Freshwater Ecosystems Linkages of Complexity and Processes

S. CARPENTER, T. FROST, L. PERSSON, M. POWER AND D. SOTO

12.1 INTRODUCTION 12.1.1 Uses of Freshwaters

Freshwater ecosystems are indispensable for life. Unlike some resources, there is no substitute for water. Its availability influences the distribution of Earth's major biomcs and the productivity of agriculture. Historically, fresh-waters have been a magnet for human settlement. Important human uses of freshwaters include drinking, fishing, industry, irrigation, recreation and transportation (Schindler and Bayley 1990). Freshwaters and their bordering riparian corridors are also crucial conduits in regional ecological and economic systems (Naiman et at. 1993).

More than 97% of Earth's water is saline (La Riviere 1989). Of that which is fresh, most occurs as ice (1.97%) or groundwater (0.61%). Only 0.014% of Earth's water occurs in the biosphere. This pool of water available for life is relatively small and distributed patchily over Earth's surface. Consequently. water is often a limiting resource (Gleick 1993). The largest impacts of global climate change on human society and the biosphere are likely to arise from shifts in the distribution and availability of freshwater (Ausubel 1992). More immediate and direct threats to Earth's freshwaters result from human overpopulation, poor land-use practices, habitat degradation and pollution (La Riviere 1989; Schindler and Bayley 1990).

12.1.2 Vulnerability of freshwater resources

Although each stressed lake or river may appear unique, the pattern of water resource degradation is global. Human overpopulation is a root force that accounts for most of the losses of water resources. Most commonly, impairments of ecological complexity and ecosystem functions in freshwater

Functional Roles oj Biodiversity: A Global Perspective

Edited by H.A. Mooney, i.H. Cusbman. E. Medina. O.E. Sala and E.-D. Sehulre (jStf) © 1996 SCOPE Published in 1996 by John Wiley & Sons Ltd m'' unh-

are driven by habitat loss and degradation, species invasions, overharvesting and pollution (Witkowski 1992; Allan and Flecker 1993). The importance of these drivers and their consequences are well documented (National Research Council 1992). Global climate change is a potential, but less certain, threat to Earth's freshwaters than direct human impacts (Carpenter et al. 1992a).

Habitat loss and degradation involve both land-use change and direct modifications of aquatic systems (National Research Council 1992; Allan and Flecker 1993). Land-use practices including deforestation, intensification of agriculture, spreading of human settlement and draining of flooded area contribute to erosion, siltation and pollution that degrade freshwaters. Aquatic systems arc especially sensitive to modifications of riparian areas (Naiman 1992; Naiman et al. 1993). Landscape biodiversity in the catchment is directly related to freshwater quality. In particular, intact riparian vegetation and upland vegetation that retard erosion and siltation are essential for maintaining freshwater quality.

The most conspicuous direct modifications of freshwaters are large dams and other major water projects. Adverse environmental impacts of such projects are well documented, but the most serious cficcts may arise many years after completion of a project and specific predictions of impacts are difficult (Rosenberg et al. 1987; Fearnside 1989; National Research Council 1992). Channel straightening and removal of riparian vegetation are less grandiose, but ubiquitous, causes of habitat degradation (Hughes et al. 1990). In arid regions, extraction of water for human use can cause simplification or even complete elimination of freshwater ecosystems (Moyle and Leidy 1992).

Species invasions may also cause enormous changes in ecological communities and ecosystem processes (Magnuson 1976; Lodge 1993). Invasions have been caused by deliberate introductions of fishes (for commercial fishing, angling, aquaculture and biological control) and inadvertent transport of organisms (Moyle and Leidy 1992; Allan and Flecker 1993). Not ail exotic species succeed in new habitats, but successful invasions can have dramatic consequences. A spectacular example occurred when opossum shrimp were introduced to Flathead Lake (Montana, USA) as food for a prior successful introduction, kokanee salmon (Spencer et al. 1991). The salmon had supported angling and large populations of eagles and grizzly bears. The introduced shrimp were voracious predators of zooplankton that had previously supported the salmon. However, the shrimp escaped from salmon predation by migrating down to deep, dark waters during the day. Food limitation caused the salmon stock to collapse, leading to declines in the populations of eagles and grizzly bears and adverse changes in angling, ecotourism and the regional economy. Another dramatic example is the introduction of Nile perch to Lake Victoria, which has caused alterations in food-web dynamics, extinction of many native haplochromine cichlid fishes and the collapse of the traditional local fishery (Witte el al. 1992).

Both commercial and sport fishing alter freshwater communities (Magnuson 1991). Effects of fishing cascade through the food web to affect ecosystem productivity and nutrient cycling (Carpenter and Kitchell 1993). Fishing has been a factor in the extirpation of slow-growing species with high economic value, such as sturgeon (Movie and Leidy 1992). Fish communities of inland waters have also reacted to fishing pressure by the disappearance of larger individuals and an increasing dominance of smaller fishes (Wclcomme 1982). The aquarium trade for wild-caught species is increasing the value of some fishes to the point where they may become endangered (Moyte and Leidy 1992).

Freshwater systems are affected by a wide range of pollutants. Effects can be tied to direct municipal and industrial discharges, agricultural pollutants (silt, fertilizers, animal wastes and pesticides), and airborne pollutants (such as acid deposition, mercury and volatile organic compounds) (National Research Council 1992). Chronic sublethal effects are probably more common than large-scale lethal effects. Unfortunately, chronic effects are far more difficult to document. Convincing whole-ecosystem studies, such as those by Schindler et al. (1985) on effects of acidification, are rare. Information on probabilities of chronic effects at modest levels of exposure is an important need for environmental risk assessments (Bartell et al. 1992; McCarty and MacKay 1993).

12.1.3 Assessing vulnerability of freshwater resources

Abused freshwater systems can have a variety of undesirable features. Excess nutrients support algal blooms and nuisance growths of higher aquatic plants. Siltation clouds water. Sedimentation impedes navigation by boats and anadromous fishes. Potability is lost due to disease organisms, silt, pollutants or algal exudates. Native species are lost. Fish may bccome toxic to higher trophic levels, including humans, due to biomagnified chemicals. Productivity of desirable fish species may decline.

The attributes of any freshwater ecosystem result from a complex interaction of the physical-chemical characteristics of a system and the diversity of the organisms that inhabit it, Undesirable attributes can be produced by human-caused shifts in those organisms that affect ecosystem properties, or by shifts in physical chemical features. Meeting the challenges of protecting or restoring freshwater ecosystems requires a detailed knowledge of the interplay between abiotic and biotic factors.

We address the link between ecological complexity and ccosystem processes in freshwater ecosystems. This topic is one where substantial scientific progress is likely, and where reduced scientific uncertainties could have important effects on management choices. The linkages of ecological diversity and ecosystem processes are crucial to the current debate about the relative merits of species and ecosystem criteria as bases for environmental policy (Franklin 1993; Losos 1993; Orians 1993). Many freshwater resources and problems directly involve species, e.g. exploited fish stocks, endangered species and nuisance invaders. In other cases, species are directly linked to ecosystem processes such as the maintenance of water quality (Kitchell 1992; Cooke et al. 1993) or the biomagnification of toxins (National Research Council 1992).

The challenges in protecting or restoring freshwaters involve both societal goals and scientific uncertainties (National Research Council 1992). Human goals frequently conflict. For example, tradeoffs may exist between riparian land use and water quality, water quality and fish productivity and hydroelectric production and riverine productivity. The data needed to assess these tradeoffs are often incomplete, and even with the best data sets the predicted consequences of management actions are uncertain.

Hilborn (1987) identifies three categories of uncertainty relevant to predicting the effects of ecosystem stress: (1) noise, the intrinsic variability we can do nothing about; (2) uncertain states of nature, identified sources of uncertainty that are not yet quantified owing to lack of experience; (3) surprise, things that we have not considered that have enormous impacts when they occur. Noise occurs so frequently that we have extensive experience of it. Scientists quantify noise routinely, and there are well-established methods for coping with noise in environmental management. Most species interactions and ecosystem processes are in the second category, uncertain but potentially quantifiable slates of nature. Examples are the structural changes that occur in pelagic communities, with major consequences for fisheries and water quality (May 1984; Carpenter 1988). We need more scientific experience with such changes to quantify their nature and probability. Species invasions are often in the third category of surprise. An example is the sea lamprey in the Laurcntian Great Lakes of North America (Christie 1974). By definition, surprises are rare and unexpected. The best we can hope for is rapid assessment and adaptive management once a surprise has occurred. Assessment may be facilitated by prior research at scalcs appropriate to management.

In this chapter we provide several illustrations of interactions that have been documented between ecological diversity and ecosystem processes. We describe situations where shifts in the abundance and interactions of species have had a profound effect on the fundamental characteristics of ecosystem function. We also consider cases where changes in physical-chemical features of ecosystems have a substantial impact on the specics that live within them. Eventually we examine the implications of how knowledge of the relationship between ecological diversity and ecosystem processes can be used to predict changes as shifts occur in systems that are subjected to changes caused by human activities.

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