Seasonal difference in the behavior of polycyclic aromatic hydrocarbons in
a large river-dominated estuary: Significance of winter sediment resuspension

Polycyclic aromatic hydrocarbons (PAHs), a significant class of persistent organic pollutants, are particularly concerning due to their carcinogenic, mutagenic, and degradation-resistant qualities (Agency, 1979; Simonich and Hites, 1994; Sverdrup et al., 2002; Yunker et al., 1993). Anthropogenic sources, including burning of biomass and fossil fuels as well as leaking crude oil, are the primary sources of PAHs (Pettersen et al., 1997). PAHs may enter the environment by a variety of routes, including riverine discharge (McCready et al., 2000), long-distance atmospheric transport (Yu et al., 2015), and subsequent dry or wet deposition (McVeety and Hites, 1988). PAHs are widely found in water, sediments, atmosphere, and other organisms worldwide (Barakat et al., 2011; Olson et al., 2016). Because of their high octanol/water partition coefficient (KOW) and low solubility, PAHs would preferen- tially accumulate in sediments and organisms once they were introduced into the aquatic environment (Tam et al., 2001). PAHs in sediments may desorb into water and enter the food chain, causing a risk to organisms through zoobenthos' consumption (Ruiz et al., 2011; Wang et al., 2014). Therefore, the accumulation and distribution of PAHs in sediments have important implications for understanding their source-to-sink processes and the ecological risk in the environments.
Estuaries, which are transitional zones between land and ocean systems, are influenced by both terrestrial and marine factors, such as winds, waves, tides, variations in river flow, and dry-wet cycles (Wang et al., 2014). As they serve as filters that capture both natural and anthropogenic materials transported from the rivers to the marginal seas, estuaries are crucial for the transport and fate of PAHs. Accord- ingly, the estuary sediments, which contain substantial organic matter pools, may serve as an important PAH repository (Hui et al., 2009; Pereira et al., 1999; Wang et al., 2015).

The Yangtze River (YR), which originates on the Qinghai-Tibetan Plateau, covers a terrestrial area of 1.8^106 km2 (approximately 20 % of the terrestrial area of China). It is the longest (6300 km) river in Asia, and it ranks 5th globally in terms of water discharge (900 km3 yr − 1) and 4th in terms of sediment load (470 Mt. yr − 1) before the operation of the Three Gorge Dam (TGD) in 2003 (Chen et al., 2001; Milliman and Syvitski, 1992; Stone, 2008; Yu et al., 2011). The YR experiences sea- sonal variations in its annually water and sediment discharge due to the influence of the East Asian Monsoon. The discharge of the YR comes mainly from rainfall. In months of the prevalence of East Asian Summer Monsoon, the Yangtze River basin receives most of its precipitation. The flood season (summer) from May to October accounts for about 70 % to 90 % of the annual precipitation, generating an intense erosional flux (Ju et al., 2014). About 70 % and 87 % of the annual water discharge and sediment flux of YR are supplied to the Yangtze River Estuary (YRE) during the flood season (Sun et al., 2022). The intertidal marshes and the river plume area of the estuary contain half of these sediments (Li et al., 2012b). Most of the PAHs released from upper and middle reaches of YR are transported to the YRE through runoff and accumulate in the estu- arine sediments. Therefore, determining the concentration levels of PAHs in sediments of YRE is vital for risk assessment and evaluation of ecosystem health.

The YRE is an energetic and highly turbid system under strong fluvial and marine forcing, including tides, waves and coastal currents (Zhu et al., 2021). These processes can modify the water-sediment interface environment in estuarine/deltaic zones, thus influencing the fate of PAHs in sediments on continental margins. The majority of sediments derived from theYR accumulated quickly in theYRE during summer due to the block of the strong Taiwan Warm Current (TWWC). In winter, under the influence of Yellow Sea Coastal Current (YSCC) and Zhejiang- Fujian Coastal Current (ZFCC), sediments previously deposited in estu- aries in summer preferentially resuspend and move southward along the shelf, and are constrained to the inner shelf of East China Sea (ECS) (Hu et al., 2012; Liu et al., 2007). Thus, PAHs previously stored in sediments in summer may experience desorption, degradation and/or trans- portation under the influence of sediment resuspension in winter. These significant seasonal variations in hydrodynamic conditions make the YRE exhibit regularly varying depositional environments, however, it remains unclear how these variations, in particular the widespread sediment resuspension in winter, influence the fate of PAHs in the es- tuary system.
According to measurements recorded at the Datong gauging station, during July 2019 (summer), the YRE experienced a water discharge of ~155.9^109 m3, accounting for 16.7 % of the year, and a sediment flux of ~27.6^106 tons, accounting for 26.3 % of the year (CWRC, 2019). The water and sediment discharge were ~ 56.7^109 m3 (6.20 % of the year) and ~ 6.98^106 tons (6.01 % of the year) during March 2015 (winter) (CWRC, 2015). These two data series provided us with excellent infor- mation to explore the impact of the Yangtze input and East Asian Monsoon on the source-to-sink processes of PAHs in the YRE. Most of the current research focuses on the occurrence and source of PAHs in the YRE and its adjacent ECS. However, the seasonal difference in the in- fluence on the behavior of PAHs in the YRE remains poorly understood. To further explore seasonal differences in the behavior of PAHs, this study aims to explore the distribution pattern of sedimentary PAHs in the YRE in the two typical seasons and the influence of hydrodynamic conditions on the behavior of PAHs. This comprehensive research can provide new insights into how sediment resuspension-deposition loops influence the transport, burial, and preservation of sedimentary PAHs within complex large river-dominated estuarine systems. This could provide important insights for understanding the source-sink processes of PAHs under the hydrodynamic influence in worldwide river-estuary- shelf systems and global biogeochemical cycling processes of PAHs.