Chemical fractionation of zinc versus cadmium among other metals nickel, copper and lead in the northern North Sea

Concentrations of dissolved Ni, Cu, Zn, Cd and Pb were measured in water sa mples collected during a cruise with R.V Pelagia (29-6/14-7-1993) in the no rthern North Sea and N.E. Atlantic Ocean. At least six depths (0-90 m) were sampled with modified Go-Flo samplers from a rubber zodiac. In the study a rea, the first 25 m were well mixed and stratification occurred below this depth. The local bloom of Emiliania huxleyi hardly affected the trace metal s concentration, except for some removal of Cd as seen from its correlation with nitrate. The mean dissolved concentrations were for Ni (3.66 nM), Cu (1.61 nM), Zn (4.5 nM), Cd (48 pM) and Pb (108 pM). These concentrations ar e among the lowest reported for the North Sea and are of similar magnitude to those found in the eastern North Atlantic at the same latitude. Zn was t he only exception with values 10 times higher compared to those in the Atla ntic Ocean, suggesting external inputs, mainly atmospheric and possibly fro m surrounding land masses. The observed ratio Zn:Cd in the North Sea and es tuaries is in between the high ratio 600-900 for continental sources and th e low ratio 5-10 for oceanic waters. Latter low ratio is consistent with th e 21-fold stronger inorganic complexation of Cd in seawater which, in combi nation with the preferential biological uptake of Zn, may lead to the obser ved about hundredfold fractionation of Zn versus Cd in the marine system. O ther processes may play a role but would need further investigation. The di ssolved Pb values tend to be lower than found before in the North Sea, indi cating decreasing inventories due to reduced anthropogenic emissions. (C) 1 999 Elsevier Science B.V. All rights reserved.