Organizing Paradigms for the Study of Inland Aquatic Ecosystems

Patrick L. Brezonik

Department of Civil Engineering and Water Resources Center

University of Minnesota

Minneapolis and St. Paul, Minnesota

SUMMARY

This paper describes the major organizing principles that have driven the development of the science of limnology for lakes, flowing waters, and wetlands. Several broad themes that contribute to the understanding of how these water bodies behave as ecosystems have played important roles in integrating the component disciplines. These include the concept of lakes as microcosms, wetlands as ecotones or "gradient ecosystems," and the River Continuum Concept. In recent years, the importance of terrestrial-aquatic interactions has been widely recognized, and a paradigm common to all classes of inland aquatic ecosystems regards them as reflections or integrators of conditions in the watershed in which they are located.

INTRODUCTION

Limnology is usually defined as the science of all inland aquatic systems: lakes, reservoirs, rivers, streams, and wetlands.1 For example, in a chapter entitled "What Is Limnology," Edmondson (1994) describes limnology as

the study of inland waters … as systems. It is a multidisciplinary field that involves all the sciences that can be brought to bear on the understanding of such waters: the physical, chemical, earth, and biological sciences, and mathematics.

1  

Some aquatic scientists include ground water in this list, but most limnologists probably regard ground water as they do atmospheric precipitation: a potential source of water (and chemicals) to their systems of interest, which may be studied in this context, but not systems that they study in their own right.



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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Organizing Paradigms for the Study of Inland Aquatic Ecosystems Patrick L. Brezonik Department of Civil Engineering and Water Resources Center University of Minnesota Minneapolis and St. Paul, Minnesota SUMMARY This paper describes the major organizing principles that have driven the development of the science of limnology for lakes, flowing waters, and wetlands. Several broad themes that contribute to the understanding of how these water bodies behave as ecosystems have played important roles in integrating the component disciplines. These include the concept of lakes as microcosms, wetlands as ecotones or "gradient ecosystems," and the River Continuum Concept. In recent years, the importance of terrestrial-aquatic interactions has been widely recognized, and a paradigm common to all classes of inland aquatic ecosystems regards them as reflections or integrators of conditions in the watershed in which they are located. INTRODUCTION Limnology is usually defined as the science of all inland aquatic systems: lakes, reservoirs, rivers, streams, and wetlands.1 For example, in a chapter entitled "What Is Limnology," Edmondson (1994) describes limnology as the study of inland waters … as systems. It is a multidisciplinary field that involves all the sciences that can be brought to bear on the understanding of such waters: the physical, chemical, earth, and biological sciences, and mathematics. 1   Some aquatic scientists include ground water in this list, but most limnologists probably regard ground water as they do atmospheric precipitation: a potential source of water (and chemicals) to their systems of interest, which may be studied in this context, but not systems that they study in their own right.

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Similarly, a recent "state of the science" assessment (Lewis et al., 1995) defined limnology as the integrative study of inland waters. It encompasses the biological, chemical and physical phenomena, as well as all levels of organization extending from individual chemical reactions or adaptations of individual organisms to the analysis of entire ecosystems. Nonetheless, many aquatic scientists, as well as the general public, associate the word limnology exclusively with the study of lakes (and reservoirs). There are historical reasons; organized studies of lakes using techniques of modern science began much earlier (late nineteenth century) than studies of flowing waters and wetlands.2 In addition, the discrete, semiclosed nature of lake basins, compared with the open, flowing nature of streams and the often diffuse boundaries of wetlands, makes it easier to view lakes as appropriate objects of scientific study. Lake studies have always been approached holistically to include physical, chemical, biological, and geological aspects, even though biologists conducted most of the early work. In contrast, the physical, geological, chemical, and biological characteristics of streams have been studied primarily within separate disciplines (i.e., as separate fields of study)—hydrology, geomorphology, geochemistry, sanitary (environmental) engineering, public health biology, fisheries science. These studies generally were pursued in separate academic departments such as civil engineering, geography, geology, public health, biology, and fisheries. To a considerable extent, this fragmentation still exists today—to the extent that stream limnology as a distinct field within the broad field of limnology might be questioned. To be sure, academic lake limnology suffers from the same fragmentation, but it is still considered a discrete field. Except for studies on wetland flora and fauna by botanists and zoologists, studies on wetlands are even more recent, especially in North America, and for the most part, comprehensive studies of wetlands as ecosystems are the product of the past 20 to 30 years. This paper examines the various paradigms that have driven the fields and subdisciplines of limnology since its founding in the late nineteenth century. As described above, the subareas of limnology differ substantially in the length of time over which they have developed, and this has important implications concerning the driving paradigms. 2   Some ecological studies on wetlands predate the beginnings of modern science in the late eighteenth and early nineteenth century (see section of this paper on Wetlands for further details), but the prevailing attitude that wetlands are wastelands limited both scientific and public interest in these ecosystems until recent years.

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology LAKES Lakes come in all sizes and shapes and have a wide range of chemical and biological characteristics. Nonetheless, they all share fundamental attributes that make it useful to define them as a class of objects worthy of study. They have well-defined physical boundaries and are semiclosed systems (in the sense that water stays within a lake's boundaries long enough to develop distinctive biological and chemical characteristics compared with the water that flows into it). Moreover, lakes are important to humans for practical, recreational, and aesthetic reasons, and this, as much as any factor, led to an early development of the science of lake limnology. Major changes have occurred in the organizing paradigms for the study of lakes over the past century, reflecting advances not only in our understanding of how these complicated natural systems operate, but also in the basic sciences that contribute to the study of lakes. Lakes as Microcosms or Integrated Ecosystems The founders of lacustrine limnology in the latter half of the nineteenth century viewed the subject expansively as the application of all (relevant) basic sciences to the investigation of lakes as fundamental objects of study. In his seminal paper of 1887, Stephen Forbes, one of the earliest limnologists, described lakes as ''microcosms," or little worlds. Although the term "ecosystem" was not introduced for another half century (by Tansley in 1935), Forbes' approach was essentially that of an ecosystem scientist. He proposed that lake studies should focus on many of the ecosystem-level processes that define the late twentieth century field of ecosystem ecology: the circulation of elements and substances (now called biogeochemistry), the production and decomposition of organic matter, food web interactions (especially predator-prey relationships) and their resulting effects on the structure of biological communities, and the effects of physical conditions and gradients on biological communities. Together, these topics define lakes as functioning, integrated systems. The paradigm of the lake as a microcosm or integrated ecosystem has pervaded the study of lakes to this day. Of course, the concept has been refined as the science of ecology has developed, and it has been supplemented by other organizing paradigms at the same scale of organization (e.g., lakes as experimental units) and some at even larger scales of organization (the more recent paradigm of the lake as an integrator or reflection of its watershed). Current limnological studies on lakes focus on lakes as "open" systems that receive inputs of water, solar energy, and chemical substances from outside the system. As implied by the second definition of limnology given in the introduction, studies within lakes at smaller scales of organization (e.g., studies of chemical reactions or populations of organisms) also are part of the field of limnology, and as described

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology below, numerous paradigms have been developed for these scales and the scientific subdisciplines of limnology. François Forel was the first scientist to use the term limnology in a publication, and his three-volume Le Leman: Monographie Limnologique ,3 published over the period 1892 to 1904, is considered to be the first book on limnology. Encyclopedic in scope, the book is divided into 14 chapters (Table 1), 11 of which define the main supporting fields of modern limnology (Edmondson, 1994). Taken together, Forbes' and Forel's contributions provide the multidisciplinary and interdisciplinary framework that still defines the science of limnology. The founders of academic limnology in North America, Edward Birge and his younger colleague Chancey Juday, continued this multidisciplinary, interdisciplinary tradition at the University of Wisconsin (Brooks et al., 1951; Mortimer, 1956; Frey, 1963). Birge was trained as a zoologist and was attracted to lake studies (in the context of zooplankton life cycles) even as an undergraduate at Williams College in Massachusetts during the early 1870s. Juday also was trained as a biologist and was first hired by Birge in 1897 to help conduct lake surveys for the newly established Wisconsin Geological and Natural History Survey, of which Birge served as the first director. However, Birge and Juday soon branched into the physics and chemistry of lakes as they realized that one could not understand the dynamics of plankton without knowledge of these subjects. Their studies on annual cycles of thermal stratification (Birge, 1898) and dissolved gases (Birge and Juday, 1911) are seminal works that provided limnologists with information essential for understanding virtually all biological cycles in lakes. Birge and Juday knew the limitations of their training and actively sought collaboration with physicists and chemists to study lake phenomena beyond their own field of expertise. Together, these scientists developed many new techniques to measure physical properties and processes (e.g., light transmission, heat transfer, and heat TABLE 1 Contents of Forel's Treatise on Lac Leman (Lake Geneva) Vol. 1 (1892) Vol. 2 (1895) Vol. 3 (1904) Geography Hydraulics Biology Hydrography Temperature Historya Geology Optics Navigationa Climatology Acousticsa Fish Hydrology Chemistry   a Not considered a subfield of modern limnology. 3   Lac Leman is Lake Geneva (Switzerland/France).

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology budgets) and many chemical characteristics of lakes, including the first methods used to analyze the nature of dissolved organic matter. The early Wisconsin limnologists collected a wealth of information on individual lakes and synthesized this at the regional scale. As practiced by these scientists and their contemporaries, limnology was essentially a passive or observational science, but this term is not meant (nor should it be taken) in a pejorative sense as merely qualitative and lacking in rigor. The knowledge gained was largely from sample collection, analysis, and interpretation rather than from controlled experiments. Quantitative analyses and interpretation of data were undertaken on various physical processes (e.g., heat budgets and light transmission) as far back as the 1920s. The regional, descriptive approach to limnology, at least in part, can be regarded as an outgrowth of the lake-as-a-microcosm paradigm, in that the observations on individual lakes usually were multidisciplinary, involving physical, chemical, and biological elements and implicitly or explicitly recognizing lakes as complex organized systems. Regional efforts during the first half of the century focused on grouping lakes into major types or classes based on a multidimensional set of descriptors. For example, in the classification scheme that categorizes lakes according to trophic state (i.e., general nutritional status), 4 a wide array of indicators was developed for classification purposes, including physical measures (transparency), chemical concentrations (nutrients), and biological characteristics (species types and abundance and primary production). In this sense, classification, which is looked upon by many current scientists as a fairly unimportant or useless exercise, was a force for integration and synthesis leading to generalizations about lakes as systems. Lakes as Experimental Systems The idea that lakes can serve as subjects for scientific manipulation is also a product primarily of the Wisconsin school of limnology. Juday apparently was the first to treat a whole lake in this way. In the 1930s, he added fertilizer to Weber Lake, a small pond in northern Wisconsin, to study its effects on plankton production and fish populations (Juday and Schloemer, 1938). Einsele (1941) did a similar experiment on a small lake in northern Germany a few years later. However, neither of these manipulations seems to have had much impact on the development of experimental limnology, perhaps because of the disruptive influence of World War II on natural science. 4    The trophic classification system groups lakes into three main categories: oligotrophic (low in nutrients and plant production), mesotrophic (intermediate in these characteristics), and eutrophic (rich in nutrients and high in plant production).

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology The later but better-known "before-and-after" experimental liming of Cather Lake in 1950 (Hasler, 1964) and "paired manipulation" of Lakes Peter and Paul by University of Wisconsin limnologist Arthur Hasler (Johnson and Hasler, 1954; Stross and Hasler, 1960) are sometimes cited as the first ventures in whole-lake experimentation. In the latter case, a small, colored, two-basin lake was separated into two lakes by an earthen dike. One basin was treated with lime in an effort to improve water clarity and increase primary production; the other basin was retained as a control. Hasler's group was involved in several other whole-lake manipulations during the 1950s and 1960s. Examples include the first whole-lake experimental acidification, which involved a small bog lake (Zicker, 1955); aeration-induced destratification experiments on several eutrophic lakes (Schmitz and Hasler, 1958); and the addition of short-lived radioisotopes to the water of stratified lakes to measure rates of water movement (e.g., Likens and Hasler, 1960). Hasler's group also pioneered the use of small artificial ponds (wading pool size) treated in various ways to simulate lakes. An earlier whole-lake radiotracer experiment by G. Evelyn Hutchinson at Yale University actually predates all of the lake manipulation experiments of Hasler's group in Wisconsin. In June 1946, he added approximately 10 millicuries of 32P-phosphate to Linsley Pond, a small lake in Connecticut. The distribution of radiophosphorus was determined in several strata of the water column and in littoral macrophytes on two dates over the following month (Hutchinson and Bowen, 1947).5 Compared with the sophistication and complexity of limnological papers published today, the 32P tracer experiment on Linsley Pond was extremely simple. It involved only a few crude measurements and only the simplest mathematical analysis of the data. Techniques available to measure the radiotracer in lake samples were very crude (a simple Geiger counter) compared with the sophisticated and highly sensitive tools available today. Also, the regulatory climate for use of radioisotopes in the ambient environment was much more relaxed in the 1940s than it is today. Today, experimental limnologists conduct at least three types of whole-lake manipulations: stress-response experiments, in which a whole lake (or one basin of a multibasin lake) is treated with some chemical stressor, such as excess nutrients or acid, and the responses of the lake system are studied; remediation and rehabilitation manipulations, involving hydrologic manipulations such as water level fluctuations to improve littoral (near-Shore) 5    According to these authors, an even earlier but unsuccessful effort was made to conduct such an experiment in 1941. A similar 32P experiment was conducted by F. R. Hayes and coworkers on a small lake in Nova Scotia a few years later (Coffin et al., 1949).

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology habitat, physical manipulations such as aeration and removal of contaminated sediments by dredging, chemical manipulations such as addition of alum to remove phosphorus or lime to neutralize acidity, or biological manipulations such as fish stocking or removal and other food web controls; and tracer additions to measure rates of physical processes, such as use of radiotracers to determine water movement and sulfur hexafluoride (SF6) and noble gases to monitor air-water gas exchange. Not all of these manipulations are done for scientific reasons; many of the remediation and rehabilitation procedures outlined above are performed for management purposes rather than to gather new knowledge. Of course, prototype studies on these methods also are done to study their effectiveness. Most stress-response manipulations of lakes are done under the lake-as-a-microcosm paradigm in that a wide range of responses are studied: changes in chemical concentrations and chemical processes; effects on individual organisms, populations, and communities; and ecosystem-level parameters. Whole-lake manipulations generally cannot be replicated, and some manipulation projects have not even included a control or reference lake. Premanipulation data can be used as a baseline, but because lakes are dynamic systems, there are always some uncertainties about this approach (i.e., if changes are observed following manipulation, would they have happened even if the manipulation had not occurred?). Because of this, a few scientists have argued that such manipulations are not true experiments, and in the sense of traditional statistical design, they are not. However, sophisticated statistical methods such as randomized intervention analysis (Carpenter et al., 1989) have been developed to analyze the data from such manipulations. In the literature of the past decade, there has been considerable discussion about the interpretation of large-scale manipulations with their common problem of limited replication (Hurlbert, 1984; Stewart-Oaten et al., 1986, 1992; Carpenter, 1990). Overall, however, there appears to be a strong consensus on the importance and utility of observing responses to manipulations made at the whole-system level. Experimental limnology on a large scale (whole lakes or large enclosures) did not play a prominent role in the science until the late 1960s and 1970s, probably because of the general lack of funding to support such initiatives. Widespread concern about lake eutrophication prompted large government research programs in the United States, Canada, and several countries in western Europe in the 1960s, and these facilitated the wider use of experimental approaches in limnology. Even since then, whole-lake experiments have been relatively few in number for two practical reasons: (1) the high cost, general difficulty, and long time required for completion; and (2) the limited number of lakes available for such

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology 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 6    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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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology change, and the ELA work also serves as an important example of the use of lakes as models for global-scale processes. Related studies at ELA during this period also produced dramatic evidence of the key role of phosphorus in lake eutrophication. Addition of phosphorus to one side of an oligotrophic lake (which had been divided into two basins by a plastic barrier) led to the rapid appearance of a dense blue-green algal bloom (Schindler, 1974). In contrast, addition of inorganic nitrogen to the other side of the lake did not produce any change in algal biomass. Evidence for the key role of phosphorus in eutrophication had been accumulating from a wide variety of other limnological studies, including laboratory experiments, in situ limiting-nutrient bioassays, nutrient budgets, lake surveys, and application of mathematical models. Nonetheless, the whole-lake results were thought by many to be incontrovertible evidence; they received much public and scientific attention, and they played an important role in government decisions to control phosphorus inputs to lakes. Lakes as Chronicles of Natural History and Evolving Ecosystems Recognition that lake sediments are a repository of information about the past conditions in lakes and their watersheds—and how they have evolved over time—dates back to the early days of limnology. Most of this early "paleolimnology" was done by geologists on lithified sediments of ancient lakes. Bradley's chapter on paleolimnology (Bradley, 1963) in the compendium Limnology in North America deals almost exclusively with such studies. Probably the first paleolimnological studies in the United States were on Pleistocene Lake Bonneville, the much larger, freshwater ancestor of the present Great Salt Lake (Gilbert, 1891), and on Pleistocene Lake Lahontan in western Nevada (Russell, 1885). Stratigraphic studies on the sediments of modern lakes to reconstruct historical conditions date back at least to the 1920s. For example, Nipkow (1920) was the first to observe and explain the existence of thinly laminated or "varved" sediments. He showed that the laminae resulted from an annual depositional cycle in which spring and summer photosynthesis causes precipitation of calcite, which settles and forms a thin carbonate layer on the sediments. Organic detritus deposited during the remainder of the year forms a dark (organic-rich) layer on top of the calcite layer. These thin annual laminae allow limnologists to count back in time to date individual strata of a sediment core. By studying plant and animal microfossil remains (e.g., pollen, diatom shells, remains of zoo plankton bodies) in various laminae, paleolimnologists thus could reconstruct historical conditions in the lake and/or its watershed. Unfortunately, relatively few lakes deposit varved sediments, and it was not until the 1950s and 1960s, when radioisotope dating methods

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology (especially 14C and 210Pb methods) were developed, that the field of paleolimnology grew to become one of the important subdisciplines of limnology. The tools of paleolimnology include radiochronometers (14C, 210Pb) that date the time of deposit for sediment strata, pollen as an indicator of terrestrial vegetation, a variety of plant and animal fossil remains (both cell fragments and molecules such as plant pigments), and more recently, various organic pollutants and trace elements whose biogeochemical cycles have been influenced by human activity. Over the past 30 years or so, stratigraphic analyses of long cores of lake and wetland sediments have provided important information on regional variations in past climatic conditions and watershed vegetation patterns, from which paleoecologists have sought to answer fundamental questions about the causes of environmental change, including questions of species extinction (e.g., Deevey, 1967). Paleolimnological studies on more recently deposited sediments have provided evidence for the timing and causes of cultural eutrophication (e.g., Brezonik and Engstrom, in press), as well as temporal patterns and geographic scales of atmospheric transport of various pollutant chemicals (e.g., Swain et al., 1992). Lakes as Chemical Reactors In the late 1960s, concern about the transport of chemical pollutants to lakes, their effects on biota and water quality, and their ultimate fate led to the idea that lakes can be modeled as chemical reactors. Especially important is the concept of the lake as a completely mixed flow-through reactor, often referred to as a CFSTR (continuous-flow, stirred-tank reactor; see Brezonik, 1994). Although Piontelli and Tonolli (1964) presented some initial thoughts and equations related to this idea, the CFSTR approach was developed primarily by Vollenweider (1969, 1975). He applied the principles of reactor kinetics from chemical engineering to lakes and developed a practical means to predict phosphorus concentrations in a lake from the phosphorus loading (input) rate and simple morphometric and hydrologic data. In turn, the model (and similar ones developed under this paradigm) led to the formulation of loading criteria or management guidelines to protect lakes from eutrophication problems. The reactor concept has been used subsequently to model the behavior of many other natural and anthropogenically derived chemicals in lakes, such as sulfate in acid-sensitive lakes (Baker and Brezonik, 1988; Kelly et al., 1988) and humic matter in colored lakes (Engstrom, 1987). Moreover, other concepts of reactor kinetics, such as water and substance residence times, have been used widely to provide simple quantitative measures of lake system dynamics. Lakes as Components of Integrated Watersheds Taken literally, the microcosm paradigm considers lakes as isolated entities and disregards the land shoreward of the water's edge, littoral

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology wetlands, tributary streams, and the lake's terrestrial watershed. In reality, however, limnologists have recognized for many decades that lakes are not isolated from the other aquatic and terrestrial components of watersheds, even though limnologists focused on processes within lakes. As concern grew during the mid-twentieth century about the effects of excessive nutrient loadings on lake ecosystems and lake water quality, engineers and limnologists began to quantify these loadings in the form of nutrient budgets. Water budgets, of course, were found to be a necessary part of such studies, and these efforts gradually led to an increased awareness of the importance and closeness of the interactions between lakes and their surrounding landscape, including littoral or contiguous wetlands (Wetzel, 1990, 1992). Today, policymakers and others promote the concept of integrated watershed planning and management, a "new paradigm" for water management that many hope will replace the "command-and-control" or top-down regulatory paradigm that has driven surface water management in the United States for the past 25 years or more. If this management paradigm is to be based on sound science, limnology must continue broadening and developing the paradigm of lakes as components of integrated watersheds (see Hynes, 1975, as an early example of concern about this issue). Academic programs in limnology will need to train students more broadly in fields such as hydrology, soil science, and landscape ecology. Research programs will have to recognize that lakes (and streams) mirror conditions in their watersheds and that land-water interactions are an inseparable component within the logical basic unit for limnological studies—the watershed (Cummins et al., 1995). Other Organizing Paradigms for Lake Studies Lewis et al. (1995) describe a variety of unifying concepts and research themes that have driven the field of limnology in recent decades. In general, these themes are not as broad as the major organizing paradigms described in previous sections, but each has been important in both a research and a management context, and several have affected the way limnology is taught in graduate programs. Rules for predicting the interaction of predator and prey is a topic that dates back to Forbes' (1887) paper on lakes as a microcosm. In recent years, this theme has focused on such topics as size-selective predation by fish and the "trophic cascade" (Carpenter and Kitchell, 1993), that is, direct and indirect controls on lower trophic levels by predators. "Top-down control" and control by resource limitation (Tilman, 1982) are competing explanations for the control of primary production in aquatic ecosystems, and the relative importance of these two mechanisms is the subject of much current research. On the practical side, the "top-down" paradigm

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology has led to the idea that the consequences of nutrient enrichment (i.e., high primary production) can be managed by food web management (e.g., Kitchell, 1992). This idea, sometimes called biomanipulation, was pioneered by Shapiro and his students (e.g., Shapiro et al., 1975). The role of organic matter in natural waters has been the subject of numerous studies over the past 25 years, and substantial advances have been made in understanding the role of natural dissolved organic matter (NDOM) in controlling light transmission, in affecting the bioavailability of metals, and more recently in stimulating photolysis of synthetic organic compounds (via the production of highly reactive photointermediates such as the hydroxyl radical and singlet oxygen following the absorption of light energy by NDOM). The importance of particulate organic matter (detritus) and NDOM in energy flow within aquatic systems also has become better understood (e.g., Wetzel, 1992), but this is still an area in need of additional research. Within the past 10 to 15 years, the principles of fluid mechanics have been applied to lakes in increasingly sophisticated ways by engineers and physical scientists, who have created the subdiscipline of environmental fluid mechanics (or environmental hydraulics). This work has helped limnologists understand the dynamics of water flow on a wide range of scales: from microscopic (e.g., nutrient depletion microzones surrounding plankton, and the patchy nature of the microhabitat and its role in maintaining multispecies communities in the face of competition for a common resource) to macroscopic (e.g., design of more efficient aerators and understanding the fate of silt-laden tributaries in reservoirs). Cycling of major and minor nutrient elements (carbon, nitrogen, phosphorus, sulfur, and minor metals) within lake basins has been a topic of research in mainstream limnology for many decades. Volume 1 of Hutchinson's (1957) Treatise on Limnology gathered and synthesized extensive amounts of both published and unpublished data regarding these cycles in lakes and is still an important resource. Concerns about eutrophication and acid deposition stimulated much additional research on these cycles over the past few decades. Studies on elemental cycling at various scales, from ecosystems to the globe, has become a recognized field of study called biogeochemistry (e.g., Gorham, 1991a; Schlesinger, 1991). In limnological biogeochemistry, lakes are used as models for larger systems (e.g., the oceans) and as convenient systems for the measurement of process rates, fluxes, and storage mechanisms. Such studies are intrinsically interdisciplinary and typically involve elements of chemistry, microbiology, hydrology, and mathematical analysis (modeling). Finally, the subject of ecosystem energetics has been a major organizing tool in ecology for more than 50 years, since Lindeman published his classic 1942 study of "trophic dynamics"—energy flow through the food web in Cedar Bog Lake. This study was the first important analysis of

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology energy flow in any type of ecosystem—an example of the leadership role that inland aquatic science has played in the earth and biological sciences. Energy transfer efficiency, relationships between production and decomposition, and physical-chemical constraints on biological production have been research topics in all types of aquatic ecosystems under this general theme. The importance of linkages between wetlands and lakes and between wetlands and streams with regard to carbon and energy flows (e.g., Wetzel, 1990) ties several major research thrusts together (e.g., biogeochemical cycling, especially carbon cycling, and ecosystem energetics), and it also could serve as a unifying theme for major subfields of limnology (i.e., lake, stream and wetland ecology). RIVERS AND STREAMS Integrative paradigms are a much more recent development in lotic limnology7 than in lake limnology. This reflects the less integrated nature of the disciplines contributing to stream science. Indeed, until recently, the physical, chemical, and biological components of stream limnology could be characterized as a set of independent subdisciplines that were associated more with their parent basic-science disciplines than with limnology. For example, studies on the physical aspects of water flowing in streams are associated with the fields of hydrology and hydraulics, which are taught in engineering and geoscience departments. Hydrologists have their own professional organizations or separate divisions within major engineering societies (such as the American Society of Civil Engineering and the American Geophysical Union). Similarly, the origin and development of stream channels and drainage networks is a well-developed topic within the field of fluvial geomorphology, which is a subdiscipline of geography and (to a lesser extent) geology. Until recently, chemical studies on flowing waters have been done primarily by geochemists, environmental engineers, and lake scientists interested in quantifying fluxes of minerals and pollutants from watersheds or into standing bodies of water (lakes and oceans). That is, stream chemistry has been of interest not so much as a subject of its own but as a means to answer questions about other natural systems. To an extent, this parallels the subservient role that chemistry played in sanitary (environmental) engineering before the 1960s. For the most part, stream limnology has been identified most strongly with stream biology, but even here, practicing stream biologists often are identified more with other ''parent disciplines" such as public health biology (sanitary microbiology) and fisheries biology. Stream biology was 7    Lotic refers to flowing waters (i.e., rivers and streams).

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology mostly descriptive and autecological through the first half of the twentieth century, focusing on the distribution, abundance, and taxonomy of stream organisms (Cummins et al., 1995). The development of stream ecology as an independent discipline is fairly recent (occurring mostly over the past 25 years) and is an outgrowth of various initiatives (described below) that began in the 1950s and 1960s. Hynes' (1970) classic book The Ecology of Running Waters may be regarded as the first book on stream ecology. Even today, many stream ecologists identify more with the discipline of ecology than with limnology. Paradigms of Subdisciplines Within Stream Limnology Organizing paradigms have been part of all the scientific disciplines that collectively constitute the field of stream limnology for many decades, as illustrated by the following examples. The concept of the watershed as the basic unit in hydrology dates back at least to the 1920s (Horton, 1931; Platt, 1993), and the watershed perspective has been used both in organizing hydrologic concepts and in data collection. Classification of flowing waters according to stream order, based on their relative position in the typically dendritic (treelike) or hierarchial network in which streams are connected in a large river basin, is a long-standing organizing principle in both hydrology and geomorphology. Chaos theory and the concept of fractal dimensions have stimulated much recent research in stream geomorphology. The modeling of streams as plug-flow reactors has been an organizing paradigm of sanitary and environmental engineering since the development of the first water quality model—Streeter and Phelps' 1925 model for dissolved oxygen concentrations in streams receiving point-source discharges of sewage. The use of benthic invertebrates as indicator organisms for organic pollution and the division of streams into zones of pollution and recovery based on the presence of indicator species or groups of organisms have been major driving forces in stream biology almost since the inception of the field. Numerous classification schemes were developed under this organizing principle, starting with the European "Saprobien" system near the beginning of the twentieth century (Kolkwitz and Marsson, 1908, 1909). This paradigm stimulated much of the biological research on the structure of stream communities through the middle of this century. It also can be considered a precursor to broader indicator or classification systems such as Karr's IBI (index of biological integrity) (Karr et al., 1986, 1987) and other indices of biodiversity and biological integrity (Karr, 1991), which are currently popular topics in stream ecology. Ecological energetics also has been an organizing concept for research in stream ecosystems for almost 40 years—since H. T. Odum's classic

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology studies on energy flow in the Silver Springs and the Silver River (Odum, 1956, 1957). Numerous studies have measured primary production, respiration, and flow of energy and materials through stream food webs, and this work has led to more recent paradigms such as nutrient spiraling (see next section). Just as lake scientists shifted from a descriptive or observational approach to a more experimental approach 30 to 40 years ago, stream limnologists also have adopted experimental approaches in recent decades; the experimental manipulation of a small stream with sucrose to enhance trout production (Warren et al., 1964) was the precursor of many later manipulations (e.g., see Lamberti and Steinman, 1993). Stream experiments have involved both artificial channels and real streams. Artificial channels have the advantage of allowing replication. They have been used to study the effects of chemical contaminants on stream ecosystems and to develop a mechanistic understanding of lotic processes at several locations in both this country and Europe (Lamberti and Steinman, 1993). Real streams have been used to conduct acidification and remediation manipulations; for example, lime has been added continuously to several small streams in New York and Sweden to study rates and mechanisms of biological recovery from acidic conditions. In addition, a few watershed-stream experimental manipulations have provided valuable information about linkages between terrestrial and aquatic components of watersheds (Bormann and Likens, 1967). Perhaps the best known involved a small forested watershed in the Hubbard Brook Experimental Forest (Likens et al., 1970; Vitousek et al., 1979). A dramatic increase in nitrate export from the watershed was observed when the forested catchment was clear-cut and treated with a herbicide to prevent revegetation. The River Continuum Paradigm Organizing paradigms that treat streams as systems and that integrate across the physical, chemical, and biological aspects of stream science are few in number and recent in origin. The River Continuum Concept (RCC) (Vannote et al., 1980) is the first and perhaps most important of these integrating paradigms. The RCC views entire fluvial systems as a continuously integrated series of physical gradients and adjustments in the associated biota (Cummins et al., 1995). Geomorphological and hydrological characteristics provide the fundamental physical template, which changes longitudinally within a drainage basin from the headwaters to the river mouth in a predictable fashion. Biological communities and associated attributes, such as the nature of the functional community groups, develop in adaptation to the fundamental physical template. The model thus has a definite watershed orientation and focuses on terrestrial and aquatic interactions. It is useful at the basin and stream scale in predicting the

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology physical and biological characteristics of a stream; it is less useful at the scale of stream reach and also has little to say about stream chemistry. Since its development, the RCC has been modified in many ways to accommodate a broader range of influences on stream ecology (e.g., climate, geology, tributary effects, and local geomorphology) (Minshall et al., 1983, 1985, 1992; Naiman et al., 1987; Cummins et al., 1989; Meyer, 1990). In addition, several organizing paradigms that were developed as alternatives to the RCC have been subsumed into a broader RCC. These include the nutrient spiraling concept, in which the unidirectional down-gradient flow of streams causes nutrient cycles to be open rather than closed, so that there is a gradual downstream displacement of nutrients, the rate of which is controlled primarily by flow. Other paradigms, such as the patch dynamics concept, which were viewed initially as counter to the RCC, were later shown to be compatible with a broader vision of the RCC that considers both temporal and spatial factors as influences on stream ecology. The patch dynamics concept is based on the idea that disturbance or temporal variation is the primary determinant of community organization in streams (Pringle et al., 1988). As Cummins et al. (1995) state, periods of high flow are a natural feature of most running waters that exhibit local or patch effects, even though they also show predictable longitudinal patterns. Finally, the flood-pulse model, an outgrowth of the patch dynamics concept, was developed to describe biological communities in rivers that regularly overtop their banks and inundate the floodplain. In such systems the cycle and extent of inundation may be the fundamental community organizer that overrides longitudinal patterns along changing stream order (Junk et al., 1989). However, Cummins et al. (1995) point out that one of the more predictable patterns of flooding along river continua is a general broadening of the floodplain along the longitudinal profile. Thus, the flood-pulse concept may be considered an extension of the RCC rather than a contradiction of it. The flood-pulse model also reminds us of the importance of stream-riparian connections; this is a topic of growing interest in stream ecology (e.g., Gregory et al., 1991). In summary, the RCC can be described as an integrative framework for conceptualizing stream-river ecosystems. It is broad enough to accommodate many other organizing concepts to account for specific physical factors or processes that affect flowing waters. Like the microcosm concept for lakes, the RCC is likely to continue to be modified and expanded rather than supplanted. WETLANDS Integrated studies using modern science techniques on wetlands as ecosystems are primarily a phenomenon of the past 25 years (e.g., Mitsch

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology and Gosselink, 1993), but the development of basic ecological concepts related to wetlands can be traced back much further. According to Gorham (1953), such studies go back at least as far as those of the mid-sixteenth century Englishman John Leland and the mid-seventeenth century Gerard Boate, "Doctor of Physick to the State of Ireland." These individuals and others in the eighteenth and early nineteenth centuries described and classified wetlands using names similar to those in use today, and they related these classes to basic hydrologic conditions. Gorham also traced the idea of wetland development and succession to an account on Irish bogs published by William King in 1685. Finally, some fundamental ideas about the chemistry of bogs were developed in the late eighteenth and early nineteenth centuries. Nonetheless, most of these pioneering studies were overlooked by twentieth century botanists and ecologists as they developed similar concepts. Several of the major paradigms described earlier for lakes and streams also are important in wetland science: wetlands as microcosms (i.e., wetlands as functioning, integrated ecosystems worthy of study as entities in their own right); wetlands as experimental systems for scientific study and manipulable systems for remediation and rehabilitation; and wetlands as repositories of historical information in their sediments and, in some cases, in their vegetation (e.g., tree rings). It is interesting to note that the use of wetland sediments for paleoecological and paleoclimatic studies actually predates the widespread use of lake sediments in this context. In addition, the trophic-dynamic concept and concepts of energy flow and nutrient cycling play the same role in wetland ecology as in lake and stream ecology. Wetland science also has developed several organizing paradigms beyond those of the disciplines on which it is based and the general paradigms of aquatic ecology: wetlands as products of delicate interactions of hydrology and vegetation to produce unique, patterned landforms; wetlands as seres or ecotones—that is, gradient ecosystems grading between terrestrial and open-water aquatic systems; and wetlands as unique repositories of organic carbon (peat) with an important role in trace-gas cycling to and from the atmosphere. In the context of global warming, there is much current interest in the role that peatlands play in fixing or releasing the "greenhouse" gases CO2 and methane (Gorham, 1991b). Finally, the idea that wetlands can provide service functions to humans has some unique aspects compared to related ideas about lakes and streams in serving human needs. Lakes and streams have been used for

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology navigation, water supply, fishing, and aesthetic and recreational enjoyment for centuries. Since the "wise use" conservationists of the late nineteenth and early twentieth centuries, our stewardship responsibilities toward lakes and streams have at least been acknowledged, if occasionally ignored. In contrast, until recently, wetlands were commonly regarded as wastelands or nuisances to be "reclaimed" by draining or dredging. Their importance in regulating hydrologic processes such as floods, in providing ecological benefits such as buffering contiguous lakes and streams from impacts of human use of uplands, and in serving as habitat for wildlife has been recognized only in the past 20 years. Ironically, as scientific and public appreciation for wetland ecosystems has grown, there also has been a trend to view them simply as natural analogues or extensions of engineered treatment systems. Thus, a paradigm has developed about wetlands as natural treatment systems . Several major research programs have been developed in association with academic institutions and water management agencies to study the effectiveness of wetland systems in removing nutrients and other contaminants from domestic waste effluent (e.g., Ewel and Odum, 1984) or in purifying stormwater runoff before it reaches lakes and streams (e.g., Olson and Marshall, 1992). Both natural and constructed wetlands now are being used for such purposes on a widespread scale within the United States. For some proponents of this paradigm, the quality and ecological integrity of the wetland itself appear to be less important than its ability to perform the desired function. Still, studies conducted under this paradigm have added to the heretofore meager understanding of wetland ecosystem functions and have provided some basis for preserving wetlands that otherwise might be destroyed by drainage and urban or agricultural development. REFERENCES Baker, L. A., and P. L. Brezonik. 1988. Dynamic model of internal alkalinity generation: Calibration and application to precipitation-dominated lakes. Water Resour. Res. 24:65–74. Birge, E. A. 1898. Plankton studies on Lake Mendota II. The crustacea of the plankton from July, 1894, to December, 1896. Trans. Wis. Acad. Sci. Arts Lett. 11:274–448. Birge, E. A., and C. Juday. 1911. The Inland Lakes of Wisc.: The Dissolved Gases of the Water and Their Biological Significance. Bull. Wis. Geol. Nat. Hist. Surv. 22, Sci. Ser. 7. Madison, Wisc.: Wisconsin Geological and National History Survey. Bormann, F. H., and G. E. Likens. 1967. Nutrient cycling. Science 155:424–429. Bradley, W. H. 1963. Paleolimnology. Pp. 621–652 in Limnology in North America, D. G. Frey, ed. Madison: University of Wisconsin Press. 734 pp. Brezonik, P. L. 1994. Chemical Kinetics and Process Dynamics in Aquatic Systems. Boca Raton, Fla.: Lewis-CRC Press. 754 pp. Brezonik, P. L., and D. R. Engstrom. In press. Modern and historic accumulation rates of phosphorus in Lake Okeechobee, Florida. J. Paleolimnol.

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Streeter, H. W., and E. B. Phelps. 1925. A study of the pollution and natural purification of the Ohio River. III . Factors concerned in the phenomena of oxidation and reaeration. U.S. Public Health Serv. Public Health Bull. 146. Stross, R. G., and A. D. Hasler. 1960. Some lime-induced changes in lake metabolism. Limnol. Oceanogr. 5:265–272. Swain, E. B., D. R. Engstrom, M. E. Brigham, T. A. Henning, and P. L. Brezonik. 1992. Increasing rates of atmospheric mercury deposition in midcontinental North America. Science 257:784–787. Tansley, A. G. 1935. The use and abuse of vegetational concepts and terms. Ecology 16:284–307. Tilman, D. 1982. Resource Competition and Community Structure. Princeton, N.J.: Princeton University Press. Vannote, R. L., G. W. Minshall, K. W. Cummins, J. R. Sedell, and C. E. Cushing. 1980. The river continuum concept. Can. J. Fish. Aquat. Sci. 37:130–137. Vitousek, P. M., J. R. Gosz, C. C. Grier, J. M. Melillo, W. A. Reiner, and R. L. Todd. 1979. Nitrate losses from disturbed ecosystems. Science 204:469–474. Vollenweider, R. A. 1969. Möglichkeiten und Grenzen elementarer Modelle der Stoffbilanz von Seen. Arch. Hydrobiol. 66:1–36. Vollenweider, R. A. 1975. Input-output models with special reference to the phosphorus loading concept in limnolgy. Schweiz. Z. Hyrol. 37:53–84. Warren, C. E., J. H. Wales, G. E. Davis, and P. Doudoroff. 1964. Trout production in an experimental stream enriched with sucrose. J. Wild. Manage. 28:617–660. Wetzel, R. G. 1990. Land-water interfaces: Metabolic and limnological regulators. Verh. Int. Verein. Limnol. 24:6–24. Wetzel, R. G. 1992. Gradient-dominated ecosystems: Sources and regulatory functions of dissolved organic matter in freshwater ecosystems. Hydrobiologia 229:181–189. Zicker, E. L. 1955. The Release of Phosphorus from Bottom Muds and Light Penetration in Northern Wisconsin Bog Lake Waters as Influenced by Various Chemicals. Ph.D. dissertation. University of Wisconsin, Madison.