Bjerregaard et al. 2023 — Mercury in blue mussels and invertebrates, Nissum Broad, Denmark
This study investigated mercury concentrations in sediment and benthic invertebrate fauna (blue mussels, periwinkles, brown shrimps, amphipods) in Nissum Broad, a coastal marine area of north-western Jutland, Denmark, that was historically contaminated by discharge from the Cheminova pesticide factory during the 1950s and 1960s (an estimated 30 tonnes of mercury released). Sampling was conducted in 2018 (sediment) and 2019 (biota). The headline finding is that blue mussels (Mytilus edulis) contained average total mercury of 169–260 ng/g dry weight across four stations, all exceeding the Environmental Quality Standard (EQS) of approximately 100 ng/g dw. Periwinkles (Littorina littorea) ranged 66–203 ng/g dw, with some stations above the EQS. Overall sediment concentrations have declined markedly since the 1980s; only one station exceeded the Background Assessment Concentration of 70 ng/g dw. The mercury lost from the highly contaminated Harboøre Tange coastline does not appear to have redistributed into the broader Nissum Broad.
Key numbers
Blue mussels (Mytilus edulis, n=20 across 4 stations):
- Range of station averages: 169–260 ng Hg/g dry weight
- Overall average across all stations: 223 ± 13 ng Hg/g dw
- EQS: approximately 100 ng/g dw; all stations exceeded EQS
Periwinkles (Littorina littorea, n=66 across 7 stations):
- Range of station averages: 66–203 ng Hg/g dw
- Overall average: 123 ± 7 ng Hg/g dw
- Some stations above EQS, some below
Brown shrimps (n=37 across 6 stations):
- Range of station averages: 58–121 ng Hg/g dw
- Overall average: 82 ± 6 ng Hg/g dw
- Not significantly different from Funen (uncontaminated) reference levels
Amphipods (n=14 across 5 stations):
- Range of station averages: 53–80 ng Hg/g dw
- Overall average: 63 ± 5 ng Hg/g dw
Sediment (upper 3–5 cm, 2018): 0.9–71 ng Hg/g dw; 1 station exceeded the Background Assessment Concentration (BAC = 70 ng/g dw).
Comparison to 1980s: Sediment mercury-organic matter regression slope halved from 20.8 to 8.8 between the 1980s and 2018, indicating markedly reduced contamination. Blue mussel concentrations in the 1980s were also higher (data shown for Harboøre Tange: far higher than Nissum Broad).
All concentrations are dry weight basis throughout. Total mercury only (no speciation); analytical method: Milestone DMA-80 Direct Mercury Analyser. CRM validation: TORT lobster hepatopancreas (certified 270±60 ng/g; measured mean 292±1.5 ng/g, within range).
Methods (brief)
Biota sampled by hand, spade, and net at coastal stations; freeze-dried before analysis. Sediment cored with Kajak sampler (coastal) or van Veen grab (deeper stations), sectioned in 1 cm fractions to 5 cm depth. Total mercury by Milestone DMA-80. Quality: TORT lobster hepatopancreas CRM, blanks in each series. n=137 total biota specimens across four species; sediment: 11 stations. Basis: dry weight throughout. No speciation performed.
Implications
Certification: This paper characterises tHg in edible shellfish (blue mussels, periwinkles, shrimps) from a historically contaminated European coastal area. Blue mussel values of 169–260 ng/g dw are relevant context for EU maximum levels (EU Regulation 1881/2006 sets 0.5 mg/kg wet weight for bivalve molluscs, approximately 2,000 ng/g ww assuming ~75% moisture; the present dry-weight values would be approximately 42–65 ng/g wet weight, below the EU regulatory limit but elevated above uncontaminated baselines). This demonstrates that legacy industrial contamination can persistently elevate shellfish mercury decades after source cessation.
Courses: Useful for illustrating legacy contamination persistence, sediment-biota decoupling, and the use of biota EQS as a sentinel for food-chain risk from historically contaminated coastal areas.
App: Direct application limited; this paper is a historically contaminated site study, not a general market-basket survey. The dry-weight mussel values, converted to wet weight (approximately 42–65 ng/g ww or 42–65 ppb ww), are below EU regulatory thresholds but inform the variance range for European mussel sourcing.
Microbiome: Not applicable.