Mercury in the food chain of the Lagoon of Venice, Italy

•Baseline δ15N derived from primary consumers improves estimation of trophic position.•Trophic position explains better methyl-mercury biomagnification than raw δ15N data.•Trophic magnification factor of methyl-mercury in the Lagoon biota is about 10.•Benthic bivalves accumulate more mercury than other animals at the same trophic level.

Sediments and biota samples were collected in a restricted area of the Lagoon of Venice and analysed for total mercury, monomethyl mercury (MMHg), and nitrogen and carbon isotopes. Results were used to examine mercury biomagnification in a complex food chain. Sedimentary organic matter (SOM) proved to be a major source of nutrients and mercury to primary consumers. Contrary to inorganic mercury, MMHg was strongly biomagnified along the food chain, although the lognormal relationship between MMHg and δ15N was less constrained than generally reported from lakes or coastal marine ecosystems. The relationship improved when log MMHg concentrations were plotted against trophic positions derived from baseline δ15N estimate for primary consumers. From the regression slope a mean MMHg trophic magnification factor of 10 was obtained. Filter-feeding benthic bivalves accumulated more MMHg than other primary consumers and were probably important in MMHg transfer from sediments to higher levels of the food chain.

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