A. Zonderman et al.
(mean of 20 pg kg7) (see Fig. 2). Due to the lack of available data
‚egarding Ga in mussels in the North Sea, it is not possible to determine
f these values are typical for the geographic location or to establish a
‚elationship of Ga levels in this study to local coastal populations. Values
are in the same order of magnitude as those reported in
M. galloprovincialis in Galicia, Spain by Costas-Rodriguez et al. (2010)
(means of 60 jpg kg} to 240 pg kg! by location) and suggested values in
mussel tissue certified reference materials BCR-668 (11 pg kg”! +9 pıg
kg!) and NIST SRM 2976 (30 pıg kg”! +7 ug kg!) (Wippermann et al.
2023). Further data over several time points in addition to relevant
background values are required for a detailed evaluation of the data.
Due to its chemical similarity to Al, Ga released from galvanic anodes
‚s expected to behave comparably to Al in the marine environment albeit
showing longer residence times in seawater (Shiller and Bairamadgi,
2006). This hypothesis is broadly supported by the strong positive correlation
between mass fractions of Al and Ga found in this study
(Spearman’'s rank correlation, r = 0.93, p <2.271%), To the best of our
knowledge, toxicity of Ga in M. edulis has not been evaluated. However,
Dased on estimates in other marine organisms, initial developmental
effects (EC10) are expected between 860 fg L7! and 6000 ig L7* Ga in
seawater (Trenfield et al., 2016; van Dam et al., 2018a; van Dam et al.,
2018b). Measured concentrations in seawater from the study area have
Jeen reported of up to 10 ng L7*, indicating that toxic effects originating
rom introduction of Ga to the water column from OWF activity are
ınlikely (BSH and Hereon, 2022).
3.2.2.4. In. In is of interest as a tracer metal because, similarly to Ga, it
is not naturally abundant in the marine environment, occurring at <0.5
ng L7! in North Sea seawater (BSH and Hereon, 2022; Ebeling et al.,
2025). However, mass fractions of In in Al-based galvanic anodes are
jetween 143 mg kg! and 231 mg kg! (Reese et al., 2020). This rangeis
narkedly higher than in Zn-based anodes, providing a hypothetical
nethod of differentiating OWF-based inputs from those of shipping and
öther maritime industries using In abundance or specific element ratios.
ın mass fractions in the investigated mussel tissues were below the
LOQ of 11.5 pg kg”! and LOD of 9.5 wg kg”. Mass fractions are in
accordance with Wippermann et al. (2023), who reported In mass
fractions in multiple seafood reference materials, including the mussel
issues BCR-668 and NIST SRM 2976, to be between 0.4 jpg kg"! + 0.3
ug kg”! and 0.8 jpg kg”! + 0.7 pg kg7*. The lack of accumulation is in
contrast with reports of In enrichment in marine amphipods exposed to
galvanic anodes, though this may be a result of several factors including
exposure level, organism metabolism, and exposure in sediment versus
water (Bell et al., 2020). Our results do not indicate significant accunulation
of In in mussel tissues when exposed to the levels present in
seawater within OWF-impacted areas at the current time (< 0.5 ng L7!)
(BSH and Hereon, 2022, Ebeling et al., 2025). However, any increased In
levels in future monitoring efforts could be effectively used to demonstrate
introduction of metals into OWF areas in comparison to established
low background concentrations.
3.2.3. Toxicologically relevant and traditionally monitored metals
ig. 2 also shows the mean mass fractions and mass fraction ranges of
che commonly monitored metals Ni, Cu, Cd and Pb, which are presented
with comparative values from other studies in Table 2 (Fryer, 2022;
ATelmholz et al., 2015; Helmholz et al., 2016; Knopf et al., 2020). Mass
fractions of all analyzed metal(loid)s can be found in Table A1 of Apjendix
A. The highlighted metals were selected based on their known
toxicity and the availability of limit and reference values.
3.2.3.1. Ni. Mass fractions of Ni in mussel tissue were found with a
mean of 0.85 mg kg”! + 0.36 mg kg”* (1 SD) and are well below the
values previously reported for the North Sea (see Table 2). On average,
values in area N-5 are somewhat higher, with a mean of 0.95 mg kg”), in
comparison to means of 0.88 me ke! inN-2 and 0.78 me ke”! in N-3. As
Marine Pollution Bulletin 218 (2025) 118216
2an be seen in Fig. 2, two OWFSs (F and G) have Ni mass fractions largely
ınder the overall mean, with average mass fractions of 0.69 mg kg”! and
).60 mg kg”*, respectively. Variation in Ni mass fractions within OWFs
broadly resembles the range of the entire dataset and there is otherwise
little differentiation. As shown in Table 2, the mean mass fraction at the
relatively unimpacted Heligoland location is given as 1.717 mg kg”! +
2.221 mg kg”*, two times that found in this study.
Ni has been measured in Al-based galvanic anodes at mass fractions
between 30.1 mg kg”! + 0.9 mg kg”! and 52.4 mg kg”! + 0.9 mg kg‘). It
has the highest mass fraction outside of the tracer metals (Reese et al.,
2020). Elevated Ni in the sediment fine grain fraction (< 20 im) in OWF
areas was reported by Ebeling et al. (2023), with values between 28.5
mg kg”! + 1.4 mg kg”! and 429.1 mg kg”! + 14.5 mg kg}. The dissolved
fraction of Ni in water in the study area has recently been reported
to be between 184 ng L”! + 37 ng L7* and 797 ng L”! +4 ngL7*
‚Ebeling et al., 2025). These values likewise correspond to existing
monitoring data in the German North Sea, where Ni concentrations are
reported to be highest near the coasts and in the Elbe plume (BSH,
2016).
3.2.3.2. Cu. Mass fractions of Cu had a mean of 6.2 mg kg”! + 2.1 mg
kg! (1 SD). Despite the wide range of mass fractions, there are no
significant differences between OWF or area and variation within both
zroups is high. The highest measured values were found in area N-3,
with a range of 2.0 mg kg !_12.5 mg kg! and mean of 6.5 mg kg ”*. The
highest values within area N-3 were found in OWF A, with a mean of 7.6
ng kg”! (see Fig. 2). The mean mass fraction of this study is comparable
to the ICES BAC of 6 mg kg”! (ICES, 2021). Furthermore, mass fractions
are in accordance with existing data for mussels near the island of
Heligoland, off the German coast, with a reported mean of 6.7 mg kg”!
‚Helmholz et al., 2016), in accordance with data from the German
Umweltprobenbank, and below the mass fractions found in mussels in
20astal areas of the North Sea (Fryer, 2022).
Zu has been measured in Al galvanic anodes at 4.9 mg kg”! to 17.4
mg kg! (Reese et al., 2020). However, Cu has numerous possible
pathways of entry to the marine environment both in the context of OWF
>perations and other anthropogenic activities, including leaching from
paints, antifouling paints, and discharge from vessels, among other
sources (Tornero and Hanke, 2016). Recent concentrations of Cu in the
North Sea water in the study area have been reported by Ebeling et al.
‘2025) to be 113 ng L”* + 16 ng L7* to 900 ng L”! + 15 ng L7. These
values agree with monitoring efforts in the North Sea, which historically
fall over OSPAR reference concentrations (50 ng L71L90 ng L9 (BSH,
2016). Therefore, it is not unexpected that Cu mass fractions are found
to be more similar to reference data used in this study than other metals
shown in Table 2.
3.2.3.3. Cd. Mass fractions of Cd had a mean of 0.46 mg kg! + 0.25
mg kg! (1 SD). Only four individuals were found to have mass fractions
higher than the ICES BAC of 0.96 mg kg”! (ICES, 2021). In comparison
to other metals, the variation of Cd is relatively low, apart from these
autliers. In fact, 90 % of the dataset features mass fractions under 0.65
mg kg7*, The only OWF with a median mass fraction above 0.5 mg kg”!
is OWF H in area N-5 (0.59 mg kg”'). Overall mass fractions are well
below previously reported values for the North Sea listed in Table 2
‘Fryer, 2022; Helmholz et al., 2015).
Reese et al. (2020) reported Cd in galvanic anodes at mass fractions
3f 0.225 mg kg”! + 0.081 mg kg”! to 6.92 mg kg! + 0.10 mg kg”! and
propose the metal as a tracer for OWF activity. Additional sources
include remobilization of older contaminated sediments, riverine inputs,
ar Zn-based galvanic anodes from shipping. Seawater concentrations in
the vicinity of OWFs are reported, between 9.49 ng L7! + 0.04 ng L7*
and 40.7 ng L1+0.4 ng L7! (BSH and Hereon, 2022; Ebeling et al.,
20251.
