Responses of lakes and streams to climate change are spatially heterogeneous. Much of the local heterogeneity depends on the type of water body being considered: lake or stream, large or small, shallow or deep, eutrophic or oligotrophic, and so forth. Spatial heterogeneity also results from spatial differences in climate drivers themselves with latitude, altitude, and distance from the coast or a large lake. Inappropriate responses with large expenditures of local resources will result unless the spatial heterogeneity of local responses is understood and can be predicted.
Two ideas that help explain local heterogeneity in responses of lakes and streams to climate change from a landscape perspective are the stream continuum (Vannote et al., 1980; Minshall et al., 1985) and the position of a lake in the landscape (Kratz et al., 1991; Kratz et al., 1997; Magnuson and Kratz, 2000; Riera et al., 2000). Both concepts are geomorphic legacies resulting from the location of the water body in the hydrologic flow field. Headwater streams will be more shielded from warming relative to lowland streams because cool groundwater sources are more important; in forest catchments, they are more shaded from radiation, and in mountain catchments they are at higher, cooler altitudes (Hauer et al., 1997). For lakes, changes in chemical inputs with changes in rainfall will be greater in upland lakes than in lowland lakes. Upland lakes are supplied more by dilute precipitation than by solute-rich groundwater or overland flow. The chemistry of headwater lakes is extremely responsive to climate-driven changes in groundwater inputs because they tend to have smaller volumes and shorter residence times (Krabbenhoft and Webster, 1995).
Differences in the extent of connected wetlands or other sources of DOC contribute to local heterogeneity in responses. Export of DOC to lakes and streams increases in wetter times and decreases during drought. This, in turn, changes light penetration and vertical distribution of solar heating and, in lakes, the depth of thermocline and the relative magnitude of cold and warm thermal habitat for fish (Schindler et al., 1996b). Penetration of UV-B, thus the damaging effects of that radiation (Schindler, 1997), also will differ among lakes. Lakes without large sources of DOC that leach in during wetter periods will respond less to climate changes in these respects.
Shorter term, more stochastic patterns in catchments alter the behavior of lake ecosystems. Consider a drought that increases the likelihood of forest fires in the watersheds of boreal lakes in Ontario (Schindler, 1997). Burned areas typically are patchy and may or may not include the catchment of a given lake. If they do include the lake's watershed, there is an initial increase in the input of solutes to streams and the lake. The lake also is under greater influence of wind mixing with trees gone, which would deepen the thermocline and again alter the relative magnitude of cold and warm thermal habitat for fish.
Predicted responses of threatened anadromous Pacific salmonid stocks in the Columbia basin of the northwestern United States are varied. Climate-related factors that are important to successful reproduction include temperature, the river hydrograph (peak and annual flow), and sedimentation (Neitzel et al., 1991). Based on expected changes in these streams and 60 stocks across the basin, impacts on 23% were judged to be negative, 37% positive, and 40% neutral.
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