The distribution of Ephemeroptera, Plecoptera and Trichoptera in a Norwegian alpine catchment in relation to environmental gradients

We investigated assemblages and distribution of Ephemeroptera, Plecoptera and Trichoptera in a mountain stream catchment and their relationship to environmental variables. Sampling stations were located along an altitudinal gradient from subalpine birch forest to low alpine tundra in the Øvre Heimdalen Valley in the Norwegian Jotunheimen Mountains. The aquatic insects were sampled using a kick net in June, July and September. At the same time environmental variables (e.g., water temperature, substrate composition) were measured and water samples taken for analysis. The mayfly, <i>Baetis rhodani</i> (Pictet, 1843) (Ephemeroptera) dominated all reaches, except the outlet of Øvre Heimdalsvatn Lake, where the trichopteran filter feeder <i>Polycentropus flavomaculatus</i> (Pictet, 1834) was recorded as the most abundant species. Along with increase in altitude, there was a decrease in taxa richness. The main environmental variables explaining species distribution were discharge, substrate size and water temperature. Canonical Correspondence Analysis (CCA) clearly separated the outlet of the Øvre Heimdalsvatn Lake, and the reach downstream of the small Brurskardtjern Lake from the other stream sites situated upstream of the Øvre Heimdalsvatn Lake. This indicates the distinction between alpine streams and alpine lake outlets where filter feeders are numerous, feeding on the abundant seston of lacustrine origin.