Mercury in the open Mediterranean: evidence of contamination?

In relation to the previously observed higher Hg concentrations in the tissues of certain Mediterranean fish (e.g. mackerel and tuna) compared with the same species in the Atlantic, the evidence for anomolously high Hg contamination of the Mediterranean has been examined. Four aspects have been addressed: the reliability of analytical measurements; the paucity of data on Hg in the Mediterranean and other oceanic areas; the actual levels of Hg in truly open-ocean waters; and ecological factors which may play a role in the enrichment of Hg in Mediterranean fish. Recent evidence suggests that former measurements of Hg in sea water are inadequate, and overestimate the concentrations of Hg in oceanic waters. Even today, intercalibration studies suggest that there are serious problems in the determination of Hg at the levels present in open oceans. A comparison of the most recent reliable data indicates that Mediterranean open-ocean waters are not enriched in Hg relative to the North Atlantic or other open-ocean areas. Similarly, no evidence exists for Hg anomalies in deep-ocean sediments in the Mediterranean, although it is apparent that localised coastal anomalies do occur as a result of natural and anthropogenic Hg sources. Generally, there is a severe lack of data on total Hg concentrations in truly open-ocean environments, and virtually no data on alkyl forms of Hg. Geographic comparisons of Hg in plankton and small pelagic fish do not indicate the presence of higher Hg concentrations in the Mediterranean Sea compared with oceanic regimes. Since these lower trophic species serve as a source of food for larger predaceous fish, e.g. tuna, the origin of the high Hg content in top-level predators is difficult to explain by evoking solely food chain relationships. In trying to resolve the question of why high Hg concentrations are found in certain Mediterranean fish, future studies should include an examination of the alkyl as well as inorganic forms of Hg, and ecological factors such as growth rates and feeding regime.