Marine Pollution Bulletin 194 (2023) 115396 10 higher density of high mass fractions within the OWF compared to outside or at the edge of the OWF. This would support the results of Wang et al. (2023) if these values are considered independently from the other campaigns. However, taking the sampling campaigns in 2018, 2021 and 2022 (Fig. 4B, E, F) into account, this observation cannot be confirmed. For samples taken in 2018 with a high sample density within the OWF, a quite homogeneous distribution of In mass fractions and overall significantly lower mass fractions than 2019 were observed. Nonetheless an enrichment of In from 2018 to 2019/2020 seems un- likely, since the latest campaigns (2021 and 2022 Fig. 4E, F) show In mass fractions comparable to 2018 and below. Overall the high spatial as well as temporal variations of In elemental mass fractions emphasize the complexity of a sound source attribution of possible metal emissions from the dissolution of galvanic anodes in OWFs. 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. Therefore, further investigations should be conducted in the future regarding the distributions of elemental mass fractions within and around OWFs in the investigated areas. This includes the ongoing monitoring of already well-studied sampling areas, but also modelling of natural and anthropogenic sedi- ment relocation. 4.3. Possible source assignments As discussed in chapter 4.1 and 4.2 area N-4 exhibits larger spatial and temporal variations of elemental mass fractions than the other investi- gated areas. To find possible explanations the analysis of Sr isotope amount ratios was included in this study. The isotopic composition of n (87Sr)/n(86Sr) (median of OWFs ranging between 0.7144 and 0.7160) of the area N-4 north of Heligoland is comparable to sediments of the Elbe estuary (0.7143 to 0.7240 (Reese et al., 2019)) as shown in Fig. 3A, indicating sediment transport from the Elbe estuary along the residual current (Bohnecke, 1922) into the sampling area (area N-4). This suggests that elemental mass fractions of sediments north of Heligoland are influenced by riverine inputs of the river Elbe, besides other anthropo- genic impacts such as the known dumping activities as well as the general high frequent ship traffic in this region (Ducrotoy et al., 2000). In order to gain more insights from the elemental data regarding influences of different sediment origins, Ga/In ratios were calculated and compared to the Ga/In of the galvanic anodes. Fig. 5A shows that Ga/In ratios of Al anodes (0.44  0.01 to 0.58  0.02 (Reese et al., 2020)) are significantly different from Ga/In ratios of the UCC (313  41 (Rudnick and Gao, 2003)), as well as the range of the Ga/In ratios of recent North Sea sediments as reported by Klein et al. (2022a) (62 to Fig. 5. A) Element ratio Ga/In in North Sea sediments in and around OWFs of the German Bight taken between 2016 and 2020 together with Ga/In for UCC (Rudnick and Gao, 2003), North Sea sediments between 2010 and 2020 (Klein et al., 2022a) and galvanic anodes (Reese et al., 2020). B) Isotope ratio n(87Sr)/n(86Sr) plotted against the element ratio Ga/In for the sediments samples taken in 2018 and (C) 2019. Error bars correspond to expanded uncertainties U(k ? 2) for isotope ratios and to combined uncertainties U(k ? 1) for Ga/In. A. Ebeling et al.