Vainio et al. 2022 — Total mercury trophic magnification in the Baltic Archipelago Sea food web
This study measured total mercury (THg) concentrations in 30 species spanning benthic, pelagic, and benthopelagic food chains in the Baltic Archipelago Sea, a brackish low-diversity ecosystem with known Hg contamination, to investigate how ecological traits (food-chain origin) and ecophysiological traits (thermoregulatory strategy) affect trophic magnification. Trophic magnification factors (TMFs) were significantly higher in pelagic food chains (3.58–4.02) than in benthic food chains (2.11–2.34) when homeotherm bird species were excluded, demonstrating that food-chain origin and thermoregulation both shape Hg accumulation patterns and that standard risk assessment models using a single TMF may underestimate risk in pelagic food chains or for homeotherms.
Key numbers
THg concentrations (µg/g dry weight; µg/g wet weight) by species:
Selected food-relevant and ecologically important species:
| Species | Common name | n | Hg dw (µg/g) | Hg ww (µg/g) |
|---|---|---|---|---|
| Clupea harengus | Herring | 10 | 0.15 ± 0.17 | 0.035 ± 0.041 |
| Salmo salar | Atlantic salmon | 1 | 0.15 | 0.055 |
| Coregonus albula | Vendace | 2 | 0.087 ± 0.013 | 0.018 ± 0.003 |
| Mytilus edulis | Blue mussel | 9 | 0.037 ± 0.013 | 0.0059 ± 0.002 |
| Esox lucius | Northern pike | 9 | 0.68 ± 0.43 | 0.15 ± 0.09 |
| Perca fluviatilis | European perch | 10 | 0.48 ± 0.38 | 0.095 ± 0.070 |
| Rutilus rutilus | Common roach | 10 | 0.15 ± 0.06 | 0.035 ± 0.014 |
| Somateria mollissima | Common eider (bird) | 13 | 0.67 ± 0.31 | 0.17 ± 0.09 |
| Haliaeetus albicilla | White-tailed eagle (bird) | 7 | 1.9 ± 1.7 | 0.51 ± 0.34 |
| Phalacrocorax carbo | Great cormorant (bird) | 6 | 1.3 ± 1.0 | 0.35 ± 0.27 |
| Zooplankton | — | 3 | 0.0059 ± 0.0015 | 0.0006 ± 0.0002 |
Trophic magnification factors (TMF) for THg dw:
- Pelagic food chain (no birds): TMF = 3.58–4.02 (one-source and two-source models)
- Benthic food chain (no birds): TMF = 2.11–2.34
- Including birds raises pelagic and benthic TMFs substantially due to homeotherms’ higher metabolic demand
EU and Nordic food safety limit for total mercury in fish muscle: 500 µg/kg ww (0.5 µg/g ww) for most species; 1,000 µg/kg for predatory species (shark, tuna, swordfish, pike). Northern pike mean THg ww of 0.15 µg/g is below the general limit but approaching it at the high end (0.15 + 0.09 × 2 = 0.33 µg/g at ±2 SD). White-tailed eagle at 0.51 ± 0.34 µg/g ww exceeds 0.5 µg/g on average, reflecting bioaccumulation at the apex.
Methods (brief)
Freeze-dried homogenized tissue samples analyzed by direct mercury analyzer (DMA-80, Milestone, Italy) at Aarhus University Trace Element Laboratory following US-EPA Method 7473. Accredited by DANAK (ISO 17025). Certified Reference Material DORM-4 (fish protein; 0.412 ± 0.036 µg Hg/g dw); long-term recovery 102 ± 6% (n=284). QUASIMEME international interlaboratory QA participation. Detection limit 0.001 µg/g dw. THg only — MeHg not speciated; given that most Hg in fish muscle is MeHg (typically >90%), ww concentrations approximate MeHg exposures for human dietary assessment. Stable nitrogen isotope (δ15N) analysis used to assign trophic positions; linear mixed models with species as random effect used to compute TMFs.
Implications
Certification: THg concentrations in edible Baltic Sea fish (herring, vendace, roach, perch, pike) are directly relevant to seafood certification standards. Pike and perch at the higher end of their ranges may present challenges against strict THg limits. EU limit for THg in most fish is 500 µg/kg ww; Baltic herring and salmon are well below this threshold in this study.
Courses: Excellent teaching case for trophic magnification: pelagic chain TMFs (3.58–4.02) are nearly double benthic TMFs (2.11–2.34), demonstrating that single-TMF risk models underestimate risk in pelagic systems. Homeotherm (bird) inclusion dramatically changes TMF estimates. Introduces fuzzy zones in benthopelagic species trophic position assignment.
App: Species-level THg ww values are relevant for seafood risk scoring. Baltic herring (0.035 ± 0.041 µg/g ww) and blue mussel (0.0059 ± 0.002 µg/g ww) show very low THg. Northern pike (0.15 ± 0.09 µg/g ww) and perch (0.095 ± 0.070 µg/g ww) show moderate but variable concentrations consistent with freshwater predatory fish profiles.
Microbiome: Not directly applicable; no gut microbiome measurements made.