Pergamon 0025-326X(95)00017-8 Marine Pollution Bulletin, Vol. 30, No. 9, pp. 604-608, 1995 Copyright © 1995 Elsevier Science t,td Printed in Great Britain. All rights reserved 0025-326X/95 S9.50+0.00 Influence of Watershed Lithology on Heavy Metal Levels in Estuarine Sediments and Organisms in Galicia (North-West Spain) E. CARRAL, R. VILLARES, X. PUENTE and A. CARBALLEIRA Area de Ecoloxia, Fac. de Bioloxia, Univ. de Santiago de Compostela, Espaha The possible influence of watershed lithology on metal levels in estuarine sediments and organisms (macro- algae, phanerogams, molluscs and polychaetes) in Galicia (north-west Spain) was investigated. In sedi- ments, background levels of Fe, Mn, Cu, Cr, Pb and Ni--but not of Co and Zn--are determined by watershed lithology. In contrast, watershed lithology is not a reliable predictor of the metal contents in organisms, although for some metals in some species lithology has a statistically significant influence on metal content. The determination of natural (background) metal levels is essential for accurate evaluation of the extent to which a particular metal has been enriched in a given environ- ment. Such assessments provide an objective basis for decisions regarding the optimum use of natural resources (Grafia & Mac/as, 1984; Jeffrey, 1987). The river basins of Galicia (north-west Spain) differ in their lithology (Fig. 1), with some being predomin- antly granitic and others predominantly basic (schist and gneiss). These types of rock have different metal contents; thus, as a result of erosion and particle transport, background metal levels in the sediments and biota of an estuary can be expected to be influenced by watershed lithology. Appropriate interpretation of background levels thus requires consideration of lithology. 1 3 1 0 rim 12 16 14 13 2 2 17 18 2O GA om 2 6 m29 I35 • L 2 1 iGRA NITE US CHIS T--G NEIS 1 Fig. 1 Sampling sites and lithological characteristics of river basins of Galicia (north-west Spain). 604