Contrasting community assembly processes structure lotic bacteria metacommunities along the river continuum.

The heterogeneous nature of lotic habitats plays an important role in the complex ecological and evolutionary processes that structure the microbial communities within them. Due to such complexity, our understanding of lotic microbial ecology still lacks conceptual frameworks for the ecological processes that shape these communities. We explored how bacterial community composition and underlying ecological assembly processes differ between lotic habitats by examining community composition and inferring community assembly processes across four major habitat types (free-living, particle-associated, biofilm on benthic stones and rocks, and sediment). This was conducted at 12 river sites from headwater streams to the main river in the River Thames, UK. Our results indicate that there are distinct differences in the bacterial communities between four major habitat types, with contrasting ecological processes shaping their community assembly processes. While the mobile free-living and particle-associated communities were consistently less diverse than the fixed sediment and biofilm communities, the latter two communities displayed higher homogeneity across the sampling sites. This indicates that the relative influence of deterministic environmental filtering is elevated in sediment and biofilm communities compared with free-living and particle-associated communities, where stochastic processes play a larger role.

Effect of climate change on environmental flow indicators in the narew basin, poland.

Environmental flows-the quantity of water required to maintain a river ecosystem in its desired state-are of particular importance in areas of high natural value. Water-dependent ecosystems are exposed to the risk of climate change through altered precipitation and evaporation. Rivers in the Narew basin in northeastern Poland are known for their valuable river and wetland ecosystems, many of them in pristine or near-pristine condition. The objective of this study was to assess changes in the environmental flow regime of the Narew river system, caused by climate change, as simulated by hydrological models with different degrees of physical characterization and spatial aggregation. Two models were assessed: the river basin scale model Soil and Water Assessment Tool (SWAT) and the continental model of water availability and use WaterGAP. Future climate change scenarios were provided by two general circulation models coupled with the A2 emission scenario: IPSL-CM4 and MIROC3.2. To assess the impact of climate change on environmental flows, a method based conceptually on the "range of variability" approach was used. The results indicate that the environmental flow regime in the Narew basin is subject to climate change risk, whose magnitude and spatial variability varies with climate model and hydrological modeling scale. Most of the analyzed sites experienced moderate impacts for the Generic Environmental Flow Indicator (GEFI), the Floodplain Inundation Indicator, and the River Habitat Availability Indicator. The consistency between SWAT and WaterGAP for GEFI was medium: in 55 to 66% of analyzed sites, the models suggested the same level of impact. Hence, we suggest that state-of-the-art, high-resolution, global- or continental-scale models, such as WaterGAP, could be useful tools for water management decision-makers and wetland conservation practitioners, whereas models such as SWAT should serve as a complementary tool for more specific, smaller-scale, local assessments.