ECOLOGY OF ALPINE STREAMS

1. This review examines ecological conditions and zoobenthic communiti es of kryal, krenal and rhithral streams of the alpine zone. Altitudin al and biogeographical faunal patterns are also analysed. 2. Kryal seg ments, fed by glacial meltwater, are characterized by low temperatures (T-max less than or equal to 4 degrees C) and large diel flow fluctua tions in summer. The water may be clear or turbid from suspended rock flour. Fishes and higher plants are absent. The macroalga Hydrurus foe tidus may be abundant in kryal and other alpine stream types of the Ho larctic. The highly restricted cosmopolitan fauna of glacial brooks co nsists of diamesine chironomids, sometimes accompanied by simuliids. S parse food resources include algae and allochthonous (aeolian) organic matter. 3. Rhithral segments in alpine catchments are characterized b y soft water, a hydrograph dominated by an extended period of snowmelt runoff, and a broader temperature range than kryal or krenal biotopes . Bryophytes, macroalgae (chrysophytes, chlorophytes, cyanophytes, rho dophytes) and epiphytic and epilithic diatoms constitute the flora. A relatively diverse zoobenthos includes four orders of insects (Plecopt era, Ephemeroptera, Trichoptera, Diptera), turbellarians, acarines, ol igochaetes and nematodes. 4. Krenal segments, fed by groundwater, are typically calcareous with summer-cool and winter-warm thermal conditio ns, high water clarity, and constant flow regimes. Bryophytes and macr oalgae are accompanied by a rich diatom flora. The zoobenthos consists of a composite of kryal and rhithral elements with few crenobionts. Z oobenthos species richness values are intermediate between those of kr yal and rhithral segments, whereas densities in perennial, well-oxygen ated springbrooks far exceed those in other alpine stream types. 5. Do wnstream faunal changes are most predictable in kryal segments where c hironomids of the genus Diamesa are the predominant, if not sole, memb ers of the zoobenthos in the upper zone of glacial brooks, the metakry al. Where T-max exceeds about 2 degrees C the transition to the hypokr yal occurs and Diamesa is co-dominant with simuliids. These largely st enozonal headwater forms decline downstream where T-max exceeds about 4 degrees C, concomitant with a marked increase in the euryzonal mount ain fauna. 6. Species occurring in alpine rhithral biotopes tend to be euryzonal forms at their upper altitudinal limits, whereas the lower elevation mountain stream fauna consists of species with narrower dist ribution limits. There is, however, a precipitous drop in mean altitud inal range from the alpine rhithral to the kryal because of the stenoz onal nature of the glacial brook fauna. 7. The view that effects of te mperature on generation time and mutation rate determine the speed at which selection proceeds is consistent with altitudinal species richne ss patterns exhibited by zoobenthos along the altitudinal gradient and may provide an evolutionary explanation for the low faunal diversity in alpine headwaters. 8. With increasing altitude, mountain 'islands' become progressively insular as area decreases and isolation increases . For a cold-adapted stream fauna the insular nature of mountain toys is greatest in the tropics. Nonetheless, alpine stream faunas generall y exhibit relatively broad geographical distribution patterns. Only un der conditions of extreme geographical and ecological isolation may th e fauna attain a high level of endemism.