Climate Change Amplifies Chronic Ammonia Risks in China's Freshwater Ecosystems

Ammonia toxicity poses a significant yet often overlooked risk to freshwater ecosystems. Conventional water quality standards rely on static concentration thresholds and fail to capture the dynamic nature of ammonia toxicity, which is strongly dependent on water temperature and pH. In this study, we combine high-frequency national water quality monitoring data with climate projections to evaluate both current and future chronic ammonia risks across China's freshwater systems. Our results reveal that existing criteria substantially underestimate ecological risks: approximately 71% of chronic exceedance events go undetected under most commonly used static thresholds. These risks exhibit marked spatial heterogeneity, with over 80% of northern river basins experiencing elevated chronic exposure, while southern basins face less than half the national average. Climate change is projected to intensify ammonia risks even in regions with historically strong pollution controls. Under the high-emission scenario, the national exceedance rate increases from around 8% to nearly 14% in summer, primarily driven by rising temperatures and increasingly extreme hydrological conditions. These findings demonstrate the critical limitations of fixed-threshold regulation and highlight the need for dynamic, climate-adaptive ammonia toxicity risk assessments. To effectively protect freshwater biodiversity and water quality in the face of climate change, regulatory frameworks must incorporate temperature- and pH-sensitive criteria alongside targeted, region-specific mitigation strategies.