Benthic invertebrate taxonomic and trait associations with land use in an intensively managed watershed: Implications for indicator identification

Abstract

Expansion and intensification of human land uses has resulted in the loss of all but remnant areas of natural cover over increasingly vast areas of land. Effective stream assessment in these regions thus requires indicators sensitive to ecosystem change in systems that are already highly stressed. The goal of this study was to determine if benthic macroinvertebrate communities in streams exposed to extensive anthropogenic stress are associated with land-use variables and demonstrate potential to be used as bioindicators for assessment of further land use modification. Using the Grand River in southern Ontario as our model, we sampled for benthic macroinvertebrates in 70 small (3–20 km2) sub-watersheds that comprised a gradient of intensive land use (65–100% developed). Redundancy analysis and variance partitioning results indicated that macroinvertebrate taxonomy and traits were only weakly associated with environmental gradients with the strongest associations occurring at the reach scale. With strong anthropogenic filters present across the watershed, it is likely that the majority of benthic macroinvertebrate communities in the Grand River have experienced a homogenization towards tolerant taxa and traits that respond more to local habitat variables. Threshold Indicator Taxa Analysis (TITAN) revealed 19 valid taxa indicators and 32 valid trait indicators for one or more of six environmental variables (percent sub-watershed agriculture, percent buffer agriculture, pool substrate, sediment reduction, natural channel, and riparian width). However, individual taxon and community change points exhibited broad quantile scores indicating a gradual or random response to environmental variables. Comparison of traits and taxonomic associations with environmental variables showed minimal difference suggesting traits may not offer increased sensitivity to land-use stress. However, several traits did show potential to serve as diagnostic indicators for specific stressors associated with agricultural land use. Our ability to identify even a few indicators in a region with intensive human activity is promising for land managers charged with detecting biotic alterations in this stressed system. Further research developing indicators sensitive to intensification of anthropogenic stress in extensively developed landscapes is critical to ensuring managers have the tools needed to protect and enhance aquatic resources.