Seasonal variability of landuse impacts on macroinvertebrate assemblages in streams of western Georgia, USA

We examined the influence of land use/land cover (LULC) on macroinvertebrate assemblages and environmental conditions in streams draining 18 small watersheds in the Southern Outer Piedmont ecoregion in Georgia, USA, over a 24-mo period of record. Specifically, we analyzed relationships among LULC categories (i.e., % impervious surface [IS], pasture, silviculture, and deciduous/evergreen forest) and hydrological, physicochemical, and benthic habitat variables, and macroinvertebrate metrics. Macroinvertebrate metrics were related primarily to % IS and % deciduous forest cover, with lowest biotic integrity (species diversity, taxon richness, biological stream condition index) found in high % IS watersheds. Biotic integrity declined with decreasing % forest cover throughout the seasons, and multiple regression models and partial correlation analysis revealed that physicochemical and benthic habitat variables explained more variation in macroinvertebrate metrics throughout the seasons than did hydrological variables at most sites. Based on nonmetric multidimensional scaling, heavily urbanized sites were strongly separated from all other sites in terms of assemblage structure. Total ordination distance among seasonal samples from the same sites increased as % forest cover increased. This pattern might have arisen because urbanized streams had high abundances of a few tolerant, persistent taxa and lacked many ephemeral taxa found in less disturbed systems. The influence of anthropogenic LULC on macroinvertebrate assemblages appears to be consistent throughout the year and reduces seasonal changes in assemblages. LULC-associated differences among assemblages are strongly associated with changes in physicochemistry and benthic habitat conditions that probably are mediated by hydrological alterations associated with altered LULC in the watersheds of this region.