Decoupling of microbial diversity and ecosystem function driven by metal ions and nutrient shifts across dry-wet seasons in a temperate river basin

Microbial communities are key drivers of biogeochemical cycles and water-quality regulation in freshwater ecosystems. However, how these communities respond to climatic and environmental changes at the watershed scale across dry and wet seasons remains poorly understood. Using metagenomic analysis, this study investigated the spatiotemporal variations in bacterial communities and ecosystem multifunctionality (EMF) in a temperate river basin across five typical scenarios during dry and wet seasons. We found that bacterial α-diversity was significantly higher in the wet season. In contrast, the abundances of functional genes related to carbon and nitrogen cycles, along with bacterial community multifunctionality and EMF, were significantly higher in the dry season. Our results revealed a decoupling between bacterial diversity and EMF across seasons. We found that metal ions (e.g., Sb, Ni) were positively associated with α-diversity, while they were negatively correlated with functional gene abundances and EMF. Those results indicated that the decoupling between bacterial diversity and EMF was primarily driven by metal ions, which enhanced α-diversity by selecting for metal-resistant taxa but concurrently suppressed functional gene expression due to enzymatic toxicity. Our results highlighted that watershed management should adopt seasonally adaptive strategies: controlling point-source organic pollutants in the dry season and mitigating metal-rich non-point source runoff in the wet season. • Metal ions boost wet-season diversity but cut ecosystem function, causing decoupling • Ecosystem multifunctionality peaks in dry season, against microbial diversity trends • Chronic industrial pollution forms adapted microbiomes for sustained high function • Management must be seasonal: control organics in dry season, metals in wet season