CHEMICAL SPECIATION OF CHROMIUM AND NICKEL IN THE WESTERN MEDITERRANEAN

The chemical speciation of dissolved Cr and Ni was measured onboard sh ip in the western Mediterranean Sea using cathodic stripping voltammet ry (CSV) during the winter of 1992 and summer of 1993. Vertical profil es for dissolved Cr(III) in the water column showed concentrations bet ween 0 and 1.4 nM, with highest concentrations in the surface layer, p ossibly caused by photochemical conversion of Cr(VI) to Cr(III) during summer periods and by atmospheric inputs during winter periods. Enhan ced Cr(III) levels also were encountered in the deeper Mediterranean w aters and were attributed to mineralisation of sinking biological mate rial and diffusion from sediments. Dissolved Cr(VI) concentrations ran ged between 2 and 3 nM, and total dissolved Cr concentrations between 2 and 3.5 nM. Lowest concentrations of total Cr and Cr(VI) occurred in the surface Atlantic waters and highest levels in the deeper Mediterr anean waters. Total dissolved Ni concentrations ranged between 1.5 and 5 nM. Between 10 and 20% of the total dissolved Ni in the western Med iterranean appeared to be strongly organically complexed. Both labile and total dissolved Ni concentrations were lower in the surface Atlant ic waters than in the Mediterranean deeper waters. Little difference w as observed between the data sets for winter and summer, indicating th at the influence of seasonal changes on the speciation of Ni was small . A clear nutrient-like behaviour was not apparent for Ni and Cr, and their vertical and horizontal distributions were mainly determined by physical processes. Dissolved budget results for Cr and Ni for the Val divia and Disco very (data in parentheses) cruises of -572 (55) and -4 39 (369) t yr(-1) were calculated, indicating that the dissolved budge ts for these elements were well balanced. The budget deficits and surp luses for dissolved Cr and Ni were small compared to the mass fluxes i nvolved in the calculation of the mass balances. (C) 1997 Elsevier Sci ence Ltd.