Spatio-temporal patterns of benthic invertebrates along the continuum of a braided Alpine river

Abstract

Abstract: Aquatic invertebrate community structure was quantified from six sites, each located in distinctly different geomorphic reaches along the Tagliamento River (N.E. Italy). Quantitative samples of benthic invertebrates were collected quarterly over one year from six sites (from 5–1100m above sea level), while physico-chemical data were collected monthly. Total abundance of benthic invertebrates was highly vari-able in space and time with a minimum density of 433 ± 158 individuals m–2 ( ± 1 standard deviation; n = 3) occurring in the main channel of an island-braided flood plain midway along the continuum after the autumnal floods (November 1998). At most sites, maximum densities (111,226 ± 45,395 ind. m–2) were observed during late summer (August 1998). In August, insects comprised more than 87 % of all taxa in each reach, with Chironomidae (Diptera) and Baetidae (Ephemeroptera) being the most abundant insect families. Abundance of Insecta, Crustacea (primarily copepods and amphipods), Oligochaeta, Nematoda, and Hydrachnidia all increased with distance downstream. In general, dominant insect taxa were represented by highly mobile forms with potential for multivoltinism (Chironomidae, Baetidae, Simuliidae). Non-insect