Restoration schemes often rely on the assumption that enhancing habitat complexity through addition of in‐stream structures such as boulders and woody debris leads to increased biodiversity, but evidence for this assumption is scarce. We compared structural heterogeneity and fish and invertebrate diversity at restored, unrestored, and reference sites on tributaries of the Ume River, northern Sweden, where several kilometers of streams have been restored from channelization through placement of boulders into the channel. Structural heterogeneity at the study sites was assessed using a contour tracer at two spatial resolutions likely to be affected by restoration. These are the patch scale (0.7 m), reflecting substratum characteristics, and the reach scale (50 m), reflecting general channel topography. Fish and invertebrate samples collected via electroshocking were used to assess taxonomic richness, taxonomic density, evenness, and assemblage composition at the study sites. Measures of structural heterogeneity were substantially higher at restored relative to channelized sites; however, components of fish and invertebrate diversity were similar between these treatments. At restored sites, measures of structural heterogeneity and fish and invertebrate diversity were consistent with, or slightly exceeded reference levels. This implies that local (patch to reach) heterogeneity did not structure fish and invertebrate assemblages in the study streams. Our results suggest that restoration might have little beneficial effect on biodiversity if the restoration schemes (and the original impact under amelioration) do not affect structural heterogeneity relevant to the target organisms.

Log floating in the 19th to mid 20th centuries has profoundly changed the environmental conditions in many northern river systems of the world. Regulation of flow by dams, straightening and narrowing of channels by various piers and wing dams, and homogenization of bed structure are some of the major impacts. As a result, the conditions for many riverine organisms have been altered. Removing physical constructions and returning boulders to the channels can potentially restore conditions for these organisms. Here we describe the history of log driving, review its impact on physical and biological conditions and processes, and predict the responses to restoration. Reviewing the literature on comparable restoration efforts and building upon this knowledge, using boreal Swedish rivers as an example, we address the last point. We hypothesize that restoration measures will make rivers wider and more sinuous, and provide rougher bottoms, thus improving land-water interactions and increasing the retention capacity of water, sediment, organic matter and nutrients. The geomorphic and hydraulic/hydrologic alterations are supposed to favor production, diversity, migration and reproduction of riparian and aquatic organisms. The response rates are likely to vary according to the types of processes and organisms. Some habitat components, such as beds of very large boulders and bedrock outcrops, and availability of sediment and large woody debris are believed to be extremely difficult to restore. Monitoring and evaluation at several scales are needed to test our predictions.

Despite long-standing research, the processes that drive community assembly remain poorly understood. We censused macroinvertebrate communities and measured flood disturbance in 17 Scandinavian mountain streams to assess the hypothesis that communities are shaped by stochastic processes under stable conditions, and increasingly by deterministic processes as disturbance becomes more severe. Each study stream was categorized as being stable (n 05), intermediate (n 07), or disturbed (n 05) depending on the severity of scouring floods. Following spring floods, the number of potential colonisers decreased with increasing disturbance, suggesting that disturbance filtered out species unable to cope with the stress involved. Communities at stable sites had the highest beta diversity, indicating that stochastic processes of community assembly were most important under the least disturbed conditions. In partial contrast with our predictions, the lowest beta diversity occurred between intermediate (not disturbed) sites, suggesting that increasing disturbance first enhances determinism but then rekindles stochasticity at severity levels beyond intermediate. Macroinvertebrate communities were shaped by deterministic processes, which recruit potential regional colonists depending on niche differences and disturbance conditions and by stochastic processes, which distribute the selected species randomly among individual localities. Although often considered opposing, stochastic and deterministic processes interact hierarchically, with relative strength modified by disturbance.

Summary 1.Increasing degradation of ecological conditions in streams because of human activities has prompted widespread restoration attempts; however, the ecological consequences of restoration remain poorly understood. We explored the effects of restoration through placement of boulders into the channel in the Ume River catchment in northern Sweden, where tributary streams were extensively channelized to facilitate the transport of timber in the 19th and early 20th centuries. Retentiveness and breakdown of coarse particulate organic matter (CPOM), two key ecological functions in low-order streams most likely to be affected by channelization, were compared between restored, channelized and unimpacted reference stream sites. 2. Artificial leaves were used to assess short-term CPOM retentiveness, while CPOM breakdown was estimated as the mass loss of alder ( Alnus spp.) leaf packs placed in coarse mesh litter bags. Also, the taxonomic richness, abundance, biomass and evenness of the leaf-eating invertebrates (shredders) on the retrieved leaf material were quantified. Detailed field measurements were carried out to identify geomorphological and hydraulic controls of CPOM retentiveness and breakdown at the study sites. 3. CPOM retentiveness reflected most strongly the density of boulders and submerged woody debris at the study sites. Restored sites were on average twice as retentive as channelized sites and significantly more retentive than reference sites when discharge was controlled. 4. Current velocity at bank-full flow was the single most important predictor of CPOM mass loss, implying that mechanical fragmentation was substantial during high flows; other apparent controls of CPOM breakdown included water temperature and shredder abundance. CPOM mass loss was similar between restored and reference sites. However, breakdown was slightly faster at most channelized sites, consistent with higher hydraulic stress during high flow conditions. The shredder assemblages that colonized the litter bags were similar in richness, abundance, biomass and evenness between treatments. 5. Synthesis and applications . In channelized forest streams, low retentiveness and fast mechanical fragmentation during high flows contribute to the rapid depletion of benthic CPOM following leaf abscission in autumn, thereby weakening the heterotrophic energy pathways that probably support much of the biological production in these systems. Our results illustrate that restoration by replacement of boulders can successfully reverse these impacts of channelization and thus contribute to the efficient ecological functioning of impacted streams.

SignificanceReliable prediction of future climate conditions requires a thorough understanding of climate variability throughout Earth’s history. Microbial molecular fossils, such as bacterial membrane-spanning tetraether lipids [branched glycerol dialkyl glycerol tetraethers (brGDGTs)], have proven to be particularly useful for the assessment of past climatic conditions, because they occur ubiquitously in the environment and show compositional changes related to temperature. However, the identity and ecology of brGDGT-producing bacteria is largely unknown, and a mechanistic basis for brGDGT-based paleoclimate reconstruction is still lacking. Here, we present insights into the ecological parameters that affect brGDGT synthesis in lakes, demonstrating that eutrophic lakes with oxygen-deprived bottom waters are the preferred sites for brGDGT-based reconstructions of continental climate.