Effects of Climate Change on In-Stream Biology and Freshwater Ecosystems

Jill S. Baron, Mid-Continent Ecological Science Center, U.S. Geological Survey

Biological Resources Division, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523

1. Current Resources and Stresses

a. Current Resources

Freshwater aquatic ecosystems are a diverse collection of landscapes and habitats, and include lakes, ponds, and reservoirs, rivers, streams, and canals, bogs, marshes, swamps, and floodplain wetlands, and groundwaters. Many regions of the United States have an abundance of freshwater, and throughout the world freshwater ecosystems are central to the socioeconomic status of nations, human health and quality of life, and environmental sustainability.

North American aquatic ecosystems support a highly diverse fauna and flora (Covich 1996, Flather et al. 1994, Dobson et al. 1997, Stein and Chipley, 1996). For some taxa and regions, such as the Southeastern United States, numbers of endemic species equal that known to occur in many tropical ecosystems (Lydeard and Mayden 1995). North America has the most diverse fauna in the world for some groups of aquatic organisms, such as freshwater bivalves (mussles) and crustaceans (e.g. shrimp and crayfish) with approximately 820 and 4,000 species, respectively. There are 815 native fish species in North America (Stein and Chipley 1996, Flather et al. 1994, Dobson et al. 1997, Covich 1996).

Aquatic organisms, both plants and animals, perform vital roles in ecosystem functioning, including decomposition of detrital material and providing food for other organisms, including humans (Covich 1993, 1996, Kitchell 1992). Many species operate in an intricate biological association with other species, with closely linked life histories. Disruption of one link in a food web or commensal species association may affect the status of many species of higher and lower trophic status. The diminished resistance and resilience to disturbance that accompanies loss of biodiversity depletes the capacity of aquatic ecosystems to both maintain ecological integrity and provide ecosystem goods and services to human populations (Covich 1993, Cushing 1997, Schindler 1997). Goods and services provided by aquatic ecosystems include maintaining and improving water quality through filtering and storing contaminants, providing food for human consumption, transportation, replenishing floodplain nutrients through periodic flooding, providing habitat and food for wildlife, and recreational opportunities. Failure of aquatic ecosystems to perform these functions can lead to degraded drinking water quality, loss of biodiversity, transmittal of infectious diseases, loss of food supplies, and costly remedial actions to counteract these problems (Cushing 1997, Schindler 1997, Nash 1993, Nielson and Lee 1987, Watson et al. 1996).

b. Stresses

Nearly all freshwater ecosystems of the United States have been modified by direct and indirect human activity. Direct activities that have altered aquatic ecosystems include: habitat destruction and fragmentation; dams, diversions, channelizations, and groundwater pumping that alter flow patterns, erosion activities, thermal regimes, and species migration routes; point and non-point introduction of a wide array of organic chemicals, toxic metals, and fertilizers such nitrogen and phosphorous; alteration of thermal regimes through damming, point source inputs of hot water, or riparian vegetation alteration; introduction of non-native plant and animal species; and over-harvesting. Indirect effects occur from land use change within watersheds, and long-range transport and deposition of acid and metal pollutants.

Wetlands area has declined by 40-60 per cent nation-wide (excluding Alaska), and in some states, such as Ohio, 90% of the wetland area has been lost (Dahl 1990, Swift 1984, Naiman et al. 1992). Wetland loss is primarily due to large-scale draining for agricultural activities; more recent losses of wetlands are due to urban expansion (Dahl 1990, Swift 1984). Riparian forests have been destroyed on 70% of rivers in the 48 contiguous states, less than 10,000 km, or two per cent, are still free-flowing and have federal protection from national park or wild and scenic river status (Naiman et al. 1992, Behnke 1990).

Organic chemicals from pesticides and herbicides that run off agricultural lands and urban lawns, and from industrial waste, can have both acute and chronic long-term effects on aquatic biota, although there is considerable uncertainty as to what the long-term risk will be (Environmental Protection Agency 1983, Naiman et al. 1992). Many of these compounds persist for decades in groundwater and sediments. They may act as hormone disruptors, and interfere with reproduction of organisms, growth and survival of young, and be passed up the food chain to top predators such as birds, salmon, and humans (Watson et al. 1996). Little is known about organismal or ecologic responses to these chemicals, and more research is needed.

Sources of metals include mine drainage, atmospheric deposition, industrial activities, stormwater runoff from urban areas, and runoff from modified watersheds. The most common pollutants in urban runoff, according to the EPA Nationwide Urban Runoff Study (1983), are heavy metals such as copper, lead, and zinc. Arsenic, mercury, and cadmium can additionally be added from mine drainage (Naiman et al. 1992). Watershed impoundment and groundwater pumping can cause metals that were previously sequestered to become available to organisms; selenium in wetlands of California, and mercury in Midwestern and Canadian fisheries are two examples (Schindler 1997). Mercury can also be deposited in atmospheric deposition, and elevated concentrations of methylmercury in freshwaters have been noted in remote aquatic ecosystems of the northern hemisphere (Wiener and Spry 1996). As of 1996 freshwater fish consumption warnings or advisories due to elevated mercury in fish have been issued in 38 states; 6 states have statewide (all freshwater) bodies. The toxicity of metals in aquatic ecosystems is closely linked with other physical characteristics, such as temperature and chemical interactions, particularly with dissolved organic carbon and pH (Wiener and Spry 1996).

Nitrogen can enter aquatic ecosystems in atmospheric deposition and as runoff from urban and agricultural activities. There is widespread evidence that nitrogen contamination is growing in severity through much of the United States (Cole et al. 1993, Moore et al. 1997). Concentrations greater than federal drinking water criteria standards (10 mg/l NO3-N) were found in more than 25% of groundwater wells sampled in some agricultural counties (Naiman et al. 1992). Nitrogen can act as a fertilizer if other nutrients are available, and lead to eutrophication of water bodies, including many streams, rivers, lakes, and estuaries. Eutrophication of estuaries is a severe and growing problem, and leads to toxic algal blooms and anoxic waters (Mulholland et al. 1997, Moore et al. 1997, Cole et al. 1993). Aquatic resources at risk of losing their acid-neutralizing capacity from excess nitrogen concentrations include the Catskill Mountains, which is the drinking water supply for New York City, and high elevation lakes and streams of the Colorado Rocky Mountains (Murdoch and Stoddard 1992, Williams et al. 1996).

Thermal alteration of even 1 degC can have significant effects on aquatic biota. Invertebrates and cold and cool water fish respond directly to temperature. Impoundments of rivers in Montana's Flathead River Basin resulted in the extirpation of caddisflies because of changes in water temperature; some of these species have since recolonized areas above and below dams according to their temperature preference (Hauer and Stanford 1982, Ward and Stanford 1982). The rates of many chemical and biological processes are temperature-dependent, including decomposition, bio-accumulation of toxins, and growth rates of organisms (Wetzel 1983). Changing the rates of these processes can have complex outcomes on aquatic ecosystems.

The spread of exotic, or non-native aquatic and riparian organisms through accidental or deliberate introduction has been a major cause of biological impoverishment of aquatic ecosystems (Stein and Chipley 1996, Naiman et al. 1992, Covich 1996). Introductions have led to competitive exclusions, in some cases, of native species by invaders (Moore et al. 1997, Makarewicz and Bertram 1991, Naiman et al. 1992). Phreatophytic plants, such as tamarisk from Asia, have altered hydrologic and geomorphological regimes of the Southwest (Grimm et al. 1997). These plants consume water and stabilize sand bars, thus removing spawning beds and drying up pools that were formerly persistent. Naive introductions of organisms thought to enhance recreational fishing have often gone awry. Species, such as the zebra mussel, purple loosestrife, eurasian watermilfoil, and exotic fishes represent major threats to the integrity of freshwater ecosystems (Naiman et al. 1992).

Aquatic organisms are directly affected by habitat degradation and destruction. A tabulation of species at risk in the United States show that the four leading groups of species that are extinct, imperiled, and vulnerable are those that rely on aquatic systems for all or part of their life cycle (Stein and Chipley 1996, Dobson et al. 1997, Flather et al. 1994). These, the freshwater mussels, crayfish, amphibians, and freshwater fishes, depend on rivers, lakes, or streams. While species have been lost or are at risk in every state and nearly every country, particularly vulnerable populations occur in certain "hotspots": the Southeast, the desert Southwest, California, and the Pacific Northwest (Flather et al. 1994, Stein and Chipley 1996, Dobson et al. 1997).

2. Additional Impacts Imposed by Climate Change

While global warming is widely presented as the major effect of elevated atmospheric concentrations of greenhouse gases, increased climate variability (prevalence of extreme events) and elevated transmission of ultraviolet radiation from loss of stratospheric ozone are also likely to occur (Watson et al. 1996). All of these will affect the structure and functioning of aquatic ecosystems, individually, and in synergism with the human-caused disturbances discussed above. Global warming will have direct effects on aquatic ecosystems, by altering water temperatures and regional water balances, or shortening the number of days lakes are ice-covered, for example (Magnuson et al. 1990, Schindler 1997). Global warming will also have indirect effects. Indirect effects on aquatic ecosystems will occur through altered terrestrial disturbance patterns and frequency, changing species distributions and assemblages, changing chemical budgets and rates of nutrient and toxin cycling (Naiman et al. 1992, Schindler 1997).

Climate warming will lead to less water throughout most of the United States, even where precipitation remains constant when temperatures rise (Watson et al. 1996, Schindler 1997, Mulholland et al. 1997). This is due to shorter winters, longer growing seasons, and increased evaporation and transpiration. Higher temperatures increase the ratio of evaporation + evapotranspiration (ET) to precipitation. A twenty year record of temperature increase in the Experimental Lakes Area of Ontario shows an average increase in ET of 9 mm per year, corresponding to an average of 35 mm per degree C per year (Schindler et al. 1996, Schindler 1997). Neither the extant precipitation records nor the projected changes suggest that precipitation will increase commensurate with the increase in ET.

Direct hydrologic responses will be seen in declining streamflow, lake levels, water renewal rates, and water levels in wetlands and groundwaters (Watson et al. 1996, Schindler 1997, Magnuson et al. 1990, Poiani et al. 1995, Poiani and Johnson 1991). This will lead to changes in the physical and chemical environments of aquatic organisms, as well as changes in their surrounding watersheds. Fire frequency increases during periods of drought, and may accelerate the warming of stream and lake waters, as well as affect long-term patterns of chemical exports (Schindler et al. 1996). Comparatively, the effects of fire are expected to be less important to aquatic systems than direct temperature warming or climatic extremes (Schindler et al. 1996).

As evapotranspiration increases and snowmelt runoff decreases, a dramatic loss of meadow and shallow marsh lands of the northern prairies is predicted (Poiani and Johnson 1991, Poiani et al. 1995, Winter 1989). Prairie wetlands provide an invaluable international resource by supplying more than half the annual waterfowl population produced in North America. Warmer temperatures will also lead to increased oxidation of wetland and lake sediment carbon stocks throughout North America, adding a potentially significant amount of carbon dioxide and methane to the atmosphere (Swanson er al. 1988, Khalil and Shearer 1993).

As climate warms, lake and stream temperatures will also warm. Warming affects rates of hatching and development, shortens life cycles of organisms, and alters the geographic distributions of many organisms. Using baseline water temperature conditions from 1,700 US Geological Survey stream monitoring stations, and extrapolations of published fish temperature tolerances, one study concluded that stream habitat for cold and cool water fish could decline across the U.S. by 50% (Eaton et al. 1995, Eaton and Scheller 1996). Habitat losses were greatest among species with smaller distributions and in geographic regions projected to show the greatest temperature change, such as the upper Midwest (Eaton and Scheller 1996). In mountainous regions warming could force cold water fish to increasingly higher elevations, leading to loss of 9-76% of current geographic range (Rahel et al. 1996). In addition to habitat loss, population fragmentation will occur (Eaton and Scheller 1996, Rahel et al. 1996, Hauer et al. 1997). Species that are not able to migrate to suitable habitats will become locally or regionally extirpated, or possibly become extinct. Migration to favorable climates is made more difficult where corridors have been modified by human activity.

Lower streamflows can lead to less export of chemicals and toxins from watersheds, although concentrations of many solutes will increase (Moore et al. 1997, Schindler 1997). Less watershed flushing reduced available nutrients over 20 years of worming in Ontario lakes (Schindler er al. 1996). In southern lakes nutrients became more available with lower flows during prolonged drought, due to increased release from anoxic bottom waters will increase, and the added nutrients and warmer temperatures will decrease the amount of available oxygen in lakes and streams (Schindler 1997, Mulholland et al. 1997, Moore et al. 1997).

3. Implications for Management

a. Vulnerable resources

The magnitude of effects due to changes in climate on water supply for human uses is small relative to changes in other variables. Water supply is already highly managed to optimize societal access to safe and sufficient water (Watson et al. 1996, Naiman et al. 1992). Natural aquatic ecosystems, unfortunately, have not such buffer between their health and directional climate and cultural change. While there is abundant evidence for dramatic climate and rapid warming from lake sediment records, tree-rings, and historical records, never before have climate changes been coincident with large scale landscape fragmentation and alteration.

Ecosystems that are most vulnerable to change include: those already at their climatic limits; ecosystem hot spots with high numbers of endemic species; regions of the country where competition for water supply between urban, agricultural, and natural uses is high or increasing; and systems where climate change will act in concert with existing human-driven stresses to worsen ecological health. These areas include, but are not restricted to, the southwestern and southeastern United Stated, prairie wetlands of the Great Plains, highly productive estuaries and wetlands such as the Chesapeake Bay and Everglades ecosystem, and Pacific coastal rivers (Naiman et al. 1992, Flather et al. 1994, Cushing 1997).

The effects of increased temperatures will be critical in some locations where extreme summer temperatures are already near the thermal maxima of some stream species. The high water temperatures, and increased evaporation and evapotranspiration is expected to lead to the elimination of some aquatic habitats and to be lethal for some species. The Kansas-Oklahoma-Texas region is one area where the combined loss of habitats and high temperatures will be compounded by increased human demand for water supplies (Covich et al. 1997).

b. Socioeconomic impacts

Decreased thermal niches for coldwater fishes in lakes and streams will be detrimental to the recreational fishing industry (Rahel et al. 1996). Similar decreases in the recreational hunting industry can be expected as enhanced evapotranspiration and reduced spring runoff dry shallow-basin prairie wetlands of the northern Great Plains, diminishing habitat for waterfowl (Poiani et al. 1990). Climate warming could lead to an increase in malaria at higher latitudes as the seasonal habitat for Anopheles mosquitos is predicted to increase (Martin and Lefebvre 1995). Other non-native and nuisance species will expand the northern limits of their ranges, which are often determined by minimum winter temperatures (Mulholland et al. 1997).

Warming appears to increase the rate of bioaccumulation of toxins in aquatic organisms, and the rate of transfer of these pollutants up the food chain (Wiener and Spry 1996, Schindler 1997, Wetzel 1983). This will have detrimental effects on fish-eating birds and mammals, and increase the numbers of fish populations that are unsafe for human consumption due to heavy metal or organic contamination.

Higher concentrations of nutrients and pollutants and less frequent flushing may affect the harvestability of fish, molluscs, and crustaceans from lakes and estuaries. On the other hand, it is possible that decreased runoff from polluted watersheds may actually increase water quality by reducing the inputs of nutrients to lakes and estuaries (Mulholland et al. 1997, Moore et al. 1997).

As freshwaters become increasingly scarce, competing demands for use will become more intense. Increasing human use of water will aggravate water quality problems, both by removal for consumption, and through wastewater returns (Schindler 1997, Golubev and Biswas 1985, NRC 1992). Water removal will be particularly detrimental to water quality by reducing dilution of effluents. In the southwestern and southeastern United States this is already occurring, and will worsen as climate warms (Mulholland et al 1997, Grimm et al 1997).

c. Options for management

As demands on water resources become increasingly great, enhanced communication among water users, managers, and scientists will become critical. We must develop the institutional mechanisms that foster information flow, tolerance, and compromise, so that the natural capital inherent in our aquatic resources is nor lost in hasty or one-sided decision making. In the face of increased variability, adaptive management will become crucial to preserving, protecting, and restoring aquatic ecosystems. Adaptive management involves prediction of all possible environmental outcomes associated with combination of management actions and the many aspects of climatic change, implementation of the most ecologically and economically rational actions, monitoring and evaluating the results of the actions, and redirecting management actions as indicated by monitoring and evaluation (Lee 193, Naiman et al. 1994).

In keeping with adaptive management, there will need to be increased flexibility in the management of water supplies. Water management policy must be increasingly guided by a whole system approach that optimizes the consequences of actions for natural ecosystem structure and function as well as for human uses.

Finally, there is a need for enhanced, government-supported, long-term environmental monitoring, so that the consequences of climatic change couple with human actions can be studied, understood, and used to drive policy. Such a strategy is essential if we are to differentiate between actual and perceived environmental issues, and address them appropriately to avoid both unnecessary regulation and serious environmental problems.

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