Sources of small-scale variation in headwater stream habitat and macroinvertebrate communities

Abstract

The extent to which small stream habitats and communities are influenced by the surrounding terrestrial environment is a function of the linkage between aquatic and terrestrial systems. The strength of that linkage is mediated primarily by topography and can affect the susceptibility of the stream to impairment by harvesting disturbance. Stream habitat and macroinvertebrate communities were studied at 30 small stream sites to characterize their associations with the surrounding terrestrial environment at the riparian and catchment scales. Local topography was described using the ‘reach contributing area’ (RCA) as a measure of the lateral terrestrial area contributing to the stream reach. In the first section of the study I examined the linkage between stream habitat characteristics and riparian and catchment scale terrestrial variables in light of differences in local topography, and assessed the effects of harvesting disturbance on stream habitat as mediated by that aquatic-terrestrial linkage. Using redundancy analysis (RDA) riparian scale factors were found to be more strongly correlated with local habitat variability than catchment scale factors in both small and large RCAs, and in both reference and harvested sites. In sites with large RCAs riparian scale variables explained 40% more variation than catchment scale variables. Aquatic habitat at sites with recent local harvesting had significantly higher temperatures and nitrogen concentrations (MANOVA p<0.05). Stream habitat variation in harvested sites was more strongly correlated with forest cover, whereas variation in reference sites was more strongly correlated with topographic variables. In the second section I characterized how macroinvertebrate communities are structured based on aquatic and terrestrial variables at 3 spatial scales, and how those influences differ based on local topography. The effects of harvesting on macroinvertebrate community structure were also examined in the final section. Local scale variables explained the most variation in taxonomic and functional invertebrate community structure (51.6% and 59.1%), followed by catchment scale variables (43.9% and 43.5%). In large RCA sites, the riparian scale variables had almost 10% more influence on taxonomic structure than catchment scale variables, and in small RCA sites catchment scale variables explained almost 23% more variation in feeding guild structure than riparian scale variables. The total abundance and richness of macroinvertebrates was significantly higher in harvested sites than in reference sites (ANOVA p<0.05), and communities differed significantly in structure (MRPP p<0.05). This study shows the effect of local topography on the linkage between aquatic and terrestrial environments. The RCA effect combined with the effects of harvesting disturbance on small streams can be combined to better understand processes driving variability in small stream habitats and communities. This understanding can then be applied to management practices to better conserve all aspects of headwater stream environments.