purposes. Whole-lake experiments of the stress-response type are practical only on small lakes with no shoreline habitation and in watersheds where public access can be limited.

For these reasons, experimental approaches on a smaller scale, such as in situ plastic enclosures (one to a few meters in diameter), often called mesocosms, limnocorrals, or limnotubes, have certain practical advantages and have been popular for field experiments both in Europe, where their use was pioneered by John Lund, and in North America. This intermediate scale of experimentation (smaller than whole lakes or lake basins but larger than laboratory microcosms) has enabled limnologists to perform experimental studies under conditions that can be replicated and subjected to reasonable control using systems more similar in complexity to whole lakes than is possible in the laboratory. Nonetheless, mesocosms cannot duplicate the complicated communities and ecosystems of whole lakes, and they are especially inadequate for studying responses of large fish over long periods (Gorham, 1992).

Despite the small number of whole-lake experiments that have been conducted over the past 30 years, they have been very important in two ways: (1) advancing understanding of fundamental limnological and ecological processes and (2) providing critical evidence for management of major pollution issues such as eutrophication6 (e.g., Schindler, 1974) and acidification (Schindler et al., 1985, 1992; Brezonik et al., 1993). Their strengths for both purposes lie in their ability to test hypotheses (Schindler, 1990) and to provide a "platform" for related laboratory or field experiments at a range of scales. Two brief examples from eutrophication studies at the Experimental Lakes Area (ELA) in western Ontario during the early 1970s illustrate both types of advances.

One of the major issues in eutrophication during this period concerned the role of carbon in limiting primary production in lakes. Rates of carbon dioxide (CO2) transfer across the air-water interface and the question of whether CO2 transfer is a simple physical process or is enhanced by chemical reactions at the air-water interface were of great interest because of the potential importance of this transfer in renewing the limited supply of CO2, especially in low-alkalinity lake waters. Field studies that included carbon mass balances on one of the ELA lakes were used by Emerson (1975) to quantify the amount of chemical enhancement in the CO2 air-water transfer process. The transfer of CO2 across the air-sea interface has been a major topic of research for many years in relation to global climate


 Eutrophication is the nutrient enrichment of lakes that results in an array of symptomatic changes, including an increase in primary production and in the abundance and composition of phytoplankton and other aquatic organisms, and a decrease in water clarity. Some of these changes are considered objectionable and limit the usefulness of the lake for recreational purposes (e.g., swimming, fishing) or as a drinking water supply.

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