PS 37-22 - Does water flow in a large, high-discharge river homogenize microbial diversity and functions in the upper sediment layer?

Abstract: Rivers are complex and extremely dynamic environments. In rivers with intermittent high-discharge events, sediment can be subjected to significant disturbance due to displacement and deposition. Massive sediment turnover could reset ecological succession and create biotic homogenization. Moreover, frequent, and intense storm disturbances can have widespread and strong effects on the biogeochemical cycles and their associated microbial communities. We tested whether high-discharge events alter species dominance and homogenize microbial diversity in sediment of a large, industrialized river. The Kanawha River watershed encompasses more than 31,000 km² of Appalachian Mountains. Yearly flow averages 470 m³/sec, but high-flow events can exceed 3000 m³/sec. Replicate sediment cores from each location were collected using an 8 cm diameter corer. Sediment microbiomes were compared between periods that represent average flow levels and high-discharge events. The same river locations were compared for 2016 and 2018. Community structure and functional diversity were assessed with Illumina sequencing of 16S rRNA following EMP protocols and metagenome sequencing. QIIME2 and SILVA 138 data base were used to analyze 16S rRNA sequences while analysis of functional gene diversity was done using KEGG orthology groups, and STAMP. Network analysis was done using Spearman correlations implemented in R. Chemical variables were measured with ICP-OES and Dionex Ion-Chromatography. Statistical analysis included CCA, NMDS, and Pearson’s correlation to evaluate effects of chemical parameters on microbial communities in the two periods. UniFrac with multivariate regression tree analysis was used to compare diversity between years. UniFrac results showed that microbial diversity from replicate sediment samples during 2016 showed much less dispersion than replicate samples from 2018. Two locations (50 km apart) were also differentiated by dominant species during average flow. However, following successive high-discharge events, communities were homogenized and different dominants emerged. KEGG level 3 and functions analysis revealed that there was also a functional homogenization between locations in 2018. Network analysis showed a large reduction (80-90%) of both positive and negative associations following high-discharge events. PCA showed that sediment geochemistry displayed spatial organization with greater similarity of chemical profiles among replicate samples from each location. In conclusion, high-discharge events homogenized the upper sediment microbiome of a large river, while lower water flow was associated with a reduction in beta diversity among samples from individual locations. Periods of lower flow allowed ecological succession to create spatially explicit patterns representing local habitat variation. Funding by USDA-NIFA-2021-38821-34706.