HEAVY METALS IN THE MARINE ENVIRONMENT Evidence of Naturally High Levels of Chromium in the Ortigueira Ria (NW Iberian Peninsula) R. Prego 1, M. Caetano 2, N. Ospina-Álvarez 1, P. Brito 2, A. Filgueiras 1, C. Vale 1 1Marine Research Institute (CSIC), Av. Eduardo Cabello, 36208 Vigo, Spain 2IPIMAR/National Institute of Biological Resources, Av. Brasília, 1449-006 Lisboa - Portugal Abstract The influence of chromium minerals (chromite and chromospinel), associated with ultrabasic rocks of Ortegal Complex (SW Europe), on the levels of Cr in water, particles and organisms from Ria Ortegal was studied. The sampling comprised 24 water samples in the tributary rivers Mera, Landoi and Lourido, 42 surface sediments in Ria and Orte- gal shelf, and 8 samples of the mussel Mytilus edulis and Fucus spp. Total Cr was deter- mined in all samples and the labile fraction determined in selective sediment samples using ICPMS and GFAAS. Levels of Cr in surface sediments (<2 mm) ranged within the intervals: 200-930, 230-1340 and 290-1660 mgkg -1 in inner, outer Ortegal ria and shelf, respectively. Labile Cr was only up to 0.3% of total Cr. Results point to 5-10 times higher levels than the background concentration of other Galician rias. Chromium in sediments correlates with Mg, which is assumed to be a tracer of particles derived from the Ortegal complex. The levels of Cr in rivers corroborate the origin of Cr: Landoi and Lourido tributaries that cross the Ortegal complex showed 7-48 and 5-81 nM of dis- solved Cr exceeding largely values registered in Mera River out of this geological complex (<0.2nM). Differences on SPM were less pronounced 0.2-0.8 and 0.2-1.3 gkg -1 against <0.3 gkg -1 ). Accordingly, increased concentrations of Cr were found in Fucus spp. (7-16 mgkg -1 ) and similar levels in mussels (2.0-3.5 mgkg -1 ) when compared to values found in other Galician Rias. This work is integrated in projects CTM2007-62546-C03-01/MAR (CICYT), 2007PT0021 and FCT/CSIC9/CSIC/08. 15 th ICHMET 577.