Abstract
Owing to the rapid progress of high‐throughput sequencing technologies, microbial assemblages have gained growing interest in environmental impact assessment. However, research on microbial community responses, particularly those of benthic biofilm, to browning (increased concentrations of dissolved organic carbon [DOC]), is scarce. We used data from 55 boreal streams to examine if biofilm bacterial communities exhibit changes in diversity and community composition along a gradient of browning (3.6–27 mg DOC L −1 ). Species richness increased slightly with increasing DOC, whereas community composition changed markedly across the gradient, especially in the active community. Pseudomonadota and Bacteroidota were overall dominant bacterial phyla. In the active community, Bacteroidota became relatively less abundant and Pseudomonadota more abundant with increasing DOC. Nitrate‐N (NO 3 ‐N) and DOC were the most important predictors of bacterial community turnover. The greatest change in community composition occurred between 75 and 100 μ g NO 3 ‐N L −1 . For DOC, the first change point was at the low‐end of the gradient, followed by a major change in strongly brownified waters (> 20 mg L −1 ). Bacterial communities became phylogenetically more similar than expected by chance as DOC increased. Concordance between bacterial and benthic invertebrate communities was very high, indicating that browning exerts a strong control over both taxonomic groups. Our results suggest that microbial communities, particularly the active portion of the community, may provide a sensitive and reliable tool for stream bioassessment. We defined a threshold‐type response in bacterial assemblages to water browning but more research is needed on microbial responses to multiple simultaneous stressors related to global warming and land‐use intensification.