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

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 5-1100 m above sea level), while physico-chemical data were collected monthly. Total abundance of benthic invertebrates was highly variable in space and time with a minimum density of 433 +/- 158 individuals m(-2) ( I 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 taxa were also well represented by high mobility (e.g., Echinogammarus spp.) and ability to rapidly reproduce (e.g., Oligochaeta, Nematoda, Copepoda), life-cycle strategies critical for persistence in a highly dynamic and physically harsh environment. Temperature, substrate size, nutrient concentrations, benthic organic matter, and biomass of epilithic algae all exhibited distinct changes along the continuum. Changes in benthic invertebrate community structure (abundance and diversity) also occurred along the longitudinal gradient and these changes were correlated with spatial and temporal shifts in various environmental factors. Longitudinal patterns revealed by Co-Inertia analysis, corroborated diversity and abundance patterns by illustrating the distinct nature of assemblages occurring in headwater reaches and the meandering lowland reach. Faunistic-to-environment concordance among reaches and dates was remarkably high and exhibited spatio-temporal dynamics. Concordance of communities with their local environment was more variable from date-to-date in lower reaches compared to headwater reaches, particularly following spring and autumn floods. Floods caused massive removal and relocation of individuals and appeared to disrupt the organizing pressures (selection pressures) experienced by communities after prolonged periods of low flow (i.e., competition, predation, and spatial organization of food and habitat resources). Patterns observed using Co-Inertia analysis were driven primarily by abundance and secondarily by community composition. In this way, it is important to recognize the complementary nature of comparing and contrasting measures of diversity (driven primarily by composition and secondarily by abundance) with multivariate analyses.