The concentrations and the seasonal changes of heavy metals and organic carbon in the sediments underlying a Zostera marina L. bed were measured monthly during one year, in two Mediterranean lagoons: Than (France) and Venice (Italy). While at Thau sediments showed Cu (18.7 +/- 3.9 mug g(-1)) and Pb (13.8 +/- 3.8 mug g(-1)) average concentrations twofold higher than at Venice (Cu: 8.4 +/- 4.8 mug g(-1); Ph: 6.1 +/- 0.70 mug g(-1)), the Italian site exhibited average concentrations of Fe (13 383 +/- 955 mug g(-1) versus 6098 1089 mug g(-1) at Thau), Mn (339 +/- 12 mug g(-1) versus 190 +/- 23 mug g(-1) at Thau), Zn (61.6 +/- 12.7 mug g(-1) versus 36.1 +/- 7.4 mug g(-1) at Thau), Cr (47.3 +/- 7.3 mug g(-1) versus 21.8 +/- 8.0 mug g(-1) at Than) and Ni (12.7 +/- 1.7 mug g(-1) versus 8.9 +/- 3.1 mug g(-1) at Thau) approximate to1.5-2 times as high as the French site. The organic carbon concentration was systematically higher at Than (1.0 +/- 0.3) than at Venice (0. 7 +/- 0.2). A significant seasonal fluctuation was found for Zn, Cu, Ni, Cr in both lagoons while no significant variations were recorded for Pb at Venice and for Cd at Than; Some of those changes appeared to be significantly correlated with the biomass of Zostera at Thau and the concentration of organic carbon at Venice. (C) 2004 Elsevier Ltd. All rights reserved.