Estuarine fine particles enhance nitrogen elevation and nutritional imbalance in coastal and offshore waters

• Gravity-settling effectively classifies SPM by particle size. • Fine-grained SPM and low salinity levels tend to accumulate N. • Fine-grained SPM tends to suspend in the water column and migrate with the river. • Migration of fine-grained SPM increases the risk of N pollution load and N/P ratio imbalance. Nitrogen (N) in suspended particulate matter (SPM) plays a vital role in estuarine and coastal ecosystems. However, the accumulation and migration processes of N in SPM and their effects on water quality evolution are still unclear. This study utilized three methods (gravity-setting, dry sieving, and wet sieving) to categorize the particle size of SPM in the Yellow River Estuary (YRE). Simulated tests and the field sampling of SPM in the YRE were conducted to examine N accumulation and migration patterns in SPM across different salinity levels, particle sizes and migration distance. Adsorption experiment was used to investigate the potential impact of N in SPM on water quality evolution. The results indicated that SPM could be effectively sorted into distinct particle size groups through gravity-setting, and fine-grained SPM contributed to accumulating N under low salinity. The SPM with the largest particle size at a salinity of 33 PSU presented the lowest TN concentration of 818.4 mg·kg −1 . In comparison, the SPM with the smallest D50 at a salinity of 0.48 PSU exhibited the highest TN (2438 mg·kg −1 ). Microbial community analysis underscores the microbial-mediated mechanisms driving N dynamics in SPM. Cyanobacteria play a crucial role in N accumulation in fine-grained SPM, while salinity and N concentration significantly influenced the distribution of key bacterial phyla (e.g. Proteobacteria and Planctomycetota ). Additionally, fine-grained SPM tends to suspend in the water column and are carried by river flow towards offshore, the sampling point located furthest from the YR mouth (N8) recorded the highest TN concentration of 1973 mg·kg −1 in SPM. During the migration process, the release potential of N in SPM has significantly increased, thereby exacerbating the risk of N pollution and N/P ratio imbalance in seawater. This study presents a novel perspective on addressing N pollution and N/P ratio imbalance, highlighting the significance of intercepting fine particles for effective N management in estuarine and coastal zones.