Biomonitoring of trace metal pollution using the Mollusc Bivalve Donax Trunculus and the surface sediment from the Mediterranean coast of Northern Tunisia

In recent years, as many other Mediterranean coasts, the Gulf of Tunis, situated in the northern eastern coast of Tunisia has been subjected to significant urban, agricultural and industrial extension that might have affected the exploitation of its maritime resources and contaminated its coastal environments. Therefore, a seasonal assessment of contamination by trace metals was made using the surface sediment and the most abundant species of bivalve living along this coast (Donax trunculus). Zinc, copper, lead, manganese, nickel, chromium and cadmium were analysed by atomic absorption spectrophotometry (Air acetylene) in three sites from the Gulf of Tunis (BorjCedria, Rades and Kalâat El Andalous). Results have shown that sediments from these coasts were considered to be polluted by lead and chromium with the highest metal concentrations found in the surface sediments’ samples taken from Kalâat El Andalous. This finding is probably attributed to the fact that Kalâat El Andalous is subject to the impact of the most regular wadi in the Gulf of Tunis, Wadi Medjerda, formerly known by intense mining activities in its catchment area. The clams, D.trunculus, commonly consumed in the region, had different amounts of metals in their tissues. Zinc was the most abundant element, while cadmium was recorded at its lowest concentrations and nickel was not at all detected. Seasonal patterns of trace metals accumulation seem to be affected not only by metal availability but also by the reproductive cycle of the species. The evaluation of the risks to human health suggested that D.trunculus from these coasts pose no health risk for moderate shellfish consumption. Furthermore, data show that measuring metals in D.trunculus themselves does not accurately reflect true contamination levels in the surface sediments from the Gulf of Tunis.