and these programs facilitated wider use of experimental approaches in lake limnology. Even so, large-scale experiments (such as whole-lake manipulations) have been relatively few in number because of their comparatively high cost, the long time (at least several years) required to complete them, and the limited availability of lakes that can be dedicated to such purposes. Consequently, experimental approaches at smaller scales using enclosures of one to a few meters in diameter—often called mesocosms, limnocorrals, limnoenclosures, or limnotubes—that are installed in the lake have become popular in Europe and North America. This intermediate scale has enabled limnologists to complete a great variety of experiments, under conditions that can be controlled and replicated, on systems more similar in complexity to whole-lakes than one can achieve in laboratory-scale systems. Nonetheless, mesocosms cannot duplicate the complicated ecosystems of whole-lakes and are especially inadequate to study populations of large fish over long periods.

Because manipulations of whole aquatic ecosystems generally cannot be duplicated, limnologists have focused considerable effort over the past decade on developing sophisticated statistical methods and other techniques to evaluate data from such unreplicated experiments (Carpenter et al., 1989; Rasmussen et al., 1993). Of special importance is the gathering of adequate baseline data prior to manipulation. Paleolimnological techniques (described later in this chapter) also can help to carry such baseline information backward in time.

Despite the difficulties involved in conducting and interpreting whole-lake experiments, a strong consensus has developed among limnologists that observing responses to manipulations made at the whole-system level is a highly useful technique. Whole-lake experiments conducted over the past 30 years have been important both in advancing the understanding of fundamental limnological processes and in providing critical evidence for the management or solution of major pollution issues such as eutrophication and acidification. Their strengths for both purposes lie in their ability to test hypotheses and to provide a "platform" for related laboratory or field experiments at a range of scales.

Paleolimnology

During the middle of this century, the field of paleolimnology (see Box 2-7) developed into one of the key subdisciplines of limnology. Paleolimnology, closely related to paleoecology and paleoclimatology, has its origin in early nineteenth century botanical and chemical studies on peat cores and late nineteenth century geological studies on lithified sediments of ancient lake beds. By the early 1920s, limnologists had begun to collect sediment cores from lakes and to interpret stratigraphic data on plant and animal fossils as a record of the lake's history. Nipkow (1920) was



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