INFLUENCE OF CHRONIC EXPOSURE TO SILVER AND MERCURY IN THE FIELD ON THE BIOACCUMULATION POTENTIAL OF THE BIVALVE MACOMA-BALTHICA

In order to assess the adaptation to metals previously observed in the bioindicator organism, Macoma balthica, subjected to chronic contamin ation by silver and mercury in the French Loire estuary, the bioaccumu lation potential of individual organisms originating from the contamin ated Loire estuary and a relatively uncontaminated control estuary (So mme) was evaluated using both radiotracers and stable isotopes of Ag ( 80 mu g Ag litre(-1)) and Hg (100 mu g Hg litre(-1)). Clams from the c ontaminated estuary were more sensitive to Ag (LT50 = 9 d) than those originating from the Somme estuary (LT50>15d), even though the former bioaccumulated Ag to a significantly lower degree. This is attributed to a consequence of the chronic stress induced by Ag while clams were living in their natural environment. Therefore, past history of trace metal contamination should be considered when evaluating the susceptib ility of M. balthica to heavy metal exposure. Lower uptake rates obtai ned for Hg (during the initial uptake phase only) and for Ag in clams from the polluted estuary suggest the presence of an adaptive trait fo r survival in contaminated areas. However, the lower degree of bioconc entration observed for Ag was not sufficiently low to reduce the sensi tivity of the organisms to Ag and allow them to resist the toxic stres s. Clams that survived Ag or Hg exposure at LT50 did not protect thems elves against metal toxicity by accumulating a significantly lesser am ount of these metals than clams which did not survive metal stress. Th e results suggest that the bioaccumulation potential of each individua l was not a factor which can explain the survival ability of M. balthi ca exposed to chronic Ag and Hg contamination in estuaries. In this ca se, cellular, biochemical and genetic levels of adaptation are presume d to be of greater importance. (C) 1998 Elsevier Science Ltd. All righ ts reserved.