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 , and air pollution . They can also be used to assess the rate of recovery from the activities once they have ended.
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 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 and deposition of 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.
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 in a continuum of overlapping sets of species. In this scenario although ecosystems change, it is not directed toward a particular 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. 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 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.
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 and 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 were to lower the water tables' moss types, characteristics of drier conditions would occur.
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. 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 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 ]
Freshwater Ecosystems: Revitalizing Educational Programs in Limnology. Washington, DC: National Academy Press, 1996.
Mitsch, W.J., and J.G. Gosselink. Wetlands. New York: Van Nostrand Reinhold, 1993.