Follow-up survey of mercury in fish from Gunneklevfjorden and nearby lakes of reference

Summary

In Norway and Scandinavia, fish is the most important human exposure route for mercury (Hg). Hg can be of both natural and anthropogenic origin, where the anthropogenic sources can have both local and global effects. In this follow-up survey of work conducted in 2013 and 2016, we investigated Hg concentrations in different environmental matrices from Gunneklevfjorden, a brackish fjord with a local historical Hg source, and from Flåte and Svanstulvatnet, two nearby freshwater lakes without any local pollution sources where the main source of Hg to the catchments is long-range atmospherically transported Hg. The work is aiming to strengthen the data basis for documentation of effects from planned polluted sediment remediation measures in Gunneklevfjorden. Samples from 2018 shows significantly higher concentrations of Hg in perch (Perca fluviatilis) from Gunneklevfjorden (0.48 ± 0.15 mg/kg) compared to populations from Flåte (0.25 ± 0.04 mg/kg) and Svanstulvatnet (0.32 ± 0.07 mg/kg). The results confirm patterns found in the survey from 2016, where the two lakes with long-range atmospherically transported Hg as the main pollution source demonstrated lower concentrations than what was observed in Gunneklevfjorden. Of perch collected in Gunneklevfjorden between 2013 and 2018, 34 out of 138 fish specimens have concentrations higher than the EU limit for protection of human health (0.5 mg/kg). For the reference lakes Flåte and Svanstulvatnet, zero and three fish demonstrate concentrations above the limit during the same period. The fish from Gunneklevfjorden belongs to significantly higher trophic levels than the fish from the two freshwater lakes and it is not surprising to find more fish with higher Hg levels in Gunneklevfjorden. Of the 319 fish collected from the three water bodies between 1991 and 2018, not one fish has Hg concentrations below the Water Framework Directive limit for protection of ecosystems (0.02 mg/kg). In the aqueous phase, differences in Hg concentration are small between the three localities, while sediment concentrations are much higher in Gunneklevfjorden compared to Flåte and Svanstulvatnet (data from 2016). Considering that levels of biomagnification and bioaccumulation are similar in the three food webs, but that Methyl-Hg (MeHg) concentrations are significantly higher in the lower parts of the food web (zooplankton and macro invertebrates) in Gunneklevfjorden, it cannot be ruled out that a potential influence from high sediment concentrations in Gunneklevfjorden on Hg concentrations in fish adds to the influence from long-range transported Hg. The data from 2013, 2016 and 2018 from Gunneklevfjorden and from 2016 and 2018 from Svanstulvatnet and Flåte is not sufficient to determine a time trend for Hg concentrations in fish. Based on the results from this survey we recommend continuous sampling of fish from the selected water bodies to be able to determine whether concentration of Hg in fish is changing over time and potential effects from sediment remediation measures in Gunneklevfjorden. In addition to remediation measures, several factors can potentially affect Hg levels in fish, such as for example changing water chemistry and climate. Hence, fish Hg concentrations often vary substantially from year to year, something which can affect the conclusions drawn from only a few years of data. In the future, yearly sampling and monitoring of Hg in fish should be combined with tracing of Hg sources in fish using stable Hg isotopes measurements and determination of MeHg and stable nitrogen isotopes in macro-invertebrates. This will provide a more detailed understanding of the transport of Hg and MeHg from sediments and catchments to the food web, and then effectively determine effects on Hg in fish from sediment remediation measures in Gunneklevfjorden.