Are the shells of Scrobicularia plana useful for monitoring trace metal pollution events?

The rupture of a mine dam in Aznalcóllar (SW Spain) in April 1998 entailed the contamination of Guadalquivir River and its estuary. To study the ability of bivalve shells to monitor the temporal changes on metal composition in the area, two year old Scrobicularia plana were obtained from two different locations of the estuary (Pantalan highly influenced and Pantoca less influenced) during the year 2000 and 2007. Co, Hg, Mo, Ni and Zn content was measured in shell and soft tissue of S. plana. Metal composition in shells sampled by micromilling and corresponding to the years 1999, 2000, 2006 and 2007 was higher in Pantalan (Co: 5.88 +/- 2.7 microg g(-1); Hg: 0.04 +/- 0.03 microg g(-1); Mo: 0.41 +/- 7.90 microg g(-1); Ni: 37.66 +/- 25.56 microg g(-1) and Zn: 9.19 +/- 8.88 microg g(-1)) than in Pantoca (Co: 3.64 +/- 0.50 microg g(-1); Hg: 0.02 +/- 0.02 microg g(-1); Mo: 4.70 +/- 1.20 microg g(-1); Ni: 7.21 +/- 13.60 microg g(-1) and Zn: 3.90 +/- 1.89 microg g(-1)). A marked temporal decrease was observed for all metals in Pantalan station with concentrations of Co, Hg, Mo, Ni and Zn varying respectively from 7.35 +/- 3.02, 0.05 +/- 0.03, 19.90 +/- 2.40, 70.58 +/- 21.94 and 18.04 +/- 0.98 microg g(-1), in 1999 to 3.07 +/- 1.08, 0.009 +/- 0.001, 2.40 +/- 1.43, 10.11 +/- 3.80 and 1.11 +/- 0.33 microg g(-1) in 2007, indicating that the effect of the mine accident had diminished significantly. Metal content in soft tissues did not follow the same decreasing trend indicating that soft tissues present a different capacity to accumulate metals from the environment. Our results confirm that micromilling shells are a suitable tool to assess bioaccumulation ofcontaminants during the entire life-span of bivalves.