Dam construction and nutrient enrichment led to simplification of riverine micro-food webs by weakening top-down...

Dam construction and nutrient enrichment are two of the most prevalent and detrimental disturbances confronting the riverine ecosystems, yet the response of riverine micro-food webs to these two disturbances remains largely unknown. Here, we collected 84 water samples from 28 sampling sites covering a 1893 km reach of the Jinsha River, one of the largest rivers in southwest China. The 28 sampling sites were divided into four regions based on the condition of dam intervention and nutrient enrichment. High-throughput sequencing analysis was performed to obtain the ASV information of multi-trophic microbial groups, encompassing bacteria, algae and fungi and protozoa. Then, we investigated the variation of alpha diversity, beta diversity and micro-food webs among the four regions. Different from prevailing co-occurrence network analysis, this study combined empirical predator-prey relationships with DNA data to construct the meta-food web that captures a more realistic representation of micro-food-web characteristics. The results indicated that dam construction and nutrients were pivotal drivers in shaping the distribution patterns of multi-trophic microbial groups. The complexity was considerably decreased (21.54 % on average) when high nutrient concentration or dam construction as two disturbances combined, yet reduced by 5.80 % on average when just one disturbance was present. The two disturbances led to the simplification of the micro-food webs by weakening the top-down control of basal resources by protozoa. When the two disturbances combined, they had a synergistic effect on simplifying the micro-food webs. This study investigated the response of a large river to changing environments from the perspective of micro-food webs, which is conducive to guiding the assessment of the health and ecological function of rivers subjected to multiple disturbances.