Sources of small-scale variation in the invertebrate communities of headwater streams

1. Our objective was to investigate the associations between benthic macroinvertebrate communities and environmental factors described at three spatial scales: local, reach and catchment. Differences in these associations, because of local topography, were determined by categorising sites into those having a large or small ‘reach contributing area’ (RCA), which is the lateral area of land contributing surface and subsurface flow of water, nutrients, organic and inorganic materials to a stream reach independent of catchment size and upstream contributions, and created by local topography. 2. Twenty-three sites were sampled in the Lake Nipigon Forest in north-western Ontario. Local variables included hydrological metrics and stream morphology characteristics. Terrestrial reach and catchment variables included forest characteristics, topography and geomorphology. Invertebrates were identified and classified into functional feeding guilds. Canonical correspondence analysis and redundancy analysis were used to determine which variables contributed most to macro-invertebrate community structure and how the RCA influenced those associations. 3. Overall, local-scale variables explained most variation in taxonomic and functional community structure (51.6 and 59.1%). Catchment-scale variables explained more variation in the community (43.9 and 43.5%) than reach-scale variables. In sites with a large RCA, the reach-scale variables accounted for almost 10% more variability in taxonomic structure than the catchment-scale variables, and in sites with a small RCA, catchment-scale variables explained almost 23% more variation in feeding guild structure than reach-scale variables. 4. Topography was important at both the reach and the catchment scales. Variables at the reach scale most associated with invertebrate community tended to be related to forest cover, whereas important variables at the catchment scale were related to topography, wetlands and lakes. 5. The difference in associations between invertebrate communities and environmental factors based on RCA indicates the influence of local topography on the linkage between aquatic habitats and the surrounding terrestrial environment. Understanding how these linkages affect aquatic communities can help to develop more effective predictive models of invertebrate community and more prescriptive conservation and management strategies for small streams.