Ecological characteristics of lake outlets in Alpine environments of the Swiss Alps

Abstract

We examined the physical, chemical, and biological attributes of temporary and permanent lake outlets situated above treeline in the Swiss Alps. Surface flow in the temporary outlet streams ceased by mid-September. High discharge in spring associated with snowmelt was an order of magnitude greater in the temporary outlets than in the permanent outlets. Periphyton biomass attained 5--9x higher levels in the temporary sites than in the permanent sites by mid-summer. Although macroinvertebrate densities were similar between temporary and permanent sites, taxon richness was 2x greater in the permanent outlets. Macroinvertebrate groups absent from temporary outlets included Ephemeroptera, Plecoptera, Trichoptera, and long-lived dipterans such as Limoniidae and Tipulidae. The semi-terrestrial gastropod Galba truncatula colonized the temporary lake outlets via the lake. Chironomidae, Oligochaeta and Simuliidae also were common and able to complete development in the temporary lake outlets. Cladocera and Copepoda originating from the lakes attained high abundances in some outlets regardless of being temporary or permanent. The harsh Alpine environment in concert with intermittency were strong determinants of ecosystem properties in the temporary outlets. Regardless of flow permanency, the lakes directly affected community dynamics of outlets by influencing physical and chemical properties, and providing colonists to outlet streams.