ASSESSMENT OF CLIMATE-CHANGE AND FRESH-WATER ECOSYSTEMS OF THE ROCKY-MOUNTAINS, USA AND CANADA

The Rocky Mountains in the USA and Canada encompass the interior cordi llera of western North America, from the southern Yukon to northern Ne w Mexico. Annual weather patterns are cold in winter and mild in summe r. Precipitation has high seasonal and interannual variation and may d iffer by an order of magnitude between geographically close locales, d epending on slope, aspect and local climatic and orographic conditions . The region's hydrology is characterized by the accumulation of winte r snow, spring snowmelt and autumnal baseflows. During the 2-3-month ' spring runoff period, rivers frequently discharge > 70% of their annua l water budget and have instantaneous discharges 10-100 times mean low flow. Complex weather patterns characterized by high spatial and temp oral variability make predictions of future conditions tenuous. Howeve r, general patterns are identifiable; northern and western portions of the region are dominated by maritime weather patterns from the North Pacific, central areas and eastern slopes are dominated by continental air masses and southern portions receive seasonally variable atmosphe ric circulation from the Pacific and the Gulf of Mexico. Significant i nterannual variations occur in these general patterns, possibly relate d to ENSO (El Nino-Southern Oscillation) forcing. Changes in precipita tion and temperature regimes or patterns have significant potential ef fects on the distribution and abundance of plants and animals. For exa mple, elevation of the timber-line is principally a function of temper ature. Palaeolimnological investigations have shown significant shifts in phyto- and zoo-plankton populations as alpine lakes shift between being above or below the timber-line. Likewise, streamside vegetation has a significant effect on stream ecosystem structure and function. C hanges in stream temperature regimes result in significant changes in community composition as a consequence of bioenergetic factors. Stenot hermic species could be extirpated as appropriate thermal criteria dis appear. Warming temperatures may geographically isolate cole water str eam fishes in increasingly confined headwaters. The heat budgets of la rge lakes may be affected resulting in a change of state between dimic tic and warm monomictic character. Uncertainties associated with predi ction are increased by the planting of fish in historically fishless, high mountain lakes and the introduction of non-native species of fish es and invertebrates into often previously simple food-webs of large v alley bottom lakes and streams. Many of the streams and rivers suffer from the anthropogenic effects of abstraction and regulation. Likewise , many of the large lakes receive nutrient loads from a growing human population. We concluded that: (1) regional climate models are require d to resolve adequately the complexities of the high gradient landscap es; (2) extensive wilderness preserves and national park lands, so pre valent in the Rocky Mountain Region, provide sensitive areas for diffe rentiation of anthropogenic effects from climate effects; and (3) futu re research should encompass both short-term intensive studies and lon g-term monitoring studies developed within comprehensive experimental arrays of streams and lakes specifically designed to address the issue of anthropogenic versus climatic effects. (C) 1997 by John Wiley & So ns, Ltd.