Benthic bacterial communities are shaped by browning in boreal headwater streams

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 (NO3‐N) and DOC were the most
important predictors of bacterial community turnover. The greatest change
in community composition occurred between 75 and 100 μg NO3‐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.