Global change and freshwater ecosystems

The biosphere contains only 0.014% of Earth's water, distributed among lakes (0.008% ), soils (0.005% ), and the atmosphere, rivers, and biota (0.001 % ) (79). An additional 2.58% of Earth's water is fresh, occurring as ice (1.97%) or groundwater (0. 61 % ) . The remainder of Earth's water is saline. Climate affects key fluxes (evaporation, water vapor transport, and precipitation) that deter­mine the amount and distribution of freshwater (79). The availability of freshwater to both ecosystems and humans is therefore sensitive to changes in climate. The small amount of water found in the biosphere has large significance for ecosystems and society. The availability of water influences the distribu­tions of major biome types and potential agricultural productivity (35). Humans depend directly on freshwater for drinking, irrigation, industry, transportation, recreation, and fisheries. Expanding human populations and changes in global climate will exacerbate already severe stress to freshwater resources in some regions (1, 79, 108, 163, 172). In others, increased availability of water will potentially mitigate stress ( 1, 15, 163, 172). This review addresses the potential effects of global climate change on lake and stream ecosystems. Concern for brevity severely constrained the topics covered. Sustained shifts in global climate will have enormous effects on distributions and interactions of species (38, 71). We limit this review to fishes, which interact strongly with lower trophic levels (18, 122), illustrate key considerations common to other aquatic organisms, and are significant to the public. Although wetlands interact intimately with streams and lakes (169), our consideration is limited to their interactions with atmospheric chemistry. The major consequences of global change for freshwater ecosystems depend on the temporal and spatial scales at which effects are assessed. We have organized this review around two broad scales of change. Transitional scales pertain to landscape-level shifts in location, morphometry, and persistence of lakes and streams and their biotas over decades to centuries. Perturbational scales pertain to events with return times of years to decades that affect entire stream or lake ecosystems, such as floods, droughts, or fish recruitment events. We then turn to ecosystem metabolism as an integrated response to climate, and to feedbacks between freshwaters and regional climate; we close the review with salient uncertainties that should influence future research priorities.