Distribution of Mercury in the Sediments and Fishes of the Lagoon of Aveiro, Portugal

Dry and wet sieving were compared on their suitability for the size fractionation of bottom sediment samples, collected from a brackish water, coastal lagoon, prior to flameless atomic absorption analysis of the sediment concentration of total mercury. The work aimed at identifying the most important point sources of mercury to the lagoon waters and at assessing the distribution, throughout the ecosystem, of the mercury discharged. The results reported in this paper refer to the lagoon sediments and fishes. Ongoing work is extending the survey to the other inland ecological niches and to the coastal waters and sediments.

A chlor-alkali plant proved to be the most important source of mercury but its contribution has not yet been disentangled from those originating at a pyrite roasting facility and a PVC factory operating in the same industrial park. Sewage discharges from the bankside communities are the other significant point sources of Hg.

Concentrations of Hg in the superficial sediments ranged from 850 mgkg⁻¹, near the chlor-alkali sewer outlet, to 0.05 mgkg⁻¹ in the intertidal sands of the coastal beaches. Sewage-affected sediments have concentrations of Hg in the range of 1 to 2 mgkg⁻¹. The concentrations observed in the water samples and superficial sediments suggest that some of the Hg historically discharged into the lagoon may have been exported to the Atlantic Ocean coastal waters.

Bioaccumulation of Hg in fish tissues shows the usual dependence on species, territoriality, feeding habits and size. Concentrations as high as 25 mgkg⁻¹ fresh weight have been observed but the average values do not exceed the EEC quality objective of 0.3 mgkg⁻¹ (EEC, 1982).