Extreme variation in the concentration of trace metals in sediments and bivalves from the Bilbao estuary (Spain) caused by the 1989–90 drought

Intertidal sediments and bivalves Scrobicularia plana from Bilbao estuary (Spain) were repeatedly sampled during and after the 1989–90 drought. The organic content (OC) and the concentration of most of the 10 metals analysed in sediments were extremely variable (e.g. Cd ranged from 4 to 112 μg g−1, ppm dry wt), and they were strongly related to an index applied to estimate the magnitude of the drought (namely the deficit flow of the main river). After OC normalisation, the correlation versus deficit was lost for some elements (Cr, Fe, Ni) but not for others (Cd, Co, Cu, Pb, Zn). This leads to the conclusion that both a natural and a man-induced process account for the observed rise and fall of the contamination pattern: the reduced river flow raised the metal-binding capacity of sediments, and restrictions in the water supply increased the corrosion of sewers and their leaching of some elements. The concentration of trace metals in local bivalves (Cd reached 100 and 458 ppm dry wt, respectively, for whole tissues and digestive gland) mirrored the sediment contamination dynamics, but at a slower pace. The means by which S. plana is able to tolerate such high Cd tissue levels remain unknown. These results constitute some prime field evidence of drought effects that should be born in mind when pollution and risk from climate change are assessed in estuaries.