Organonitrogen pesticides in Beibu gulf seawater: spatiotemporal distribution, key influencing factors, sources, and ecological risks.
Organonitrogen pesticides (ONPs) are widely used in agriculture and can enter the ocean via surface runoff, posing potential threats to marine ecosystems. This study systematically investigated the occurrence and spatiotemporal distribution of 49 ONPs in seawater of the Beibu gulf, China, examined their key influencing factors, and conducted source apportionment and ecological risk assessment using multiple statistical models. The results showed that target ONPs were ubiquitously detected, with total concentrations (Σ49ONPs) ranging from 14.3 to 38.0 ng/L (mean: 20.2 ng/L). Spatially, higher concentrations were observed in the northern and central gulf, and lower in the southern area. Seasonally, the Σ49ONPs in summer 2022 (10.7-37.2 ng/L) were significantly higher than in winter (6.36-14.2 ng/L, p < 0.01), but comparable to summer 2021 (14.3-38.0 ng/L), with this seasonal pattern primarily linked to variations in riverine discharge. Correlation and random forest analyses identified terrestrial inputs and their interactions with suspended particulate matter (SPM) as critical drivers of ONP distribution and fate. Seawater Si:P ratios and Chl a concentrations serve as effective indicators of ONPs levels, while SPM acts as both a carrier and a sink. These findings underscore the necessity of integrating runoff management with pesticide application controls in coastal pollution mitigation. Multi-model source apportionment indicated that ONPs mainly originated from local agricultural applications and rubber industrial processing activities, with additional modulation by ocean currents and water exchange rates. Ecological risk assessment revealed that total ONPs concentrations posed moderate-to-high risks to marine organisms across trophic levels at most sampling sites, deserving particular attention.