Shifts in the Distribution and Fate of Sedimentary Polycyclic Aromatic Hydrocarbons Within the Estuarine‐Inner Shelf Areas of the East China Sea: Response to Human‐Induced Catchment Change

Abstract

Human-induced changes in river catchments, such as damming, can affect estuarine depositional settings and further affect the transport of sediments and associated pollutants. Polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the East China Sea (ECS) at two time nodes (2006 and 2018) were compared to understand the response of PAHs to human-induced catchment changes. PAH concentrations in the ECS ranged from 8-414 ng g −1 (dry weight), with a mean value of 112 ± 77 ng g −1 , relatively low than that in 2006 (38-308 ng g −1 , with a mean of 122 ± 60 ng g −1 ). Sharp decreases in sediment loads have triggered erosion in subaqueous delta and changed the distribution of sediment components, which may eventually influence the distribution pattern of PAHs. The obvious spatial differentiation of PAHs between 2006 and 2018 suggested that the depositional center of PAHs shifted from the estuary to the inner shelf area. PAH deposition patterns in the ECS were primarily influenced by riverine input before 2006 but are now dominated by winnowing processes related to long-distance transport due to sharply decreased sediment loads. Dam construction in the river catchment intercepted large amounts of sediments and PAHs, shifting the Changjiang-derived PAH depositional center from the ocean to reservoirs. Overall, depositional patterns of PAHs in the ECS were largely altered by human-induced catchment changes, which may cause significant impacts on the region's biogeochemical cycles and ecosystem health. Plain Language SummaryCoastal oceans are usually considered important sinks of terrestrial materials, especially river-dominated coastal margins. However, river-derived terrestrial materials have decreased drastically in the last several decades worldwide, due to intensified human activities that have changed the local sedimentary environment. In this study, changes in the surface sediment grain size within the East China Sea from 2006 to 2018 indicated that the sedimentary environment in this region was clearly altered. Similarly, the distributions and compositions of PAHs in surface sediments changed from 2006 and 2018. Based on combined analysis of grain size and PAHs, we found that PAH dynamics in this region were primarily influenced by riverine inputs before dam construction in the Changjiang (Yangtze) River basin but are now dominated by winnowing processes related to long-distance transport due to sharply decreased sediment loads. Therefore, intensified human-induced catchment changes have significant influenced the fate of PAHs in the East China Sea.

Reservoir Construction Has Reduced Organic Carbon Deposition in the East China Sea by Half Since 2006

“…While the variations in the sedimentary OC can be partly attributed to the reduction in the terrestrial POC input, the hydrodynamic process is also considered a factor affecting the fate of sedimentary OC in marginal seas (Bao et al, 2018, 2019; Bianchi et al, 2018; Bröder et al, 2016, 2018). We have confirmed that the sedimentary environment in the study area changed due to the sharp reduction in the sediment load (Wang, Hao, Feng, Zhang, Li, et al, 2020), which would inevitably affect the dispersal and burial of POC. The sediment flux discharged from the Changjiang River to the ECS dropped from 340 Mt yr −1 in 1986–2002 to 145 Mt yr −1 after the TGR impoundment, which is insufficient to maintain the geomorphic stability of the CSDMD (Yang et al, 2018).…”

“…Therefore, the seabed sediment in the CSDMD was eroded and transported southward by coastal currents. Long periods of insufficient sediment supply have caused erosion to spread further into the ZMCMB (Gao et al, 2019; Wang, Hao, Feng, Zhang, Li, et al, 2020). Large amounts of fine‐grained particles in the seabed sediment have been washed away, which has significantly changed the sediment grain sizes and components in the study area.…”

“…The Changjiang River annually discharges ~478 Mt of sediment into the ECS (Milliman & Farnsworth, 2011) and forms the Changjiang subaqueous delta mud deposits (CSDMD) and the Zhe‐Min coastal mud belt (ZMCMB) (Figure 1; Gao & Collins, 2014; Liu et al, 2007). Human activities, such as in the case of the construction of the Three Gorges Reservoir (TGR, construction of the dam began in 1994 and was not completed until 2006, impoundment began in 2003), significantly impact the transport of terrestrial materials, which further changes the sedimentary environment and biogeochemical cycles in the ECS (Gao et al, 2019; Wang, Hao, Feng, Zhang, Li, et al, 2020; Wang, Hao, Feng, Zhang, Gao, et al, 2020). In addition, the POC flux decreased from 10.9 ± 2.6 Mt yr −1 in the period 1952–1986 to 1.9 ± 1.0 Mt yr −1 in 2003–2008 (Li et al, 2015).…”

Chlorinated Paraffin Pollution in the Marine Environment

“…In the marine environment, the water transport ability of CPs mainly depends on their discharge, deposition, transfer, mobility, and persistence. ,,, A large amount of CPs in the water body was input into coastal waters, resulting in higher levels of CPs than in offshore waters. ,− For example, high concentrations of SCCPs and MCCPs were detected in the Pearl River Delta, China, with river runoff as the primary source . After being discharged into the ocean, high concentrations of CPs in the coastal ocean can be subject to long-range transport and were then potentially brought into the offshore waters via the carrier of the ocean and tidal currents. ,,,− On the other hand, CPs are readily adsorbed by sediments due to low water solubility and high organic carbon partition coefficient. , The transport ability of CPs by ocean currents varies in different CP congeners due to the diversity of persistence, volatility, and solubility.…”