Aquatic ecosystems are critical components of the global environment. In addition to being essential contributors to biodiversity and ecological productivity, they also provide a variety of services for human populations, including water for drinking and irrigation, recreational opportunities, and habitat for economically important fisheries. However, aquatic systems have been increasingly threatened, directly and indirectly, by human activities. In addition to the challenges posed by land-use change, environmental pollution, and water diversion, aquatic systems are expected to soon begin experiencing the added stress of global climate change.

“Aquatic Ecosystems and Global Climate Change” is the seventh in a series of reports examining the potential impacts of climate change on the U.S. environment. It details the likely impacts of climate change over the next century on U.S. aquatic ecosystems. Report authors, Drs. N. LeRoy Poff, Mark Brinson, and John Day, Jr. find:

• Increases in water temperatures as a result of climate change will alter fundamental ecological processes and the geographic distribution of aquatic species. Such impacts may be ameliorated if species attempt to adapt by migrating to suitable habitat. However, human alteration of potential migratory corridors may limit the ability of species to relocate, increasing the likelihood of species extinction and loss of biodiversity.
• Changes in seasonal patterns of precipitation and runoff will alter hydrologic characteristics of aquatic systems, affecting species composition and ecosystem productivity. Populations of aquatic organisms are sensitive to changes in the frequency, duration, and timing of extreme precipitation events, such as floods or droughts. Changes in the seasonal timing of snowmelt will alter stream flows, potentially interfering with the reproduction of many aquatic species.
• Climate change is likely to further stress sensitive freshwater and coastal wetlands, which are already adversely affected by a variety of other human impacts, such as altered flow regimes and deterioration of water quality. Wetlands are a critical habitat for many species that are poorly adapted for other environmental conditions and serve as important components of coastal and marine fisheries.
• Aquatic ecosystems have a limited ability to adapt to climate change. Reducing the likelihood of significant impacts to these systems will be critically dependent on human activities that reduce other sources of ecosystem stress and enhance adaptive capacity. These include maintaining riparian forests, reducing nutrient loading, restoring damaged ecosystems, minimizing groundwater withdrawal, and strategically placing any new reservoirs to minimize adverse effects.

The authors and the Center gratefully acknowledge the input of Drs. Virginia Burkett, Judy Meyer, Elizabeth Strange, and Alan Covich on this report. The Center would also like to thank Joel Smith of Stratus Consulting for his assistance in the management of this Environmental Impacts Series.

Executive Summary

Climate change of the magnitude projected for the United States over the next 100 years will cause significant changes to temperature regimes and precipitation patterns across the United States. Such alterations in climate pose serious risks for inland freshwater ecosystems (lakes, streams, rivers, wetlands) and coastal wetlands, and they may adversely affect numerous critical services they provide to human populations.

The geographic ranges of many aquatic and wetland species are determined by temperature. Average global surface temperatures are projected to increase by 1.5 to 5.8oC by 2100 (Houghton et al., 2001), but increases may be higher in the United States (Wigley, 1999). Projected increases in mean temperature in the United States are expected to greatly disrupt present patterns of plant and animal distributions in freshwater ecosystems and coastal wetlands. For example, cold-water fish like trout and salmon are projected to disappear from large portions of their current geographic range in the continental United States, when warming causes water temperature to exceed their thermal tolerance limits. Species that are isolated in habitats near thermal tolerance limits (like fish in Great Plains streams) or that occupy rare and vulnerable habitats (like alpine wetlands) may become extinct in the United States. In contrast, many fish species that prefer warmer water, such as largemouth bass and carp, will potentially expand their ranges in the United States and Canada as surface waters warm.

The productivity of inland freshwater and coastal wetland ecosystems also will be significantly altered by increases in water temperatures. Warmer waters are naturally more productive, but the particular species that flourish may be undesirable or even harmful. For example, the blooms of “nuisance” algae that occur in many lakes during warm, nutrient-rich periods can be expected to increase in frequency in the future. Large fish predators that require cool water may be lost from smaller lakes as surface water temperatures warm, and this may indirectly cause more blooms of nuisance algae, which can reduce water quality and pose potential health problems.

Warming in Alaska is expected to melt permafrost areas, allowing shallow summer groundwater tables to drop; the subsequent drying of wetlands will increase the risk of catastrophic peat fires and the release of vast quantities of carbon dioxide (CO2) and possibly methane into the atmosphere.

In addition to its independent effects, temperature changes will act synergistically with changes in the seasonal timing of runoff to freshwater and coastal systems. In broad terms, water quality will probably decline greatly, owing to expected summertime reductions in runoff and elevated temperatures. These effects will carry over to aquatic species because the life cycles of many are tied closely to the availability and seasonal timing of water from precipitation and runoff. In addition, the loss of winter snowpack will greatly reduce a major source of groundwater recharge and summer runoff, resulting in a potentially significant lowering of water levels in streams, rivers, lakes, and wetlands during the growing season.

The following summarizes the current understanding regarding the potential impacts of climate change on U.S. aquatic ecosystems:

1. Aquatic and wetland ecosystems are very vulnerable to climate change. The metabolic rates of organisms and the overall productivity of ecosystems are directly regulated by temperature. Projected increases in temperature are expected to disrupt present patterns of plant and animal distribution in aquatic ecosystems. Changes in precipitation and runoff modify the amount and quality of habitat for aquatic organisms, and thus, they indirectly influence ecosystem productivity and diversity.

2. Increases in water temperature will cause a shift in the thermal suitability of aquatic habitats for resident species. The success with which species can move across the landscape will depend on dispersal corridors, which vary regionally but are generally restricted by human activities. Fish in lowland streams and rivers that lack northward connections, and species that require cool water (e.g., trout and salmon), are likely to be the most severely affected. Some species will expand their ranges in the United States.

3. Seasonal shifts in stream runoff will have significant negative effects on many aquatic ecosystems. Streams, rivers, wetlands, and lakes in the western mountains and northern Plains are most likely to be affected, because these systems are strongly influenced by spring snowmelt and warming will cause runoff to occur earlier in winter months.

4. Wetland loss in boreal regions of Alaska and Canada is likely to result in additional releases of CO2 into the atmosphere. Models and empirical studies suggest that global warming will cause the melting of permafrost in northern wetlands. The subsequent drying of these boreal peatlands will cause the organic carbon stored in peat to be released to the atmosphere as CO2 and possibly methane.

5. Coastal wetlands are particularly vulnerable to sea-level rise associated with increasing global temperatures. Inundation of coastal wetlands by rising sea levels threatens wetland plants. For many of these systems to persist, a continued input of suspended sediment from inflowing streams and rivers is required to allow for soil accretion.

6. Most specific ecological responses to climate change cannot be predicted, because new combinations of native and non-native species will interact in novel situations.Such novel interactions may compromise the reliability with which ecosystem goods and services are provided by aquatic and wetland ecosystems.

7. Increased water temperatures and seasonally reduced streamflows will alter many ecosystem processes with potential direct societal costs. For example, warmer waters, in combination with high nutrient runoff, are likely to increase the frequency and extent of nuisance algal blooms, thereby reducing water quality and posing potential health problems.

8. The manner in which humans adapt to a changing climate will greatly influence the future status of inland freshwater and coastal wetland ecosystems. Minimizing the adverse impacts of human activities through policies that promote more science-based management of aquatic resources is the most successful path to continued health and sustainability of these ecosystems. Management priorities should include providing aquatic resources with adequate water quality and amounts at appropriate times, reducing nutrient loads, and limiting the spread of exotic species.

Overall, these conclusions indicate climate change is a significant threat to the species composition and function of aquatic ecosystems in the United States. However, critical uncertainties exist regarding the manner in which specific species and whole ecosystems will respond to climate change. These arise both from uncertainties about how regional climate will change and how complex ecological systems will respond. Indeed, as climate change alters ecosystem productivity and species composition, many unforeseen ecological changes are expected that may threaten the goods and services these systems provide to humans.