Marine Pollution Bulletin 194 (2023) 115396 2 relocation (Ducrotoy et al., 2000; BSH, 2016). Ocean currents in the German Bight are directed anti clockwise with water masses entering the bight from western directions and flowing north into the Skagerrak re- gion via the Southern Jutland Current (Brückner and Mackensen, 2006). The German Bight within the North Sea is considered anthropogenically affected, which becomes particularly evident within long-term moni- toring data (BSH, 2016). Mass fractions of legacy pollutants like Zn, Pb and Hg often exceed background and even effect threshold values (i.e. NOAA effect range low). In many cases elevated mass fractions are observed near the coast, which indicates possible discharges by rivers or other near-shore anthropogenic sources (BSH, 2009, 2016). Currently, only a few studies were dedicated to the investigation of the fate of metal emissions from galvanic anodes into the marine envi- ronment. Lab-scale dissolution experiments of Zn anodes found an in- crease of dissolved Zn concentrations, as well as increased Zn mass fractions of marine surface sediments (Rousseau et al., 2009). A case study within the port of Le Havre (France) evaluating the dissolution of Al anodes showed that whilst anodic dissolution does not significantly increase the concentration of Al in the water, both enrichment and increased mobility of Al were evident in sediments sampled from the vicinity of the galvanic anodes (analyzed elements: Ca, Fe, K, Mg, Al and Si) (Gabelle et al., 2012). Furthermore, a five-year monitoring in the port of Calais (France) showed that the dissolution of Al anodes resulted in a higher Zn than Al enrichment of sediments. Results suggest a po- tential toxic effect for marine organisms with regards to the discovered Zn level, especially for confined areas with low water exchange (Caplat et al., 2020). In addition, species-specific studies showed, that the release of elements from galvanic anodes can pose effects on the species of oysters (Caplat et al., 2012; Mottin et al., 2012; Golding et al., 2015; Levallois et al., 2022), blue mussels (Mao et al., 2011; Golding et al., 2015), sea urchins (Caplat et al., 2010; Golding et al., 2015), amphipods (Bell et al., 2020), diatoms (Golding et al., 2015; Bell et al., 2020) and bacteria (Bell et al., 2020) and that in combination with other stressors such as noise the effect of a metal exposure is enhanced for biota such as lobsters (Stenton et al., 2022). In contrast, another recent study in La Rochelle (France) concludes that effects on the health of scallops cannot clearly be appointed to be caused by galvanic anodes (Barbarin et al., 2023). Moreover, the impact of climate change itself could increase metal release and the mortality of biota inhabiting wind farms due to higher water temperature and lower pH (Voet et al., 2022). Recently, Wang et al. (2023) applied geo-accumulation index, contamination factor and pollution load index to sediments taken in the Putidao OWF (China). First hints on heavy metal pollution (Cu, Pb, Zn, Cr and Cd) caused by OWFs were found (Wang et al., 2018). Moreover, the authors suggest future long-term monitoring for a sound assessment of the impact of offshore wind farms on the marine environment. Therefore, the analysis of elemental mass fractions in sediments is one important aspect for the assessment of long-term or emerging contamination of aquatic systems potentially caused by the dissolution of galvanic anodes in connection with the ongoing large-scale devel- opment of renewable energy production worldwide. We previously suggested a multi-tracer approach for the investigation of OWF-induced emissions from galvanic anodes consisting of the elements Cd and Pb, as ecotoxicologically relevant legacy pollutants, the previously discussed main components Al and Zn and the technology-critical elements (TCEs) In and Ga. Moreover, the application of Pb isotope ratios and element ratios was suggested (Reese et al., 2020). Despite an increasing scientific interest in and usage of the elements Ga and In, these elements still show a lack of knowledge regarding their (eco)toxicity, their respective environmental speciation and potential impact on the (marine) envi- ronment (Filella and Rodriguez-Murillo, 2017; Romero-Freire et al., 2019). In addition to elemental analysis metal stable isotope ratios have been shown to be a powerful tool for environmental source and process tracing (Wiederhold, 2015). The isotope amount ratio n(87Sr)/n(86Sr) can be used as a measure for the age and/or type of rocks and sediments (Capo et al., 1998), as the relative abundance of the radiogenic isotope 87Sr increases by the radioactive ? -decay of 87Rb (t1/2 ? 4.88 ? 1010 a) (Brand et al., 2014; de Laeter et al., 2003). Typically, finer sediment fractions are more radiogenic than sandy sediments, due to an enrich- ment of Rb-rich minerals in the fine sediments (Bayon et al., 2021). Several studies have investigated Sr isotope ratios for provenancing and characterization of sediments, e.g. (Andrews et al., 2016, Reese et al., 2019, Bayon et al., 2021, Deng et al., 2021). This study provides a first baseline dataset regarding the spatial and temporal variation of galvanic anode tracer elements of German North Sea sediments inside and within the surroundings of different OWFs over the course of seven years. A multi-element and Sr-isotope tracer approach was applied to assess possible inputs of metal contaminants originating from OWFs and to differentiate from other sources. This work can be used as the basis of future studies on potential mid- and long-term chemical emissions and environmental effects related to the ongoing development of renewable offshore energy production. 2. Material and methods 2.1. Reagents and materials All preparatory laboratory work was performed in a class 10,000 or class 1000 clean room. Type I reagent-grade water (18.2 M? cm) was obtained from a Milli-Q Integral water purification system equipped with a QPod-Element polishing system (Merck-Millipore, Darmstadt, Germany). Analytical grade HNO3 (w ? 65 %, Fisher Scientific, Schwerte, Germany) and analytical grade HCl (w ? 30 %, Carl Roth, Karlsruhe, Germany) were further purified either by double sub-boiling in perfluoralkoxy-polymer (PFA)-subboling stills (DST-4000 & DST- 1000, Savillex, Minnesota, USA) or by double sub-boiling using a cascade of two quartz stills (AHF Analysentechnik, Tübingen, Germany) operated under clean room conditions. HBF4 (w ? 38 %, Chem-Lab, Zedelgem, Belgium) was used in ultra-pure quality for sample diges- tion without any further purification. External calibration standard solutions for quantification (all trace- able to NIST standards) were prepared from custom-made multi-element standards of different composition (Inorganic Ventures, Christiansburg, USA). The reference marine sediments GBW 07313 (National Research Centre for Certified Reference Materials, Beijing, China) and the refer- ence stream sediment GBW 07311 (National Research Centre for Certified Reference Materials) were used for method validation. 2.2. Sampling A total of 215 sediment samples were obtained from OWFs in the German North Sea during six sampling campaigns with the research vessels Atair (Federal Maritime and Hydrographic Agency, BSH) in 2018, 2019, 2021 and 2022 and the Ludwig Prandtl (Hereon) in 2016 and 2020. Due to logistical challenges and weather conditions not all sampling stations were sampled in every year. Samples were taken via a box-corer in and around selected OWFs. Due to safety reasons and legal restrictions it was not possible to obtain samples directly next to the foundations or in certain corridors due to the presence of the OWF un- derwater power cables. The top layer (upper 5 cm) of three or more individual box-cores from the same sampling station were combined, homogenized and stored at 20 C until further analysis. A list of all samples including GPS coordinates can be found in the electronic sup- plementary material (ESM) Table A4. A map of all sampling locations for the six cruises is shown in Fig. 1. Table 1 gives an overview over the number of samples taken in the respective OWF areas per year. A. Ebeling et al.