key questions become apparent only after detailed long-term study. Through observation of aquatic ecosystems, patterns or relationships are discovered that cannot be readily explained. Even the seemingly simple determination of whether an observed pattern is caused by hydrologic, chemical, or biological processes may be difficult. Incorrect assumptions about unexplained patterns or observations can lead to confusion.

An inherent challenge in limnology is "scaling up" from controlled laboratory experiments to real aquatic ecosystems. In studies of the biodegradation of organic contaminants or of many physical processes, for example, it may be true that a beaker is equivalent to a lake or an experimental flume is equivalent to a stream. Conversely, field experiments, such as ecosystem manipulations, are difficult to replicate and may be influenced by uncontrolled variables.

Within this context of limnology as an integrative science addressing aquatic systems, it is a challenge to distinguish which questions are fundamental and which are merely intriguing. A fundamental question may lead to a domino effect, where one answer illuminates answers to many other questions. Further, a fundamental question may or may not be a "big" question, that is, an area in which we recognize a gaping void in understanding or knowledge. For instance, the role of peatlands in the global carbon cycle is a fundamental question, but a big question that is not fundamental in the same sense is just how large the peat reservoir is, particularly in Russia. We know that Russia has enormous deposits of peat, especially in the west Siberia Plain, but the numbers on both peatland area and mass of peat are of questionable accuracy.

INLAND AQUATIC ECOSYSTEMS

For the purposes of providing a background for this study, we have chosen to pose questions at a level relevant to the full range of inland aquatic ecosystems (lakes, wetlands, streams, big rivers, ponds, etc.). An ecosystem is defined by its boundaries, and an inland aquatic ecosystem can range in size from a small transient desert pool that forms after a storm to the watershed of the Colorado River. Boundaries can be set based on their utility for studying a given process or interaction. For example, defining an aquatic ecosystem as an entire watershed may be useful for studying responses to acidic deposition, but it may not be useful for studying the daily migration of zooplankton between depths in a lake within that watershed.

Several approaches can be used to classify inland waters. To illustrate their diversity and distribution, we have broadly classified inland waters based on the physical or chemical characteristics that exert major influences



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