Optimizing stream bioassessment: habitat, season, and the impacts of land use on benthic macroinvertebrates

Bioassessment of running waters should ideally be optimized to include sampling of the biota when and where they are most sensitive to anthropogenic disturbances, but direct comparisons of the responses of biota across habitats and seasons are lacking. We sampled benthic macroinvertebrates from nine boreal streams situated along an agricultural land use gradient in two seasons (spring and autumn) and two habitats (pools and riffles). Univariate (e.g., diversity) and multivariate (ordination scores) metrics, as well as biological traits, were used to assess changes in assemblage composition associated with agricultural land use. Abundances were generally higher in agricultural compared to forested streams, and in riffles compared to pools. Spring samples had lower mean abundances of several insect taxa (e.g., chironomid midges) compared to autumn samples, while abundances of non-insects (e.g., oligochaetes and Pisidium spp.) remained unchanged. Community turnover (correspondence analysis) had higher precision and sensitivity compared to diversity metrics, and samples from the spring and from riffles responded more to the land use gradient than those from autumn and pool habitats, respectively. The finding that catchment land use resulted in macrohabitat differences and, ultimately, differences in taxonomic composition between agricultural and forested streams and between pool and riffle habitats can be used to optimize future bioassessment based on macroinvertebrates.