Fundamental Research Questions in Inland Aquatic Ecosystem Science

Diane M. McKnight

Water Resources Division

U.S. Geological Survey

Boulder, Colorado

Elizabeth Reid Blood

J. W. Jones Ecological Research Center

Newton, Georgia

Charles R. O'Melia

Department of Geography and Environmental Engineering

The Johns Hopkins University

Baltimore, Maryland

SUMMARY

This background paper presents one possible set of fundamental questions for inland aquatic ecosystems. There are other questions that may be more fundamental or more pressing for meeting the needs of society in managing water resources. The general view we would like to convey is that although there is a need for additional monitoring, better analytical techniques, and more extensive systematics and classification, aquatic scientists are constrained most by incomplete understanding of the aquatic ecosystems themselves. Scientific progress and improved management of aquatic resources will be achieved by improving understanding of aquatic systems and by applying and communicating that understanding.

INTRODUCTION

Limnology is an integrative science, combining a knowledge of physical, chemical, and biological processes to promote understanding of aquatic systems. For any lake, stream or wetland, there are innumerable unanswered questions; some are obvious, others are more subtle. Some



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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Fundamental Research Questions in Inland Aquatic Ecosystem Science Diane M. McKnight Water Resources Division U.S. Geological Survey Boulder, Colorado Elizabeth Reid Blood J. W. Jones Ecological Research Center Newton, Georgia Charles R. O'Melia Department of Geography and Environmental Engineering The Johns Hopkins University Baltimore, Maryland SUMMARY This background paper presents one possible set of fundamental questions for inland aquatic ecosystems. There are other questions that may be more fundamental or more pressing for meeting the needs of society in managing water resources. The general view we would like to convey is that although there is a need for additional monitoring, better analytical techniques, and more extensive systematics and classification, aquatic scientists are constrained most by incomplete understanding of the aquatic ecosystems themselves. Scientific progress and improved management of aquatic resources will be achieved by improving understanding of aquatic systems and by applying and communicating that understanding. INTRODUCTION Limnology is an integrative science, combining a knowledge of physical, chemical, and biological processes to promote understanding of aquatic systems. For any lake, stream or wetland, there are innumerable unanswered questions; some are obvious, others are more subtle. Some

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology 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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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology on ecosystem properties (see Table 1). We have also indicated the general occurrence of such ecosystems in different regions of North America (Table 1 and Figure 1). The three main classes (streams or rivers, lakes, and wetlands) represent a gradient in hydrologic residence time from rapid to slow. It is important to remember, however, that there are not always clear distinctions between these ecosystems because of their interconnections. General types of inland waters are identified in the next level of classification. Saline lakes and blackwater rivers have chemical characteristics with a pervasive influence on ecosystem properties. For example, the limited transmission of light in blackwater systems restricts photosynthesis. Saline lakes are closed-basin lakes, with high concentrations of dissolved solids. Prairie wetlands and intermittent streams are distinguished because the climatic sensitivity of their hydrologic regimes can cause TABLE 1 Occurrence of Various Classes of Inland Waters in Regions of North America Classification Region   1 2 3 4 5 6 7 8 Streams/rivers Blackwater rivers — — — — X — — — Alpine streams — X X — — X — — Intermittent streams — — X — — X X X Perennial streams X X X X X X X X Lakes Large lakes X X X — X X — — Saline lakes — X — — X X X X Small lakes/ponds X X X X X X X X Reservoirs X — X X X X X X Wetlands Large riparian wetlands X X X X X X X — Large swamps — — — — X — — — Prairie wetlands — — — — — X X — Small bogs/fens X X X X X X — — Large peatlands X X — — — — — — Coastal zone areas X X — X X X — X NOTE: Numbers correspond to the following regions: 1 Laurentian Great Lakes and Precambrian Shield of the United States and Canada. 2 Arctic and subarctic areas of Canada. 3 Rocky Mountains in the United States and Canada. 4 Mid-Atlantic and New England area of the United States. 5 Southeastern United States and coastal Mexico. 6 Pacific coast mountains and western Great Basin. 7 Great Plains of the United States and Canada. 8 Basin and range regions and adjacent arid and semiarid regions of the United States and Mexico.

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology FIGURE 1 Regions of North America corresponding to the designations in Table 1. SOURCE: American Society of Limnology and Oceanography 1994). © by the American Society of Limnology and Oceanography. profound annual variations in these ecosystems. Fresh deep lakes show physical properties that are unique because the temperature of maximum density (which does not exist in salt water) is dependent on pressure and salinity. Thus, it appears that the circulation of deep water in deep lakes (e.g., Lake Baikal in Russia) is a physical mystery that may be explained by the theory of neutral surfaces and by the phenomena of caballing and thermobaricity (D. Imboden, Swiss Federal Institute of Technology, personal communication, 1994). Despite its simplicity, this classification illustrates the similarities and differences in the occurrence of inland aquatic ecosystems at the regional scale for the North American continent. Three types of inland waters are commonly found throughout the continent: (1) small lakes and ponds, (2) large rivers, and (3) perennial streams. On the other hand, large peatlands, large swamps, prairie wetlands, very large lakes, and blackwater rivers are less widely distributed in North America. Insight into one type of aquatic ecosystem can be gained from study

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology of another. Similarly, study of processes occurring in lakes can provide insight into processes occurring in oceans. Two examples of processes that are important in marine systems and can be studied readily in lakes are turbulent mixing at boundaries and sediment-water interactions, including the microstructure of the sediment-water interface and the resulting microchemical environments. Other examples of transferability of knowledge between aquatic ecosystems are complementary studies of the hyporheic zone (the area under and adjacent to the open channel), which have been conducted in both small first-order mountain streams and large rivers. FUNDAMENTAL QUESTIONS To provide a structure for this chapter, we have grouped the fundamental questions identified into three broad categories relating to (1) the temporal and spatial dynamics of inland aquatic ecosystems (understanding time and space), (2) aquatic organisms occurring in inland aquatic ecosystems (the ecological significance of the species), and (3) interactions involving both major and trace chemical constituents of natural waters (natural water—a chemical world). Within the three categories, questions related to several topics are raised. Understanding Time and Space For terrestrial ecosystems, the land surface can be considered stationary during the life span of most of the organisms within the ecosystem. However, for aquatic ecosystems, climatically driven water (the milieu within which the organisms live) is moving through the ecosystem, as driven by the hydrologic cycle. Aquatic ecosystems are therefore highly variable over the full range of temporal and spatial scales. In this section, questions going from long-term to daily time scales and then questions going from very small scales to regional scales are considered. Paleolimnology A historical perspective can provide insight into contemporary phenomena. In limnology, history is archived in lake or floodplain sediments and in peatlands. Environmental change, such as change in nutrient concentration, climatic conditions, or hydrologic regime, has the potential to affect chemical, physical, and biological processes of lakes, streams, and wetlands. For example, materials entering a lake from inflowing streams, atmospheric deposition, or biomass produced within the lake can settle to the lake bottom. When changes in inflow or in lake productivity occur, evidence of the change may become preserved in the sediments. Paleolimnologists

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology can examine an integrated record of lake and watershed history, often in relatively undisturbed sediments. The complexities of a three-dimensional, dynamic aquatic system are compressed into layers of mud. The challenge in paleolimnology, therefore, is in interpretation. The primary goal is often to reconstruct the physical or chemical record from the biological record, such as diatom frustules, algal pigments, or ostracod fossils. This primary goal of detecting a signal can be obtained by a variety of tools; the more tools used, the more robust is the interpretation. Multiple lines of evidence can be used to detect often subtle environmental change. Question: Are all important limnological characteristics associated with an event or a gradual shift recorded in sediments? How can interpretations of the sediment record be strengthened? The role that time, location, stability, and complexity play in creating an entire system all enter into consideration. Further, temperate lakes move through large fluctuations during a year (ice cover, spring overturn, summer stratification, and fall overturn). The more subtle year-to-year fluctuations are superimposed upon the signal left by this intra-annual cycle. The impact and detectability of an event are dependent on the time scale over which the event occurs. Some events are abrupt or even catastrophic, such as those associated with major floods or volcanic eruptions. Other events are more gradual or prolonged, such as droughts or annual variations driven by ENSO (El Niño Southern Oscillation) events. The stability of a given parameter in relation to change can vary. For example, arctic diatom species may respond to small changes in nutrient concentrations but be insensitive to temperature changes of several degrees. Complex systems can respond to change in unexpected ways, and deciphering the complexity is the major task ahead. Question: Is a climatic signal local to one particular lake, or does it operate over a regional scale? The location of a lake is important in determining the wider implications of its sedimentary record. One successful strategy is to study sediment cores from many lakes in a region and look for correlations in the records. Careful study of just one core using a variety of methods is a very labor-intensive task; studying numerous cores from a region requires a monumental effort. Advances in analytical methods and data analysis may make the application of paleolimnological techniques more practical for regional issues. Importance of Winter As we recognize that human activities have the potential to change global climate, there is an interest in understanding the responses of

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology freshwater ecosystems to all types of temperature ranges and climate scenarios. Question: What are the critical events and conditions that control autotrophic and heterotrophic processes during winter? Field studies of inland aquatic ecosystems are not distributed evenly among the seasons of the year because the ''field season" for limnologists is generally late spring and summer. For many ecosystems, the dominant processes operative in summer have already been studied in detail. For example, the dynamic succession of phytoplankton from spring through summer into fall has been studied in numerous temperate lakes. In contrast, much less is known about the dynamics of inland aquatic ecosystems during winter because it is clearly more difficult to conduct research under winter conditions. For the northern areas of North America, winter is a time when many lakes, streams, and rivers are covered by ice and snow, and wetlands are frozen and snow covered. In lakes, for example, light transmission through the ice cover can vary during the winter with the accumulation and dispersal of snow. Therefore, photosynthesis by phytoplankton under the ice may be temporally dynamic, and the sequence of phytoplankton species dominance may vary from one year to the next. Even though there is an awareness that aquatic ecosystems are not in suspended animation under the ice and snow, these logistical difficulties combined with the constraints of academic calendars tend to discourage studies of winter dynamics. Question: What critical processes may occur in winter that control the behavior of an ecosystem in the subsequent summer and spring? Primary production refers to the rate at which algae and aquatic plants assimilate carbon dioxide and produce biomass through photosynthesis, whereas heterotrophic degradation refers to the rate at which algal and plant biomass is consumed by nonphotosynthetic organisms, from micro-organisms to fish, producing carbon dioxide. Because of decreased light intensities associated with snow and ice cover, the ratio of primary production to heterotrophic degradation undoubtedly decreases greatly in many aquatic ecosystems in the transition from fall to winter. In temperate streams, for example, the extent of heterotrophic degradation during winter of fallen leaves and other (nonliving) organic material from the watershed may depend on hydrologic and climatic conditions, such as the timing and duration of ice cover. The amount and quality of this detrital material available to benthic invertebrates at the beginning of the summer thus may vary between years. For wetlands, the timing of the freeze and thaw of the wetland surface may control the extent of heterotrophic degradation and annual production as well.

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Morning, Noon, and Night The water that surrounds an aquatic organism is continually changing, in both physical and chemical properties. For this reason, the solar cycle has a pervasive influence on aquatic ecosystems, raising a fundamental issue. Question: How do the diverse 24-hour (diel) cycles driven by the solar cycle fit together to structure the daily pulse of an aquatic ecosystem? Photosynthesis and respiration obviously vary during the day, but these are not the only processes driven by the solar cycle. Examples of other biological processes include nutrient uptake and cell division by algae, both of which are controlled by circadian clocks synchronized to the solar cycle. Processes controlling the chemical speciation of biologically reactive constitutents are also driven by sunlight or by changes in pH and oxygen concentration associated with photosynthesis. For example, photoreduction of particulate iron and manganese oxides in the water column or in the streambed can release sorbed phosphate, which is then assimilated by algae. Cascading chemical reactions involving photoproducts (such as hydroxyl radicals) and other metals or organic compounds can also occur. It is important not only to become aware of these diel variations, but also to understand in more detail how the "clockwork" controls the ecosystem and drives changes that occur over weeks and seasons. We should expect that aquatic ecosystems would have evolved a temporal "fine structure," and it could be that this fine structure is where the system is most vulnerable to ecological insult from anthropogenic contaminants. Living at the Interface The hydrologic sciences have developed with an emphasis on understanding hydrologic processes at the macroscopic scale. This is the scale most useful for understanding the development of stream networks, for predicting flood responses and bed load transport, and for determining the transport of pollutant discharges. Averaged parameters, such as a roughness coefficient and hydraulic conductivity, are effective in describing physical interactions between the streambed and the flowing water and ground water in aquifers. Key features of the habitat may not be reflected in the lumped or averaged parameters developed to answer larger-scale questions. Question: How do fluid mechanics in fluvial systems determine the habitat as experienced by the organism? Many aquatic organisms live on a surface exposed to moving water. Examples range from a bacterial cell adhered to the surface of a suspended

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology particle to a mayfly grazing on an algal-covered rock. A quantitative understanding of the physical behavior of this habitat requires an approach to environmental fluid mechanics at a smaller scale. A sound physical understanding is a necessary basis for the study of many chemical and biological processes, such as microbial degradation of contaminants sorbed on particulate or sediment surfaces and particle feeding by caddis and blackflies. Regional Scales, Spatial Heterogeneity and Ecosystem Linkages Large discrepancies exist between the scale of information needed to manage aquatic resources effectively and the scale at which current research and management occur (Repetto, 1986). Question: How can we develop an understanding of inland aquatic ecosystems at regional scales? Most current ecological studies are conducted at local to watershed scales. Resource management activities often operate independently of an ecological perspective or other concurrent management activities. Current resource management activities are carried out at the "end-of-the-pipe" to subwatershed scale. In contrast, interactions among human, ecological, and atmospheric systems largely occur at the regional scale (areas larger than ecosystems or landscapes but subcontinental), and these interactions drive local and regional structure. Environmental policy, economic incentives, and return rates are governed by political units that correspond to the regional scale. Cultural and social traditions that control the ethics of land use also occur at regional scales. Atmospheric scientists have suggested that air-parcel dynamics and interactions with characteristics of the earth's surface that occur at the "mesoscale" (e.g., region) are key processes determining regional climate patterns. Collectively, interactions among human, ecological, and physical forces both define and structure regions and require interdisciplinary study. At a minimum, these issues require that local or site-specific studies be conducted in a manner that incorporates regional influences, designed to be ''scalable" to a regional perspective. Question: How does spatial heterogeneity within an ecosystem influence its dynamics, and what are the critical ways ecosystems are linked together at the larger scale? Scalable research and management activities require the identification of fundamental units that can be combined functionally and structurally into larger functional units. Inherent in the identification of fundamental units is a thorough understanding of the important spatial and temporal patterns associated with the aquatic system under study. Spatial heterogeneity in aquatic systems results from chemical, physical, and biological

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology structures, which in turn regulate the associated ecosystem processes and patterns. As an example, thermal stratification in lakes controls biotic habitat, biological structure, and biogeochemical cycles. The timing and coherence of the stratification varies among lake systems. Geomorphic characteristics of streams interact with streamflow to create a variety of biotic habitats and control biological structure and function. Several important paradigms in riverine ecology (river continuum, riparian corridor, hyporheic corridor, and river discontinuity theories) are based on the relationship of ecological processes to geomorphic characteristics of streams and their watersheds. As a further example, in boreal peatlands, the landscape patterns reflect the areas of upwelling of base-rich ground water. Spatial heterogeneity occurs at the micro- to meso- (intermediate) scale. At the microscale, currents can control the biological and geochemical processes occurring in the layer of algae, fungi, and bacteria that covers rocks in a streambed. Interactions occurring at the mesoscale include benthic invertebrates and predatory invertebrates or fish, and ecosystem responses to episodic disturbances such as floods and fires. To develop useful concepts describing the structure and functioning of aquatic systems, it is necessary to identify fundamental units (such as the streambed), based on an understanding of the spatiotemporal patterns. The heterogeneity within an ecosystem influences the linkages to other adjacent ecosystems. Aquatic systems can be viewed from a four-dimensional framework (Ward and Stanford, 1991). Aquatic systems are affected by vertical linkages to the ground water; lateral linkages to the watershed; longitudinal linkages across the aquatic system, and temporal variation in physical, chemical, and biological processes. In addition, significant transformation may occur as these waters pass through boundaries between the surface water and source water, such as the hyporheic zone (where ground water and surface water interconnect) and the riparian zone (land surface adjacent to a stream). The relative importance of these ecotones and linkages will vary with location in the drainage basin, characteristics of the system under study, and historic land management activities. This four-dimensional framework is critical in understanding aquatic and wetland systems and in formulating scalable research and management, yet few studies have attempted to incorporate it into a holistic ecological view. In most flowing aquatic systems, repeatable four-dimensional units (defined as "reaches") can be identified and form the fundamental basis for scaling up to a watershed or region (Frissell et al., 1986). They are hierarchical, readily expanded, and collapsed in scale depending on the research question being addressed. Gregory et al (1991) stressed the importance of the four-dimensional perspective for understanding the linkages between land and water in the structuring and functioning of stream ecosystems. Streams and their

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology riparian zone structure and function result from geomorphic, hydrologic, topographic, edaphic, and biotic processes occurring within the watershed. Gregory and colleagues defined reaches by channel and valley floor geomorphic features and as having distinct topographic, edaphic, and disturbance regimes. These reaches had distinct longitudinal, lateral, vertical, and temporal characteristics. Ecological Significance of the Species For understanding many biological phenomena, such as evolution, genetics, and the growth and development of organisms, the species, defined as genetically related organisms that can produce offspring, is the fundamental unit. As discussed previously, the ecosystem is another fundamental unit governing biological processes. The definition of boundaries of an ecosystem can be designated to facilitate study of particular processes, whereas the definition of a species is more exact. Questions arise as we try to conceptually relate species and ecosystems. Species, Niches and Ecosystem Function Aquatic ecologists strive to understand the controlling relationships within an ecosystem. Two relationships that are currently under active research are (1) grazing on microorganisms by microheterotrophs (referred to as "the microbial loop"), and (2) complex, multiple pathways linking primary producers and top predators (sometimes referred to as "top-down or bottom-up" controls). These are not distinct issues, because the nature of the microbial loop in a lake ecosystem could influence the importance of higher trophic-level organisms in regulating the ecosystem. Although questions may be posed at the level of trophic interactions, in order to examine these interactions in detail in any particular aquatic ecosystem, the questions become focused on which species is doing what, how fast, and when? Question: How do characteristics and habitat requirements of individual species fit together to control ecosystem-scale processes, such as productivity and resilience? Aquatic ecosystems often contain many species within each trophic level. The magnitude of the challenge presented to the researcher by this diversity varies with trophic group and aquatic environment. For some groups, species-level identification may be very difficult to obtain. Even the knowledge of species may seem to be insufficient because it does not carry with it detailed knowledge of habitat requirement or ecological function. Individual benthic invertebrate species have been categorized according to their ecological function as scrapers (on periphyton), shredders (of leaves), predators (on other invertebrates), etc. Therefore, a list of the

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology distribution and abundance of benthic invertebrates can convey a picture of ecosystem interactions in a stream and constrain the hypotheses about ecosystem processes. For example, many species of mayflies graze on algae growing on rocks in the streambed (periphyton), whereas many species of stoneflies shred and ingest leaves. The presence of both mayflies and stoneflies in a stream indicates significant quantities of organic material, either produced in the stream by periphyton or entering the stream from trees adjacent to it in the riparian zone. For algal species, the ecological relevance of species data may be useful, but less refined than for benthic invertebrates. Hutchinson (1961) asked why so many algal species are present in a 1-liter sample of lakewater, a seemingly uniform environment. He referred to this question as the "paradox of the plankton." Whether or not this is a true paradox can be debated. For algal species, some general habitat requirements and ecological functions are known at the division level (i.e., diatoms require silica and heterocystous cyanobacteria can fix atmospheric nitrogen). However, more detailed habitat information is inferred from distributional patterns rather than by direct study of individual algal species. The significance of interannual shifts in algal species, whether within a lake or stream ecosystem, or the presence of particular rare species, may be more difficult to interpret. Question: Is understanding of important microbial processes in aquatic ecosystems (the microbial loop, heterotrophic degradation of natural organic material or organic contaminants) constrained by lack of species-level information? For bacteria, the species concept may seem hardly relevant to ecological studies. In the 1970s and 1980s, microbial ecology classified bacteria functionally, rather than by detailed, recognizable, morphological traits. Their habitat requirements were described broadly (e.g., aerobic, anaerobic, facultative, obligate). However, more current laboratory studies of isolates of aquatic bacteria show that genetic characteristics may be significant in showing relatedness among bacterial strains, but that functional characteristics are not unique. For example, some species of sulfate-reducing bacteria may be capable of reducing ferric iron under certain conditions. Current cutting-edge research is examining genetic characteristics in the context of the individual and populations; the context of communities or ecosystems is probably the next frontier. This is an important complement to another cutting-edge research area with much practical significance: the development of engineered bacterial species to remediate certain compounds in situ. This research area has much promise and is a certain future direction of applied limnology. Question: How does biodiversity in aquatic ecosystems develop and change? How much time is needed?

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Although the vulnerability of ecosystems to species loss through habitat modification and contamination is widely acknowledged, we lack a coherent data base to document changing biodiversity of inland aquatic ecosystems. An understanding of species habitat requirements and interspecies interactions is fundamental. The dynamic changes in biological populations can occur over different time scales, which will control how ecosystems adapt and evolve. Some current federal programs may provide biodiversity data for some groups and for some ecosystems in the future. Yet even if we had that data now, we might not be able to interpret the data in terms of causative factors. For example, trout fishermen and others claim that there has been a loss in diversity of benthic invertebrates in western streams. Without a knowledge of the habitat or species interactions and other factors that would maintain diversity in benthic invertebrate communities, such changes cannot be attributed to habitat degradation, chemical contamination, or overfishing. Intraspecies Genetic Variation For both unicellular and multicellular organisms, genetic variations occur within a species. These genetic variations (genotypes) give rise to different physical characteristics (morphotypes). Under certain environmental conditions, some morphotypes may have a greater probability of survival than others; thus, the distribution of genetic characteristics within a population can shift. The expression of genetic characteristics can also be regulated by environmental conditions, which is referred to as "plasticity." Modern biochemical techniques allow for characterization of genetic variation within a population of a given species, but interpretation of this information in terms of genetic shifts and adaptations to environmental conditions is a challenge. Question: How much plasticity and genetic variation is there for different species, and does that influence ecosystem stability? This is a "big" question, in that there is tremendous gap in knowledge, and a "domino" question, in that answers have the potential to increase understanding of many limnological processes. Questions related to genetic characteristics may seem esoteric at first, but they have important consequences for water quality issues. The following examples involving phytoplankton illustrate this point. Copper sulfate treatment is used routinely as a toxicant to control the growth of nuisance algal species in drinking water reservoirs. In a study of copper sulfate treatment, 90 percent of the population of the target species Ceratium hirundinella was killed by the treatment, and many cell fragments were observed in the reservoir; however, 10 percent of the population

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology remained with no impairment of motility or other functions. If such survival is attributable to intraspecies variation in tolerance to copper, such genetic variation may contribute to the annual recurrence of this nuisance species in the reservoir. Another example related to water quality is the development of toxic blooms of cyanobacteria (blue-green algae). Studies have shown that within a population, some strains produce neurotoxins and others do not. Toxic blooms occur when neurotoxin-producing strains become dominant. Occurrence of toxic blooms in farm ponds can cause severe illness or death in livestock drinking at the pond. Understanding the circumstances that favor toxin-producing strains would be useful in managing this problem. Question: Can generalizations be made about genetic variations within particular classes of organisms? There are many potential directions for research on genetic variation. Tolerance and bioaccumulation of contaminants is an issue for organisms at all trophic levels. If there are general patterns of genetic variation, such as more variation in tolerance in diatoms compared to cladocerans (water fleas), knowledge of these patterns would help in understanding ecosystem response to environmental stress. Natural Water—A Chemical World "Water quality" is a phrase used commonly in discussing water resources, but the meaning of water quality can be a source of confusion. Quality has many meanings, the principal ones being "peculiar and essential character" and "degree of excellence." The first refers to objective features of water; the second, to value judgments about the water for particular human uses (Averett and Marzolf, 1987). This distinction acknowledges that scientific inquiry into water quality issues (first meaning) is essential for the appreciation, definition, and management of water's utility for human uses (second meaning). The topics discussed below address water quality issues from the second perspective; specifically, they address the "so-what" question that can be asked about data sets of water quality parameters. Major Cations, Anions, and Alkalinity The major cation and anion concentrations vary greatly among inland waters, from saline to very dilute. There are many potentially significant questions. Addressing them could yield important insights and allow for a meaningful interpretation of the extensive water quality data that routinely are collected around the country and archived in computer data

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology bases with no more analysis than computation of average values. Such questions include the following: Questions: What is the significance of variations within the more typical range for determining species distributions of microorganisms, aquatic plants, vertebrates, or invertebrates? How does the abundance of monovalent or divalent cations affect the electrical properties of the layer around the cell? How does that in turn affect solute transport (e.g., nutrient uptake) at the cell membrane? Are natural organic polyacids, such as fulvic acid, also important in this context, and do they cross the cell membrane? Effects of Many Different Constituents The major nutrients for growth required by autotrophic microorganisms are nitrogen and phosphorus. These nutrients are dilute in the aquatic medium relative to major cations and anions; autotrophic microorganisms therefore must be able to assimilate these and other required solutes effectively at low concentrations. Microorganisms as well as multicellular organisms are also sensitive to toxicants at low concentrations and can release organic constituents to the medium that have specific functions (e.g., vitamins). Question: Which of these natural and anthropogenic trace constituents are critical in controlling inland aquatic ecosystems? Chemical regulation of aquatic ecosystems can occur through limitations on the concentration or bioavailability of particular chemical species or through the toxic effects of naturally present constituents at elevated concentrations or of xenobiotic compounds. Iron is a metal that is required by microorganisms and may be limiting in some aquatic ecosystems because of its inherently low solubility at the neutral and higher pH values of many natural waters. Ecotoxicology should be studied and applied in the broadest sense, including the effects of concentrations far below lethal doses. How do molecules released by anthropogenic activities interfere with the capacity of individuals and (more important) entire populations to adapt to a changing environment? Environmental estrogens are an example of compounds that may not be directly toxic but may limit the ability of populations to reproduce and survive. A fundamental research question that would lead to better water quality standards for contaminants in the future involves the mechanism of contaminant uptake and biological magnification by organisms. For many important hydrophobic contaminants, for example, a simple hydrophobic partitioning model does not adequately explain uptake. Questions: How reliable are present data for trace metals in fresh waters? What

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology applications of current analytical capabilities can be used to provide new ways of studying chemical (also physical and biological) processes in inland aquatic environments? Can new in situ methods be devised? In many water quality problems, hydrologic and chemical processes are tightly linked, and aquatic systems inherently are temporally dynamic and spatially heterogeneous. For some contaminants, a single analysis can be laborious and expensive for a low concentration that is nonetheless sufficient to have biological effect. Because of this, it is difficult to make the numerous chemical analyses that would be required to link hydrology to chemistry. This limitation may be overcome in the future by going online and in situ to enhance resolution in time and in space. Detrital Organic Material In aquatic ecosystems, detrital organic carbon often is the largest pool of organic carbon, typically exceeding the biomass of living algae or macrophytes by several orders of magnitude. In many systems, this organic material is allochthonous (originating in the surrounding terrestrial environment) rather than autochthonous (produced within the lake or stream). Degradation of this detrital organic matter has the potential to exert strong controls on the ecosystem, such as the development of reducing conditions in retention zones within stream ecosystems. There are many different forms of detrital organic carbon in aquatic ecosystems and an array of acronyms for each class. For particles, the distinctions are primarily based on size. CPOM (coarse particulate organic matter) can be composed of twigs, leaves, etc., and FPOM (fine particulate organic matter) can be small bits of shredded leaves. Organisms will have different physiological adaptations to feed on CPOM and FPOM. Another meaningful size distinction is particulate and colloidal, in that different processes control their transport and deposition. The term DOC (dissolved organic carbon) typically is used in a nominal sense for organic matter passing through a 0.4-µm filter and includes colloidal and truly dissolved forms. Question: How does the chemical composition of this abundant detrital organic material influence heterotrophic degradation and, in turn, the nature of the ecosystem? These size-based distinctions have their utility but do not address the quality of the material as a substrate for growth by either multicellular or unicellular heterotrophic organisms. For example, DOC includes dissolved glucose, which would be assimilated readily by microorganisms, and dissolved fulvic acids, which are biologically recalcitrant organic acids persisting for many years. A DOC measurement is the quantitative sum of many different forms of carbon. Trying to understand heterotrophic

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology processes with only DOC measurements is comparable to trying to understand the inorganic geochemistry of natural waters with only measurements of conductivity. Question: What are the different pathways by which detrital organic carbon is respired in aquatic systems? How does the source of the detrital organic carbon determine the quality of the substrate? Importance of Long-Range Transport of Constituents Question: What are the mechanisms controlling interactions of watersheds with overlying airsheds? The transport and deposition of atmospheric acidic constituents, SOx and NOx, affects the weathering rates of soils and nutrient conditions on land and in surface waters, with consequent effects on biological processes and ecosystems. For example, approximately one-third of the present nitrogen input to the Chesapeake Bay is estimated to arise from atmospheric deposition in the watershed. Another example is tropospheric ozone: ozone production in regional airsheds such as the Baltimore-Washington area involves photochemical reactions between NOx and hydrocarbons from anthropogenic and terrestrial sources to yield atmospheric ozone concentrations many miles from urban sources that exceed allowable limits. Chlorinated organic substances such as PCBs (polychlorinated biphenyls), DDT, PAHs (polycyclic aromatic hydrocarbon), and dioxin are transported to aquatic systems primarily by the air route, and may have important effects on the organisms in these systems. Mercury is also transported atmospherically; however, the available data base for metals in freshwater ecosystems may be limited to recent years because of potential contamination of samples collected earlier. The interactions of sulfur, nutrients, synthetic organic substances (including but not limited to pesticides), and many metals during transport through the vegetation canopy, across soil surfaces, through the highly reactive surficial zone of soils, and downward into aquifers is a challenging research area of environmental concern that requires an integration of chemical and hydrologic concepts and field measurements. A Changing Climate Question: What is the relative importance of direct physical changes compared to indirect biogeochemical changes in determining responses to changing climate? The effect of global climate change on regional hydrology and, in turn, on vegetation and on wetlands is a very important concern requiring substantial attention. Changes in hydrologic regimes may alter the extent

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology or timing of biogeochemical processes that control the flux of solutes, particularly nutrients. Such biogeochemical changes may have a greater effect on the receiving lakes and streams than direct physical effects of warmer water temperatures, for example. Humans: A Keystone Species Humans play a significant role in all aquatic systems. They alter the physical, chemical, and biological structure and function of aquatic systems through direct interactions (e.g., harvesting fish, discharging chemicals, thermal discharges from industry) and indirectly through altering watersheds, airsheds, and in-stream processes (e.g., dams, reservoirs, introduction of exotic species). Although much alteration of aquatic systems has occurred, few follow-up studies have fully explored the ecosystem response to those changes. Humans have been viewed by ecologists as primarily "drivers" or "receivers" in relation to ecological systems and not as an integrated part of the aquatic system. Basic research is needed to understand the fundamental role that humans play as a keystone species in aquatic systems. Fundamental Associations Between Humans and Aquatic Systems Question: Is there a common paradigm between human and aquatic sciences that can be used to relate limnology and sociology? High-quality fresh water will become a limited resource in the next century. Human-environmental interactions are causing the greatest rate of change in land, water and air resources that the earth has seen in more than 10,000 years (Silver and DeFries, 1990; Turner et al., 1990). Projected growth and redistribution of human population centers will result in increased urbanization of rural areas. Land-use changes, waste generation, and hydrologic alteration will significantly modify aquatic systems. These extensive human effects on aquatic systems and associated watersheds have been proceeding without a commensurate understanding of limnological consequences or integrated (including ecological, social, political, and regulatory) management strategies. Our understanding is limited by the complexity of the problem and the lack of appropriate spatial and temporal information. Complicating the situation is the lack of societal and institutional mechanisms for addressing these problems and of appropriate mechanisms for transferring "scientific knowledge" into adaptive holistic resource management. New mechanisms (e.g., technology transfer or training), tools (e.g., integrated models), and multidisciplinary approaches are needed to solve these problems. Questions: Are human associations more than "drivers" or "receivers"? How do

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology human metrics relate to the linkages between humans and aquatic systems? What ecological metrics relate to the linkages between humans and aquatic systems? Defining the Roles Humans Play as the Keystone Species Interactions between natural (e.g., environmental) and human (e.g., socioeconomic) sciences are not well understood. Integrated, interdisciplinary approaches have been applied only infrequently to problem solving. Ecologists tend to model ecological systems, while social or economic scientists tend to model human systems or human values. These different views are linked sometime later, rather than building an integrated model early on as part of a focused effort. Extrapolating from two separate approaches cannot compare with a truly integrated approach taking all relevant factors into account. Site-specific to local information must be developed into an integrated regional perspective using scalable research activities. What is needed is a coherent, integrated theory that fundamentally links human and aquatic systems, and this theory must be a fundamental component of future aquatic research. Questions: What ecological and human metrics are important in quantifying or defining the keystone species role in aquatic systems? Is the ecological metric structure, function, flux, rate, timing, or variability? How important are human metrics such as perceptions, myths, socioeconomic status, cultural values, education, and governmental policy in defining the association with aquatic systems? In order to stimulate the development of integrated human and aquatic research, a number of fundamental needs must be considered. These needs relate to the availability of information in a usable form, availability of knowledge and expertise, ability to develop a common language for effective communication, and identification of a common currency for model validation. We must move from viewing humans as peripheral to viewing humans as a fully integrated part of any aquatic system. Rambo (1983) developed a general systems theory for humans that might prove useful in integrated research. These systems exist in ''a complex, dynamic relationship with multi-causal, multi-directional exchanges of energy, material, and information. Each system is open to external influences through diffusion, migration, and colonization" (Turner et al., 1990). Changes in the system may be sudden or gradual and adaptive, with evolution expressed as survival of species and choices of individuals institutionalized as social norms. This theory incorporates nature's constraints on human behavior and human behavior's feedbacks to the environment. It focuses on connectivity and mutual causality among natural and human components. It positions humans in the natural environment and allows us to understand what we are doing and why. Natural sciences are focused on the quantification

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology of energy, materials, and information with an emphasis on interconnections, dynamics, and exchanges with the external environment. Organisms are linked through the exchange of energy according to the laws of thermodynamics and materials through biogeochemical cycles. A dynamic equilibrium prevails with changes occurring through evolution and succession. SIGNIFICANCE OF BASIC RESEARCH There is a greater awareness in the water resource management community that the piecemeal approaches of the past are insufficient. The new ecosystem assessment bandwagon is in some respects a "let's be smart about it" approach, where the relevant knowledge is applied and critical missing information is obtained. Obviously, greater knowledge and understanding of aquatic ecosystems will contribute to the success of these efforts and of this ecosystem-based approach. A simple example of a remediation effort for an acid mine drainage stream in the Rocky Mountains illustrates how the questions identified above are relevant to addressing important water resource and environmental issues. In the 1800s and early part of this century, there was a mining boom in the Rocky Mountains, and this boom spurred the development of the western states. Many of these mines are now abandoned, but their legacy includes an almost uncountable number of acidic, metal-enriched water discharges into mountain streams, with elevated metal concentrations in larger rivers. The ecological consequences of the acid mine drainage are not subtle. In the headwater streams, the streambed is typically covered with hydrous iron oxides, known as yellowboy; in the larger rivers, fish populations have high body burdens of metals, which limit their survival. In the Arkansas River, which drains the Lake County mining district in the Colorado Rockies, trout do not survive beyond four years because of accumulated metals in their tissues. In many of these areas, there are public and legal incentives to clean up such sites. For example, in the summer of 1994, 268 members of Volunteers of Colorado came to the Pennsylvania mine site on Peru Creek in Colorado to help restore the creek by shoveling manure into two large plastic-lined pits. These pits will become artificial wetlands to treat the mine effluent. The pits are located adjacent to a liming facility that was built in the 1980s and proved to be a failure. Such treatments are being tried in many areas to ameliorate both coal and metal mining wastes. Although the volunteers at Peru Creek were enthusiastic and hopeful, from a scientific perspective can we be sure that this approach will work in either the short or the long term? Many of the issues, challenges, and unanswered questions raised in the preceding section come into play in a more specific way in this example

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology of remediating acid mine drainage. Questions related to temporal and spatial dynamics are clearly important. If wetlands are effective, they essentially accumulate and store metal contaminants alongside the stream. What are the interactions between the stream and its watershed that may influence the long-term stability of the wetland, and are there hydrologic connections between the watershed and the stream that would develop as different routes of metal transport? For the treatment to be effective it must also work during the winter, when the snow is meters deep in the valley, and better understanding of wetland and stream ecosystems during the winter would be useful in this regard. Because of the abundant iron oxides accumulated on the streambed, there may be substantial diel variations in the stream driven by photoreduction of the iron oxides. Questions related to the occurrence of particular species are also relevant. Should the success of the treatment be evaluated in terms of the return of ecosystem function, as measured by rates of primary production by periphyton, or should success be based on the return to the remediated stream of species of algae, benthic invertebrates, and fish normally found in pristine streams in the region? The chemical considerations relate to both the reduction of contaminants in the effluent and any new chemical species that may be released by the organic-rich artificial wetland. This example also highlights the importance of understanding the changing human dimension and its relationship to water resources and aquatic ecosystems. The miners who caused many of these problems were doing well just to survive in the harsh mountain environment. It was a "boom-or-bust" industry and there was not much emphasis on planning for the future. The environmental ethic of stewardship that now motivates some members of the public, especially the 268 volunteers, may be a relatively new ethic for Western culture. ACKNOWLEDGMENTS We acknowledge contributions of ideas and comments to this chapter from our colleagues S. Spaulding, R. Runkel, A. Covich, D. Macalady, Dieter Imboden, Lynn Roberts, Laura Sigg, Alan Stone, Werner Stumm, Bernhard Wehrli, M. Gordon Wolman, and Alexander Zehnder. REFERENCES American Society of Limnology and Oceanography. 1994. Regional Assessment of Freshwater Ecosystems and Climate Change in North America Symposium report, October 24–26, 1994, Leesburg, Va. Averett, R. C., and G. R. Marzolf. 1987. Water quality. Environ. Sci. Technol. 21(9):827 Edmondson, W. T. 1991. The Uses of Ecology: Lake Washington and Beyond. Seattle: University of Washington Press. Environmental Protection Agency (EPA). 1980. Urban Storm Water and Combined Sewer

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