local topographic and drainage conditions often mask
climatic shifts.
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5. While this evidence supports the continuum assumptions that other environmental factors
overwhelm the vegetation effects, there is evidence within the bogs that says that vegetation
can have a large impact on the character of the landscape. In particular, this is evidenced by
the patterned landscape of these peatlands as the water flow is changed due to the
vegetation. It seems that in the wetlands or bogs themselves there can be changes in
vegetation that occur over time as a result of succession. However, these changes do not
necessarily point toward a terrestrial climax. In fact, pollen profiles indicate that a bog, not
some type of terrestrial forest, is the endpoint. These studies confirmed conclusions from a
classical study on the Lake Aggassiz Plain (Minnesota and south-central Canada) that
indicates most of the peatlands developed during a moist climate about 4,000 years ago when
surface water levels rose about 12 ft (4 m). This implies that while there may be changes
within the peatlands, they are stable, so the central concepts of succession in this case are not
supported.
Examples of the findings and results of some significant types of paleolimnological studies
show how they are useful in evaluating changes and as indicators of potential problems due to
human activities. The Red Lake peatland in northwestern Minnesota has been used as a study
site to document the occurrence of acid rain (/science-and-technology/biology-and-
genetics/environmental-studies/acid-rain) and global warming (/science-and-
technology/biology-and-genetics/environmental-studies/global-warming). Based on past
changes in the type of vegetation and changes in pH , future impacts on the bog surface can
be evaluated. If acid deposition were to lower the pH of the bog significantly, different types of
mosses would be evident. Likewise, if global warming (/science-and-technology/biology-and-
genetics/environmental-studies/global-warming) were to lower the water tables' moss types,
characteristics of drier conditions would occur.
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6. Using sediment cores from lakes and observing the changes in flora , fauna , and chemistry
allows the evaluation of impacts of human settlement on lakes in the upper Midwest of the
United States (/places/united-states-and-canada/us-political-geography/united-states). A lake
studied in northern Minnesota on the iron range shows these changes. There was a distinct
increase in hematite (iron) grains in the sediments as mining began and a decrease as it
declined. Settlement was also marked by a rise in the concentration or ragweed pollen which
reflects the replacement of the forests with agricultural fields. There were also changes in the
type of diatom shells reflecting the nutrient enrichment of the lake due to the discharge of
sewage directly to the lake. By analyzing the pH preferences of individual species of diatoms
and algae the past pH conditions in a lake can be determined. Using the results of these
studies it has been demonstrated that lakes in the Adirondack Mountains (/places/united-
states-and-canada/us-physical-geography/adirondack-mountains) were not naturally acidic.
Current monitoring suggests that these lakes that were acidified due to acid rain have not yet
responded reductions in the amount of sulfate deposition.
[James L. Anderson ]
RESOURCES
BOOKS
Freshwater Ecosystems: Revitalizing Educational Programs in Limnology. Washington, DC:
National Academy Press, 1996.
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7. More From encyclopedia.com
Mitsch, W.J., and J.G. Gosselink. Wetlands. New York (/places/united-states-and-canada/us-
political-geography/new-york): Van Nostrand Reinhold, 1993.
Environmental Encyclopedia
(https://www.encyclopedia.com
and-environment/ecology-and-
environmentalism/environmenta
studies/lakes)
Lakes
(https://www.encyclopedia.com
and-environment/geology-and-
oceanography/geology-and-
oceanography/lake)
Lake
(https://www.encyclopedia.com
and-environment/ecology-and-
environmentalism/environmenta
studies/limnology)
Limnology
(https://www.encyclopedia.com
states-and-canada/us-physical-
geography/great-lakes)
Great Lakes
(https://www.encyclopedia.com
physical-geography/lake-
natron)
Lake Natron
(https://www.encyclopedia.com
states-and-canada/us-physical-
geography/lake-huron)
Lake Huron
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Paleoecology/Paleolimnology
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