Trace metals in the NE Atlantic coastal zone of Finisterre (Iberian Peninsula): Terrestrial and marine sources and rates of sedimentation

Abstract

The biogeochemical patterns of bioactive metals (Cd, Co, Cu, Ni, V and Zn) and Pb were studied in the Corcubión Sound, an area close to Cape Finisterre (NW Iberian Peninsula). Seasonal patterns of dissolved and particulate trace metals in freshwater and seawater, as well as in phytoplankton were determined during three upwelling events in the winter, late spring and summer of 2005. Metals were also analyzed in the surface and to 1m-depth in the sediments. Upwelling patterns permit the discrimination between those of the outer Sound and Corcubión Inlet. Dissolved trace metal concentrations in the seawater of the outer Sound were in the range of those reported for the Northeast Atlantic Ocean. Mean values of dissolved metal concentrations in the upwelled ENACW were: 0.07±0.03nMCd, 0.11±0.02nMCo, 2.6±1.6nMCu, 4.6±0.6nMNi, 0.20±0.10nMPb, 31±2nMV and 3.8±2.3nMZn. Higher values for dissolved Co, Cu and Zn were measured in two inflowing streams and in the Inlet. A severe–moderate contamination of these metals and Pb was detected in the sediments of the Inlet. Three different sediment layers were observed in cores located at the outer-Inlet boundary. The most recent showed the highest mass accumulation rates, mainly of Cu (e.g. 428mgCu·m−2·yr−1), Pb and Zn, during the last decade. The intermediate unit corresponds to the 20th century (e.g. 39mgCu·m−2·yr−1) and the deeper layer from the 19th to 14th centuries can be regarded as representing the pre-industrial period, and shows the lowest rates of metal accumulation (e.g. 7mgCu·m−2·yr−1). Diatoms were the main autochthonous source of metals in the sediments. The genera Chaetoceros and Lauderia dominate the overlaying seawater during winter and Pseudonitzschia and Leptocylindrus were typical of summer blooms while in the sediments Chaetoceros, Paralia, Leptocylindrus and Thalassionema were the main diatom genera. The comparison of the normalized metal vs. Si in the phytoplankton and vs. Si in the biogenic opal of the sediment shows a prevailing lithogenic–anthropogenic origin of metals such as Cu, Fe, Ni, Pb and V while the upwelling was the main source for Cd in the sediments because of the settling of diatoms. The continental influence and metal contamination are restricted to the innermost part of Sound during the last century where the anthropogenic activities supply Cd, Cu, Pb and Zn. Seasonally, dissolved trace metal concentrations in the outer Sound are within the normal range measured in the north-eastern Atlantic Ocean. The Sound system is pristine compared with the neighboring and anthropogenically disturbed rias, and may be a good reference of natural trace metal levels along the north-eastern Atlantic coast.