TRACE-METALS IN THE CENTRAL AND SOUTHERN NORTH-SEA

Measurements of both dissolved and particulate trace metals (Cd, Co, C u, Fe, Mn, Ni, Pb and Zn) were made on samples collected simultaneousl y in the central and southern North Sea (below 56 degrees N), during f our cruises covering the seasonal cycle. Analyses of trace metals in b oth phases were undertaken using similar analytical protocols, and the data subjected to rigorous quality controls. Clean techniques were us ed throughout sampling and analysis. Trace metal concentrations ranged from those characteristic of waters entering the shelf region from th e North Atlantic Ocean to values which in some cases were two orders o f magnitude higher. Marked increases in concentrations were often asso ciated with waters directly influenced by riverwaters, but the results showed that the distributions could be influenced by processes other than fluvial inputs. The redox-sensitive metals, Fe and Mn, were influ enced at some sites, including those overlying areas of fine-grained s ediments, by benthic inputs associated with seasonal changes in oxidat ion-reduction conditions in the benthic interfacial zone. The behaviou r of metals which are not redox-sensitive (Cd, Zn) in some cases paral leled those of Fe and Mn, suggesting an association with Mn- or Fe-ric h solid phases, or with organic matter. However, seasonal variations i n concentrations of Cd and Zn were not directly linked to the cycle of biological utilization and regeneration of nutrients, suggesting that their geochemistry is more influenced by boundary inputs. Resuspensio n of Pb-rich sediments, long-range transport of Pb-rich suspended part iculate matter (SPM), and enhanced, though diffuse, atmospheric inputs of lead were considered to have contributed to the elevated concentra tions of particulate Pb (mass metal/mass sediment) in the water column during winter. Estimates of partitioning (K(D)s) were derived from th e dissolved and particulate data for each of the metals over the seaso nal cycle. For the more particle-reactive metals (Go, Fe, Mn, Pb), tra nsport in the suspended particulate phase is a significant, sometimes dominant factor. In contrast, for Cd, Cu and Zn, transport in solution will dominate under almost all conditions in the southern North Sea. Furthermore, the values of K-D, for a particular metal, varied widely, partly in response to seasonal differences in the relative contributi ons of lithogenous and biogenous particles to the particles in suspens ion. Only for Cu and Zn during the winter could the partitioning be de scribed by a narrow range of K-D values covering the observed concentr ations of SPM. The data from this study give unique information concer ning the contemporaneous distribution of dissolved and particulate tra ce metals in the southern North Sea and they provide a baseline agains t which the effects of reductions of inputs into the North Sea may be more accurately assessed. Viewed in combination, the dissolved and par ticulate data have yielded new insights into mechanisms affecting trac e metal cycling and transport, essential in the development of coupled hydrodynamic-geochemical models. (C) 1995 Academic Press Limited