Marine Pollution Bulletin 194 (2023) 115396 8 older sediment reservoirs (legacy pollution) due to changing current conditions in the direct surrounding of the individual foundations might be possible. Temporal variations of the measured Zn mass fraction are very small for the areas N-2, N-3 and N-6 over the studied timeframe indicating no increase of mass fractions of Zn in sediments within the vicinity of OWFs. The largest variations can be observed for area N-4. Indeed, Zn mass fractions in most of the analyzed sediment samples are above the NOAA ERL indicating potential effects on the marine environment. Some marine species are reported to be very sensitive to increased Zn concentrations in the water phase (Peganova and Eder, 2004), including changes in growth of marine cyanobacteria (Sarker et al., 2021). The continuous release of anode material has the potential to further in- crease environmental concentrations. As Zn is the second most abundant element in Al based galvanic anodes a release of 1600 kg to 3600 kg Zn in each OWF area can be conservatively estimated, considering five years and 150 offshore wind turbines per OWF area. Zn is a so-called recycled or nutrient type element in the context of seawater, hence, long residence times in seawater (103–105 years) are typical (Kremling et al., 1999) and enrichments are more likely to be observed in the water phase. Long-term monitoring data by the BSH (2016) shows that elevated mass fractions of Zn in the German North Sea were observed even before the construction of the first OWFs in 2012 (BSH, 2016), likely due to other sources such as shipping (OSPAR, 2009), river dis- charges (Reese et al., 2019; Zimmermann et al., 2019a), or, especially for OWF area N-4 located north of Heligoland, historical dumping of dilute acid waste until 1989 (BSH, 1991; Pickaver, 1982). Due to the very complex source situation and the long residence time of Zn in seawater, the current Zn pollution in the OWF areas under consideration cannot be clearly attributed to new emissions of OWFs. In contrast to the legacy pollutants Cd, Zn and Pb, the TCEs Ga and In can be considered emerging contaminants with almost no currently known anthropogenic sources into the marine environment (Romero- Freire et al., 2019). Both elements are added to galvanic anode alloys to avoid passivation of the Al surface and hence promote the dissolution of the anode instead of the steel structure. For a five year protection of one OWF area of 150 coated offshore wind turbines the release of 5 kg to 8 kg Ga and 9 kg to 14 kg In can be estimated (Reese et al., 2020). Like Pb, Ga and In are assumed to be particle-active elements in seawater, meaning that these elements are transported over the particulate matter into the sediments with short resident times in seawater (Klein et al., 2022b; Kremling et al., 1999; Romero-Freire et al., 2019). All median mass fractions of Ga are within the range of North Sea sediments as published by Klein et al. (2022a). Median mass fractions are higher in the areas N-2 and N-6 for 2021, but decrease again in 2022. The highest inter-year variability for Ga can be observed for area N-4. Very similar observations can be made for In: All In median mass fractions are within the range of North Sea Sediments as published by Klein et al. (2022a). The highest inter-year variability for In can be observed for area N-4 with the highest values in 2020, decreasing in 2021 and 2022. Similarly, area N-2 features highest mass fractions in 2021, but decreased to mass fractions similar to 2018 and 2019 in 2022. This might be also caused by the high known sea bed dynamics in the German Bight (Zeiler et al., 2008), which results also in the spatial transport and dispersion of fresh pollutant-loaded sediments. 4.2. Spatial distribution of Indium mass fractions The spatial distribution of elemental mass fractions in and around OWFs area N-4 will be discussed in detail in this chapter, as it features the highest sample density over the longest time period during this study, combined with the highest intra-year variability for the tracer elements. As In is the most promising tracer for anode emissions, the spatial distribution is discussed for In, as shown in Fig. 4. Corresponding maps for all other tracer elements (Cd, Pb, Zn, Ga) can be found in ESM Figs. A2, A5, A8, A11. As can be seen from the first sampling campaign in 2016 (Fig. 4A) the distribution of In mass fractions seems to follow a North-South gradient with higher elemental mass fractions in the northern region (140 ?g kg 1  30 ?g kg 1 to 190 ?g kg 1  40 ?g kg 1) compared to the southern region (80 ?g kg 1  20 ?g kg 1 to 130 ?g kg 1  10 ?g kg 1). However, this was not necessarily true for all five following sampling campaigns in 2018, 2019, 2020, 2021 and 2022 (Fig. 4B–F). Recent literature indicates an increase of metal concentra- tion with decreasing distance to the wind farm center (Wang et al., 2023). In our study the highest In mass fractions occurred in 2020 (marked dark red in Fig. 4D). High In mass fractions (light red) were found in 2016, 2019, 2020 and 2021, again outside or at the edge of the OWF, except for 2019. Indium mass fractions in 2019 (Fig. 4C) show a Fig. 3. (A) Isotope ratio n(87Sr)/n(86Sr) plotted against the inverse Sr mass fraction. The purple area corresponds to data from Elbe estuary sediments (Reese et al., 2019). (B) Share of the <20 ?m grain size fraction in the sediment samples plotted against the isotope ratio n(87Sr)/n(86Sr). The data points are grouped into the respective OWF areas. Error bars correspond to expanded uncertainties U(k ? 2) for isotope ratios and inverse mass fractions and to standard deviation (n ? 3) for the grain size fraction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) A. Ebeling et al.