Coupled effects of landscape structures and water chemistry on bacterioplankton communities at multi-spatial scales

Riverine bacterial communities are more shaped by species sorting in intensive urban and agricultural watersheds

Bacterial communities play a crucial role in maintaining the stability of river ecosystems and driving biogeochemical cycling, exhibiting high sensitivity to environmental change. However, understanding the spatial scale effects and assembly mechanisms of riverine bacterial communities under distinct anthropogenic disturbances remains a challenge. Here, we investigated bacterial communities across three distinct watersheds [i.e., intensive urban (UW), intensive agricultural (AW), and natural (NW)] in both dry and wet seasons. We explored biogeographic patterns of bacterial communities and the influence of landscape patterns at multi-spatial scales and water chemistry on bacterial communities. Results showed that α diversity was significantly lower in UW and AW compared to NW, particularly in the dry season. A gradient of β diversity with NW > UW > AW was observed across both seasons (p < 0.05). Pseudomonadota, Bacteroidota, and Actinobacteriota were the most abundant phyla across all watersheds, with specific taxa enriched in each watershed (i.e., the class Actinobacteria was significant enrichment in UW and AW, and Clostridia in NW). The influence of landscape patterns on bacterial communities was significantly lower in human-disturbed watersheds, particularly in UW, where this influence also varied slightly from near riparian buffers to sub-watershed. Homogeneous selection and drift jointly dominated the bacterial community assembly across all watersheds, with homogeneous selection exhibiting a greater influence in UW and AW. Landscape patterns explained less variance in bacterial communities in UW and AW than in NW, and more variance was explained by water chemistry (particularly in UW). These suggest that the stronger influence of species sorting in UW and AW was driven by more allochthonous inputs of water chemistry (greater environmental stress). These findings provide a theoretical foundation for a deeper understanding of riverine bacterial community structure, spatial scale effects, and ecological management under different anthropogenic activities.

Environmental DNA metabarcoding reveals the impact of different land use on multitrophic biodiversity in riverine systems

Human-induced changes in land use drive an alarming decline in river biodiversity and related ecosystem services worldwide. However, how different land use shapes aquatic multitrophic communities is still not well understood. Here, we used the biodiversity dataset from bacteria to fish captured by the environmental DNA (eDNA) approach in the four riverine systems with spatially different land use (i.e., Slightly disturbed group, Upstream disturbed group, Downstream disturbed group, and Strongly disturbed group) to reveal the changes in multitrophic biodiversity in relation to human land use. Firstly, our data showed that spatially different land use determined the pollutant loads of the riverine systems, most pollutants (e.g., TN and NH₃-N) had significant differences among the four riverine systems. Secondly, taxonomic α diversity across multitrophic levels did not necessarily change significantly, yet the change in community structure can be considered as a more sensitive indicator to reflect different land use, because different land use shaped the unique structure of multitrophic communities, and the dissimilarity of community structure was closely associated with land use gradient (e.g., positive relationships in the Slightly disturbed group, negative relationships in the Strongly disturbed group). Thirdly, different land use induced the shifts of key taxa, resulting in the variation of community structure and the change of co-occurrence network. Overall, these findings suggest that spatially different land use plays a critical role in shaping aquatic multitrophic communities, and an in-depth understanding of the interdependences between biodiversity and land use is a critical prerequisite for formulating river management strategies.

Sources and migration similarly determine nitrate concentrations: Integrating isotopic, landscape, and biological approaches

Rapid land use change has significantly increased nitrate (NO₃^-) loading to rivers, leading to eutrophication, and posing water security problems. Determining the sources of NO₃^- to waters and the underlying influential factors is critical for effectively reducing pollution and better managing water resources. Here, we identified the sources and influencing mechanisms of NO₃^- in a mixed land-use watershed by integrating stable isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), molecular biology, water chemistry, and landscape metrics measurements. Weak transformation processes of NO₃^- were identified in the river, as evinced by water chemistry, isotopes, species compositions, and predicted microbial genes related to nitrogen metabolism. NO₃^- concentrations were primarily influenced by exogenous inputs (i.e., from soil nitrogen (NS), nitrogen fertilizer (NF), and manure & sewage (MS)). The proportions of NO₃^- sources seasonally varied. In the wet season, the source contributions followed the order of NS (38.6 %) > NF (31.4 %) > atmospheric deposition (ND, 16.2 %) > MS (13.8 %). In the dry season, the contributions were in the order of MS (39.2 %) > NS (29.2 %) > NF (29 %) > ND (2.6 %). Farmland and construction land were the original factors influencing the spatial distribution of NO₃^- in the wet and dry seasons, respectively, while slope, basin relief (HD), hypsometric integral (HI), and COHESION, HD were the primary indicators associated with NO₃^- transport in the wet and dry seasons, respectively. Additionally, spatial scale differences were observed for the effects of landscape structure on NO₃^- concentrations, with the greatest effect at the 1000-m buffer zone scale in the wet season and at the sub-basin scale in the dry season. This study overcomes the limitation of isotopes in identifying nitrate sources by combining multiple approaches and provides new research perspectives for the determination of nitrate sources and migration in other watersheds.

Effects of land-use patterns on the biogeography of the sediment bacteria in the Yarlung Tsangpo River

Bacteria communities, as key drivers of energy flow and nutrient recycling in rivers, usually consist of a few abundant taxa and many rare taxa. During the last decades, rivers on the Tibetan Plateau have experienced dramatic land surface changes under climate change and anthropogenic disturbances. However, the responses of abundant and rare taxa to such changes and disturbances still remains unclear. In this study, we explored the biogeography and drivers of the abundant and rare bacteria in Yarlung Tsangpo River sediments on the Tibetan Plateau. Our study demonstrated that changes in surrounding land-use patterns, especially in forest land, bare land and cropland, had profound influences on the distribution of the abundant and rare sediment bacteria in the Yarlung Tsangpo River. Although both communities exhibited significant distance-decay patterns, dispersal limitation was the dominant process in the abundant community, while the rare community was mainly driven by heterogeneous selection. Our results also revealed that the abundant bacteria exhibited stronger adaptation across environmental gradients than the rare bacteria. The similar biogeographic patterns but contrasting assembly processes in abundant and rare communities may result from the differences in their environmental adaptation processes. This work provides valuable insights into the importance of land surface changes in influencing the biogeographic patterns of bacteria in fluvial sediments, which helps to predict their activities and patterns in Tibetan rivers under future global climate change and anthropogenic disturbances.