Variation factor of polycyclic aromatic hydrocarbons in surface seawater of the Sea of Japan during 2017–2022

Polycyclic aromatic hydrocarbons (PAHs), originating mainly from the incomplete combustion of fossil fuels and biomass and the leakage of petroleum products, are hazardous pollutants with adverse carcinogenic and mutagenic potential for marine organisms. The Yangtze River estuarine–inner-shelf area in the East China Sea is one of the largest reservoirs of PAHs and connected with the semi-enclosed Sea of Japan through the shallow, narrow Tsushima Strait. The main sources of PAHs in the Sea of Japan are the East Asian landmass and East China Sea, with the major PAH transport pathways being atmospheric transport of the westerly and northwesterly winter monsoon and marine transport in the Tsushima Warm Current. Investigation of the environmental behavior of PAHs in the Sea of Japan is critical in assessing the ecological risks of PAHs, as the sea is among the most fertile fishing grounds, however, the contributions of surface PAHs transported via atmosphere and ocean current are unclear. This study aimed to elucidate variation factor of dissolved PAHs in the surface Sea of Japan during 2017–2022, and analyzed 1) surface PAH distribution, 2) PAH source, 3) air-sea PAH exchange flux, and 4) ocean current driven PAHs flux through the Tsushima Strait. Surface seawater samples of 10–15 L were collected at 148 sites in the Sea of Japan, and its adjacent seas including the Tsushima Strait and Kuroshio Current area (Sesoko, Okinawa Island) from 2017 to 2022. Particulate and dissolved phases were separated by filtration through 0.5 µm glass fiber filter (GFF), and the latter was concentrated in C18 disks. Particulate and dissolved PAHs were respectively extracted from the GFF and C18 disks with dichloromethane and measured by HPLC system with fluorescence detector. Total concentrations of the 14 dissolved PAHs (Σ₁₄PAH_diss) in the central Sea of Japan were in range 0.26–5.87 ng L⁻¹, with the mean of 3.26±0.36 ng L⁻¹ in 2017, 3.70±1.44 ng L⁻¹ in 2019, 0.51±0.10 ng L⁻¹ in 2020, 1.29±0.53 ng L⁻¹ in 2021, and 0.69±0.35 ng L⁻¹ in 2022. In 2020, although atmospheric PAH deposition flux were increased to 4.0 ton month⁻¹ from 2019 (3.4 ton month ⁻¹) at the Sea of Japan, mean Σ₁₄PAH_diss in the surface seawater were parallelly decreased to 0.51 ng L⁻¹ at the central Sea of Japan, 0.86 ng L⁻¹ at the Tsushima Strait, and 0.46 ng L⁻¹ at the Kuroshio Current area. The contribution of PAH-rich and low salinity continental shelf water from the East China Sea in the Tsushima Strait was decreased to 36% in 2020 from 58% in 2019 on the basis of salinity mass-balance and ocean current simulation. Moreover, PAHs inflow flux through the Tsushima Strait were in range 2.2–8.8 ton month⁻¹, and decreased to 2.2 ton month⁻¹ in 2020. Thus, the decrease in surface Σ₁₄PAH_diss levels at the central Sea of Japan in 2020 was mainly caused by the decrease in PAHs flux via the Tsushima Warm Current, which flow into the Sea of Japan from the East China Sea.