Total mercury in the water column near the shelf edge of the European continental margin

Abstract

The purpose of this paper is to investigate the distribution and the relative importance of processes that affect mercury distribution in the water column at the shelf edge of the Celtic Sea, on the western European continental margin. The water column, down to 4500 m, was sampled during two cruises, one in winter 1994 and the other in June 1995, on eight stations. Total mercury concentrations ranged from 0.29 to 9.37 pM with the extreme values in the first 200 m. The highly variable concentrations in the surface layers are attributed to intense atmospheric-sea water exchange processes. The low concentrations encountered in the shelf water prelude any significant influence of continental waters in this area. In the mixed layer, biological uptake and regenerative processes appear to dominate in controlling the vertical mercury distribution. The intermediate and deep-water mercury distribution is mainly governed by the hydrographic features. In summer, a mercury enrichment was observed in the upwelling zone of the shelf break. Mean mercury concentration in the Lower Deep Water (LDW) was typically 1.96±0.23 pM and in the Labrador Sea Water (LSW) 1.97±0.63 pM. The Mediterranean Intermediate Water (MIW) revealed similar levels, with a mean mercury concentration of 2.04±0.84 pM. Between LDW and LSW, the Iceland–Scotland Overflow Water (SOW) exhibited higher concentrations (2.76±0.35 pM). The upper part of the North East Atlantic Central Water (NEADW) and lower part of the North Atlantic Deep Water (NEACW) were characterized by marked peaks of concentration, which are interpreted in term of atmospheric influence and particle regeneration.