Mercury in the southern North Sea and Scheldt estuary

Dissolved and particulate mercury and methylmercury concentrations were det ermined in the Southern Bight of the North Sea and the Scheldt estuary in t he period 1991-1999. Mercury and methylmercury concentrations are higher be fore 1995 than after 1995, especially in the fluvial part. The North Sea: In the offshore stations, dissolved Hg concentrations are ge nerally higher in winter than in summer while the reverse is true for parti culate Hg K-D values (K-D = the concentration of particulate Hg (Hg (p) in pmol kg(-1)) divided by the concentration of dissolved Hg (Hg-D in pmol 1(- 1))) range from 100,000 to 1000,000 1 kg(-1). Dissolved methylmercury conce ntrations vary from 0.05 to 0.25 pmol 1(-1) in summer and from d.l. to 0.23 pmol 1(-1) in winter and particulate methylmercury concentrations from 1.8 to 36 pmol g(-1) in summer and from 0.9 to 21 pmol g(-1) in winter. The K- D ranges from 9,000 to 219,000 1 kg(-1). The Scheldt estuary: In winter, dissolved Hg concentrations are elevated in the upper estuary, decrease exponentially in the low salinity range follow ed by a very slow decrease towards the mouth. In summer, they are low in th e fluvial part, increase in the low salinity range or in the mid-estuary an d sometimes show an increase in the lower estuary. Particulate Hg concentra tions do not show any seasonal trend. Dissolved MMHg concentrations are much lower in winter, when maximum concen trations are found in the upper estuary, than in summer. In summer, the MMH g concentrations are low at low salinity, they show a first increase in the salinity range from 3 to 12, a decrease in the mid-estuary and a second in crease in the lower estuary. The highest particulate MMHg concentrations are found in the upper estuary, while in the lower estuary generally lower and more constant values are ob served. The ratio of dissolved MMHg to dissolved Hg (cruise averages betwee n 1.3% and 20%), is higher than the ratio of particulate MMHg to particulat e Hg (cruise averages of 0.27-0.90%). The K-D values for MMHg are lower in the summer (30,000-65,000) than in autumn and winter (77,000-114,000). The Scheldt river: In the fluvial part of the Scheldt, dissolved increases in the most upstream stations, while particulate Hg shows no particular pat tern. Dissolved MMHg ranges from 0.1 to 0.39 pmol 1(-1) and particulate MMH g from 3.1 to 43.5 pmol g-1. The MMHg concentrations are comparable to thos e found in the estuary and no seasonal variations could be observed. (C) 20 01 Elsevier Science B.V. All rights reserved.