Watershed-scale pollution pattern shifts affecting the structure, function, and state of riverine microbiota: The...

Legacy effects, defined as the persistent impacts of historical disturbances, are critical for revealing how river ecosystems respond to catchment-scale nitrogen pollution shifts. Given the inconsistent nitrogen stressor-response relationships within river microbiota, this paper integrated a hydrological model with microbial analyses to evaluate the legacy effects of nitrogen pollution pattern changes on riverine microbiomes. Compared with point sources, non-point sources generated stronger material legacies (legacy nitrogen contributed up to 65.84 % to current river nitrogen inputs) and stronger information legacies (species life-history traits contributed up to 73.93 % to current microbial composition). We also documented a positive relation between these two types of legacies. Besides, microbial taxonomic and functional divergence between different pollution sources was amplified when pollution regimes remained consistent across seasons. Specifically, streams with a longstanding history of agricultural dominance exhibited the highest microbial taxonomic diversity but the weakest nitrification, dissimilatory nitrate reduction to ammonium, and denitrification capacities. Conversely, reaches with a long-term history of point-source dominance showed the lowest α-diversity but the strongest nitrogen-cycling potential. Further, an alteration of microbial stable state, which refers to a sudden and obvious shift in microbial ecosystems, was more likely to occur in rivers with contrasting seasonal pollution patterns. The highest probability of state shifts (41.7 %) occurred where pollution patterns shifted from agricultural contamination (wet season) to rural domestic wastewater (dry season). Our findings highlight the need to incorporate legacy effects from pollution source changes into river remediation strategies to improve predictions and management of real-time ecosystem responses to environmental stressors.