Coupled acidification-nitrification dynamics in eutrophic estuarine waters

The interplay between acidification and nitrification in estuarine systems could have profound effects on coastal biogeochemistry and ecosystem health. However, the lack of integrated field research risks oversimplifying their relationships in complex ecosystem dynamics. This study investigates the spatiotemporal covariations of acidification sensitivity and nitrification rates derived from observed inorganic carbon and nutrients data along a land-sea continuum. In the middle estuary, estuarine pH exhibited the highest sensitivity to ammonium concentration, coinciding with maximum nitrification rates. The coupling effect intensified by 40 % during the transition from dry to wet hydrological conditions. Despite that microbial network complexity generally decreased with increased acidification sensitivity, ammonia-oxidizing bacterial communities are more sensitive to acidification in estuarine water compared to ammonia-oxidizing archaea. Conversely, in the lower estuary, acidification was associated with a decline in nitrification activities. Machine learning-based models suggest that climate change scenarios could exacerbate acidification and nitrification in the Pearl River Estuary, potentially amplifying their coupling effect in the middle estuary. This holistic approach not only advances our fundamental understanding of estuarine processes, also provides critical insights for policymakers and coastal managers striving to maintain the ecological integrity of these vital ecosystems in an era of rapid global change.