Assessing stream condition using macro invertebrates and macrophytes:: concordance of community responses to human impact

Freshwater bioassessment is typically based on well known taxonomic groups. Such as fish and macroinvertebrates, while less studied groups are frequently excluded front bioassessment programs. We examined the community variation of stream macroinvertebrates and macrophytes (mainly aquatic bryophytes) in near-pristine reference sites and sites impacted by agriculture, forestry. or in-stream habitat alteration in western and central Finland. We specifically examined whether these taxonomic groups exhibited concordant variation in community Structure and whether they could be used as Surrogates for each other in stream bioassessment. The effects Of human disturbance on taxonomic composition and community Structure were investigated using canonical correspondence analysis and RIVPACS-type predictive models. The community concordance between macroinvertebrates and macrophytes was higher when test sites were included, suggesting that environmental degradation may increase community concordance between different biotic groups. However, despite increased concordance, macroinvertebrates and macrophytes responded to different stressors. The sensitivity of the two groups to human disturbance also differed markedly. The macroinvertebrate model determined 13 (with P-1 = 0+ as the threshold for species inclusion) and 12 (with P-t = 0.4) Out of 35 test sites as altered, whereas the macrophyte model detected impairment for only 7 sites (both probability levels), likely because of the low species richness of macrophytes in our study streams. Low richness of macrophytes may thus hinder their use in bioassessment based on taxonomic completeness, suggesting that measures that do not rely solely on species loss might perform better in at least high-gradient boreal streams.