Seawater organic pollution across three European Seas: insights from extra-large volume sampling and target analysis to inform policy prioritisation

Marine pollution by chemical contaminants is a major environmental concern in European waters. So far, most investigations have focused on pollution in specific marine regions, at varied temporal scales and using different analytical methods. This hinders comparability of marine pollution data across European regional seas and the identification of substances of concern at the European level. Furthermore, there is only limited information on contaminant concentrations in seawater, particularly in the mobile phase transporting the contaminants and indicating direct exposure. Here we provide the first pan-European survey on seawater concentrations of compounds of proved or potential concern over a short temporal window (three weeks in October 2021) using large-volume (700-800L/sample) transect sampling aboard the Oceanographic Research Vessel (RV) Belgica (now RV Boris Aleksandrov). The survey was organised by the European Union-funded project EU4EMBLAS, implemented by the United Nations Development Programme (UNDP). The seawater concentration of 332 organic substances was quantified in nine consecutive offshore transects through the North-Atlantic, the Mediterranean, and the Black Sea, via target and wide-target-screening analytical techniques, using Gas Chromatography-High Resolution Mass Spectrometry (HRGC-HRMS magnetic sector) and Gas Chromatography-Quadrupole Time of Flight (HRGC-QToF) technology. Overall, we detected 121 substances, of  which 69 were present in all transects, including pesticides (e.g., α-, β-, γ-hexachlorocyclohexane, HCH; triallate; chlorpyriphos; biphenyl; anthraquinone), phosphorus flame retardants and plasticizers (e.g., tris(1-chloro-2-propyl) phosphate, TCPP), brominated flame retardants (e.g., 2,2,4,4'-tetrabromodiphenyl ether, BDE-47), industrial chemicals (polychlorinated biphenyls, PCBs) and fossil carbon combustion products (polycyclic aromatic hydrocarbon, PAHs). Seawater concentrations of detected substances ranged from 0.0002 pg L^-1 for tris(2-isopropylphenyl) phosphate (TIPPP) in the Central Mediterranean Sea, to 16 ng L^-1 and 20 ng L^-1 for anthraquinone and biphenyl, respectively, both in the Celtic Sea/Bay of Biscay. Transects with the highest diversity of detected compounds were located in the English Channel (99 substances) and in the Black Sea (98 substances). The Black Sea transect had the highest concentrations of several substances, including atrazine (15 ng L^-1), β-HCH (2.1 ng L^-1), terbuthylazine (1.8 ng L^-1), diphenamid (1.3 ng L^-1) and tebuconazole (1.1 ng L^-1). The spatial pattern of HCH isomers was noteworthy and unexpected, with 200-fold higher concentrations of β-HCH in the Black-Sea than in the Mediterranean, suggesting a Black-Sea specific β-HCH pollution spreading into the Mediterranean through the Turkish Straits. Large-scale surveys on marine pollution led by pan-European collaborations such as the one presented here have the potential to be very effective in informing the selection of substances with marine relevance for future monitoring programs under the Marine Strategy (MSFD) and Water (WFD) Framework Directives. Harmonised, efficient monitoring of marine contaminants in different environmental matrices across the EU is critical to assist Member States in achieving Good Environmental Status with respect to marine pollution.