Marine Pollution Bulletin 194 (2023) 115396 5 pollutants in North Sea sediments only the <20 ?m size fraction of the sediment was analyzed. Current monitoring reports include information on the six elements Cd, Cu, Hg, Ni, Pb and Zn (BSH, 2016). Measured elemental mass fractions of the OWF areas were compared to three types of benchmark mass fractions: geogenic background con- centrations, recent North Sea sediment data and (no-) effect concen- trations, as summarized in Table 2. Geogenic background concentrations (BSH BC), derived from the fine grain fraction (grain size <20 ?m) of older, presumably largely unpolluted sediment core samples from the German Exclusive Economic Zone (EEZ), was utilized for Cd, Pb and Zn (BSH, 2016). Recent North Sea sediment data is available for the elements Cd, Pb and Zn from the BSH monitoring program for the years 2008–2011 (BSH, 2016). For Ga and In, no BSH BC nor compa- rable region-specific background is currently available. Therefore, Ga and In data of this study are compared to the abundances in the upper continental crust (UCC) according to Rudnick and Gao (Rudnick and Gao, 2003), as well as Ga and In data for recent North Sea sediments published by Klein et al. derived from the retrospective analysis of sediments sampled between 2010 and 2020 (Klein et al., 2022a). The US National Oceanic and Atmospheric Administration (NOAA) derived Ef- fect Range Low values (NOAA ERLs) for Cd, Pb and Zn as a numerical quality criterion and are derived from biological effect data from the “NOAA Sediment Quality Guidelines”. In the OSPAR region the NOAA ERLs are applied as criteria to assess the environmental status of metal concentrations. It represents a pollutant concentration below which only 10 % of the systems show effects (Macdonald et al., 1996). No NOAA ERLs are available for In and Ga. However, the European Chemicals Agency ECHA derived a Predicted No-Effect Concentration (PNEC) for In in (marine) sediments. For Ga no such concentration was defined yet. 2.5.4. Data presentation The measured sediment data is presented in box plots, indicating the median of the respective data volume (line, P50) with 98 % confidence interval, the interquartile range (P25 – P75) limited by the lower and upper limit of the box and the extreme values of the respective data series. Measured values with a distance of 1.5 interquartile ranges that exceed the interquartile limits are identified by dots as outliers. Numbers in the box plots indicate minimum and maximum values excluding outliers. 2.5.5. Distribution maps Geographical maps showing selected parameters were created using ArcGIS® software by Esri. ArcGIS® and ArcMap™ are the intellectual property of Esri and are used herein under license. 3. Results 3.1. Elemental mass fractions In total, elemental mass fractions of 55 elements were measured in 215 North Sea sediment samples (<20 ?m size fraction). The complete data set for all measured elements and grain size distributions can be found in ESM Tables A4 and A5. The following section describes the distribution of selected elements (Cd, Ga, In, Pb, Zn) in samples obtained during the six sampling campaigns. The selection of the presented ele- ments is based on work by Reese et al. (2020) who suggested Al, Cd, Ga, In, Pb, Zn as element tracers for potential emissions from galvanic an- odes applied for corrosion protection of OWFs. Although Al is the main component of galvanic anodes, it is not further discussed individually, since due to its high natural background in North Sea sediments and a naturally high variability (in 2010/2011 between 6.1 % and 7.7 % in <20 ?m in the German Bight, BSH, 2016), it is currently not possible to differentiate individual sources (BSH, 2016; Reese et al., 2020). Distri- bution maps for the selected five elements can be found in the ESM Figs. A1, A4, A7, A10 and A13. 3.1.1. Cadmium (Cd) The mass fractions of Cd (see Fig. 2A, the distribution map in ESM Fig. A1, a detailed distribution map for area N-4 in ESM Fig. A2 and a high-resolution boxplot in ESM Fig. A3) range between 88 ?g kg 1  44 ?g kg 1 and 1150 ?g kg 1  130 ?g kg 1. Even though the measured Cd mass fractions (except area N-6 (2019)) exceed the BSH BC (80 ?g kg 1 to 260 ?g kg 1), most samples are within the known mass fraction range of German North Sea sediments (150 ?g kg 1 to 870 ?g kg 1 (BSH, 2016)) and are below the NOAA ERL of 1200 ?g kg 1 (Macdonald et al., 1996). However, area N-2 (2021), area N-3 (2022) and area N-4 (2022) feature values close to the NOAA ERL, even though their median values are within the mass fraction range of North Sea sediments. Indeed, this can only be observed for a very limited number of samples. 3.1.2. Lead (Pb) The Pb mass fractions (see Fig. 2B, the distribution map in ESM Fig. A4, a detailed distribution map for area N-4 in ESM Fig. A5 and a high-resolution boxplot in ESM Fig. A6) range between 47 mg kg 1  1 mg kg 1 and 420 mg kg 1  40 mg kg 1. Measured mass fractions are higher than the BSH BC of 12 mg kg 1 to 38 mg kg 1 and higher than the NOAA ERL of 46.7 mg kg 1 (BSH, 2016; Macdonald et al., 1996). However, median mass fractions of most areas are within the known variability of German North Sea sediments ranging from 66 mg kg 1 to 182 mg kg 1 (BSH, 2016). Slightly higher median mass fractions are found for area N-4 in 2016 and 2019 (median: 199 mg kg 1 and 194 mg kg 1, respectively). Overall, area N-2 (2018 and 2019), area N-3 (2019) and area N-4 (2016–2020) show values above the variability of North Sea sediments (BSH, 2016). Latest mass fractions (samples from 2021 and 2022) are within the variability of North Sea sediments for all areas, indicating a steady pollution load for areas N-2 and N-3 and suggesting slightly decreasing mass fractions for areas N-4 and N-6. 3.1.3. Zinc (Zn) The Zn mass fractions (see Fig. 2C, the distribution map in ESM Fig. A7, a detailed distribution map for area N-4 in ESM Fig. A8 and a high-resolution boxplot in ESM Fig. A9) range from 108 mg kg 1  1 mg kg 1 to 820 mg kg 1  36 mg kg 1. Almost all samples exceed the BSH BC of 81 mg kg 1 to 116 mg kg 1 and the NOAA ERL of 150 mg kg 1 (BSH, 2016; Macdonald et al., 1996). Samples from area N-6 are slightly above the BSH BC range and slightly below the NOAA ERL. Nevertheless, median mass fractions of all other areas are within the known variability of German North Sea sediments ranging between 158 mg kg 1 and 864 mg kg 1 (BSH, 2016). Area N-2 and area N-3 feature significantly lower Zn mass fractions and a lower variability than area N- 4. Table 2 Criteria for evaluating the metal content in marine sediment. Geogenic back- ground values (BSH BC (BSH, 2016) or UCC (Rudnick and Gao, 2003)), recent elemental mass fraction ranges in the German EEZ (<20 ?m) ((BSH, 2016; Klein et al., 2022a)) and ecological (no-effect) evaluation criteria (NOAA ERL (Mac- donald et al., 1996) or PNEC (ECHA, 2020)). Zn/mg kg 1 Ga/mg kg 1 Cd/mg kg 1 In/mg kg 1 Pb/mg kg 1 Geogenic background 81–116a 17.5  0.7c 0.08–0.26a 0.056  0.008c 12–38a Recent North Sea sediment 158-864a 14-23d 0.15–0.87a 0.073- 0.237d 66-182a No-effect concentration 150b – 1.2b 5051e 46.7b a (BSH, 2016). b (Macdonald et al., 1996). c (Rudnick and Gao, 2003). d (Klein et al., 2022a). e (ECHA, 2020). A. Ebeling et al.