Summary Forestry can have detrimental impacts on stream ecosystems, particularly via excessive sedimentation. A key challenge to stream management is therefore to identify the best restoration practices to mitigate the harmful impacts of fine sediments on stream biodiversity and ecosystem functioning. We studied the effects of restoration of sediment‐impacted headwater streams on the habitat structure, hydrologic retention, biodiversity (microbes, bryophytes, benthic macroinvertebrates, riparian plants) and ecosystem functions (periphyton accrual rate and leaf breakdown) by comparing four treatments: wood‐restored, boulder‐restored, impacted (by fine sediments) and near‐natural streams. The restored streams were sampled 3–7 years post‐restoration. Restoration by wooden or boulder structures aimed to reduce deposited sediments and increase channel heterogeneity and hydraulic retention. Wooden structures were ineffective in removing fine bed sediments and did not induce positive responses in aquatic biota. Boulder additions reduced substrate limitation and thereby proved beneficial for aquatic bryophytes. Benthic macroinvertebrates were clearly impaired by sedimentation but responded weakly to restoration. Leaf‐decomposing microbes and ecosystem functions were unresponsive to restoration but neither did they differ between near‐natural and impacted streams, suggesting that they were little harmed by sedimentation. Wood addition enhanced hydraulic retention, and riparian plant assemblages along wood‐restored streams resembled those in near‐natural streams, suggesting that increased retention re‐established a more natural flood regime. By contrast, riparian plant assemblages in boulder‐restored streams did not differ from those in impacted streams. Synthesis and applications. Restoration improved several aspects of stream and/or riparian biodiversity, but had limited effects on ecosystem functions. Different restoration measures resulted in differing biodiversity outcomes: boulder addition was more effective at restoring in‐stream heterogeneity and aquatic biodiversity, whereas wooden structures helped restore channel hydrology and retentiveness, and, consequently, riparian vegetation. Therefore applying both measures in the restoration of forested headwater streams with naturally stony substrates enhances stream habitat variability at the watershed scale, providing the most promising scenario for biodiversity benefits in broad‐scale restoration designs. In‐stream restoration that increases hydraulic retention has impacts that extend beyond ecosystem boundaries, reinforcing the need to restore, manage and protect streams and their riparian forests in an integrated effort.

The spatial structure and underlying assembly mechanisms of bacterial communities have been studied widely across aquatic systems, focusing primarily on isolated sites, such as different lakes, ponds and streams. Here, our main aim was to determine the underlying mechanisms for bacterial biofilm assembly within a large, highly connected lake system in Northern Finland using associative methods based on taxonomic and phylogenetic alpha- and beta-diversity and a large number of abiotic and biotic variables. Furthermore, null model approaches were used to quantify the relative importance of different community assembly processes. We found that spatial variation in bacterial communities within the lake was structured by different assembly processes, including stochasticity, species sorting and potentially even dispersal limitation. Species sorting by abiotic environmental conditions explained more of the taxonomic and particularly phylogenetic turnover in community composition compared with that by biotic variables. Finally, we observed clear differences in alpha diversity (species richness and phylogenetic diversity), which were to a stronger extent determined by abiotic compared with biotic factors, but also by dispersal effects. In summary, our study shows that the biodiversity of bacterial biofilm communities within a lake ecosystem is driven by within-habitat gradients in abiotic conditions and by stochastic and deterministic dispersal processes.

Aim Climate change and anthropogenic environmental deterioration strongly affect aquatic microbial communities. Although microbes have irreplaceable roles in various ecosystems, the spatial variation in microbial communities has received less attention in comparison to macro‐organisms. Studies aiming to disentangle the effects of local environmental, catchment and climatic variables on microbial communities are also rare. Here, we disentangled the effects of local, catchment, spatial and climatic variables on boreal stream diatom and bacterial communities. Location Western Finland Methods We sampled 21 boreal river basins comprising 105 study sites spanning 520 km in north‐south direction and 330 km in east‐west direction in western Finland. We used principal coordinates of neighbour matrix analysis (PCNM), redundancy analyses (RDA), variation partitioning, boosted regression trees (BRT) and regression analyses to examine variation in community composition and species richness. Results Water chemistry and physical variables had significant effects on the community composition of both microbial groups. Catchment level variables explained a slightly larger amount of variation in diatom community composition than local level variables. Agriculture was the most significant determinant of variation in diatom community composition among catchment level variables and was also related with variation in the richness of both groups. Of spatial and climatic variables, growing degree days (GDD) and spatial variables were the most significant drivers determining diatom community composition. GDD was also positively associated with the richness of diatoms and bacteria. Unique effects of spatial and climate variables accounted for the largest amount of variation in the community composition of both diatoms and bacteria. Main conclusions Our results highlight that aquatic microbial communities can exhibit biogeographical variation at regional scales due to the joint influence of local, catchment and climatic variables, and possibly because of dispersal limitation. Catchment properties, especially agriculture, can be used as a proxy for the effects of landscape alteration on aquatic microbial communities.

Context One approach to maintain the resilience of biotic communities is to protect the variability of abiotic characteristics of Earth’s surface, i.e. geodiversity. In terrestrial environments, the relationship between geodiversity and biodiversity is well recognized. In streams, the abiotic properties of upstream catchments influence stream communities, but the relationships between catchment geodiversity and aquatic biodiversity have not been previously tested. Objectives The aim was to compare the effects of local environmental and catchment variables on stream biodiversity. We specifically explored the usefulness of catchment geodiversity in explaining the species richness on stream macroinvertebrate, diatom and bacterial communities. Methods We used 3 geodiversity variables, 2 land use variables and 4 local habitat variables to examine species richness variation across 88 stream sites in western Finland. We used boosted regression trees to explore the effects of geodiversity and other variables on biodiversity. Results We detected a clear effect of catchment geodiversity on species richness, although the traditional local habitat and land use variables were the strongest predictors. Especially soil-type richness appeared as an important factor for species richness. While variables related to stream size were the most important for macroinvertebrate richness and partly for bacterial richness, the importance of water chemistry and land use for diatom richness was notable. Conclusions In addition to traditional environmental variables, geodiversity may affect species richness variation in streams, for example through changes in water chemistry. Geodiversity information could be used as a proxy for predicting stream species richness and offers a supplementary tool for conservation efforts.

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