Influences of a point source on the microhabitat distribution of stream benthic macroinvertebrates

We examined how community measures (density, biomass, and richness) and community composition were related to microhabitat variables (including hydraulic parameters, substrate, and food resources) in La Tordera stream in Catalonia (NE Spain). We collected macroinvertebrate samples on six dates between November 2001 and September 2002 upstream and downstream of a point source input. Macroinvertebrate density and biomass were positively correlated with food resources and complexity of habitat architecture (benthic organic matter, chlorophyll-a, vascular plants, and mosses) while taxa richness was negatively correlated with conventional (water velocity and depth) and complex hydraulic parameters (Froude number, Reynolds number, roughness shear velocity). Inorganic substrate exerted a minor influence on macroinvertebrate distribution. Ordination analysis revealed that the microhabitat variables of major significance at the two reaches were CPOM, chlorophyll-a, filamentous algae, and maximum velocity. Sand coverage was only relevant for macroinvertebrates at the upstream reach, and moss at the downstream reach. The number of significant correlations between macroinvertebrates and microhabitat variables was higher at the upstream reach than at the downstream reach mainly because of higher taxa richness.