Osama E. M. Rizk ^a， Jingwen Shi ^a, Ye Li ^a, Lin Zhang ^a, Kazi Belayet Hossain ^a, Hongwei Ke ^a, Minggang Cai ^a*
a  State Key Laboratory of Marine Environmental Science & College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102, China
*Corresponding author: Dr. Minggang Cai, Professor
E-mail: mgcai@xmu.edu.cn
Polycyclic aromatic hydrocarbons (PAHs) in estuarine systems are frequently described in terms of spatial and temporal distribution, yet the physical and biogeochemical processes that govern their fate often remain unresolved. Here we move beyond a descriptive characterization toward a mechanistic understanding of dissolved PAH dynamics along the Jiulong River Estuary (JRE)–Xiamen Bay (XMB) continuum, based on seasonal sampling of nine stations during 2022. Seven dissolved PAHs were quantified at the surface and bottom layers across four seasons. Seasonal mean PAH concentrations followed the order winter ≈ , autumn > spring ≫ summer, with concentrations in summer roughly one-eighth of the concentrations in the cold seasons. PAH concentrations were strongly and negatively correlated with water temperature (r = −0.71), whereas the correlation with salinity was negligible (r ≈ 0.03), indicating that conservative mixing between freshwater and marine end-members was not the primary control on dissolved PAH concentrations. A conservative mixing-line analysis confirmed that observed concentrations deviated systematically from theoretical dilution, while a seasonal spatial reversal, with JRE-dominant loadings in autumn shifting to XMB-dominant loadings in winter, further pointed to process-driven rather than dilution-driven behavior. Together, these results indicate that temperature-driven loss processes, namely volatilization and microbial biodegradation, constitute the dominant control on dissolved PAH fate along the continuum, producing the pronounced summer minimum. This study reframes dissolved PAH behavior in a subtropical estuary from a descriptive pattern to a mechanistic, temperature-mediated process, offering a basis for predicting PAH fate under changing thermal regimes.
Keywords
Polycyclic aromatic hydrocarbons (PAHs); Dissolved phase; Jiulong River Estuary; Xiamen Bay; Temperature-driven loss; Mechanistic understanding
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (U25A20790 and 42576044 ), the Industry-University-Research Joint Innovation Project of Fujian Provincial Department of Education (2025Y4002), and the Guangxi Key Research and Development Program (AB25069113). Thanks for the support by CEES Open Cruise for Jiulong River Estuary–Xiamen Bay, and the data service provided by the Data Center of National Observation and Research Station for the Taiwan Strait Marine Ecosystem (T-SMART), and thanks are given to Shuiying Huang and Shaowei Zheng for their invaluable assistance with data compiling. We thank Ms. Jun Ye, Xuehong Zheng, and Chunhui Wang for their help in lab management.

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