Increased suspended particulate matter differentially stimulates ammonium consumption in a eutrophic estuary

Riverine inputs of ammonium (NH₄⁺) and suspended particulate matter (SPM) from human activities have increased significantly over the past few decades, greatly affecting coastal nitrogen cycling and water quality. However, it remains poorly understood how NH₄⁺ consumption in eutrophic estuaries responds to increasing SPM levels. Using ¹⁵N-labeling techniques, we quantified ammonia oxidation (AO) and ammonium uptake (AU) during SPM addition experiments across the salinity gradient of the Jiulong River Estuary, a typically human-impacted, eutrophic, and turbid estuary in China. The results showed that adding in situ SPM stimulated AO at the freshwater end (salinity 1.1), but had minimal impact on AO or AU in regions with intermediate salinity. At the seawater end (salinity 28.0), in situ SPM increased both AO and AU, especially enhancing AU. Furthermore, adding riverine SPM collected from freshwater, mainly of terrestrial origin, to seawater caused weaker stimulation of NH₄⁺ consumption, particularly in AU. These varied responses suggest that NH₄⁺ consumption pathways are controlled by substrate availability, particle source, and the composition of ammonia-utilizing microbial communities in the estuary. This study provides new mechanistic insights into how particles influence NH₄⁺ dynamics in eutrophic estuaries, with important implications for nitrogen cycling and water quality under increasing human impacts.