|Mercury in the southern North Sea and Scheldt estuary
coastal seas; estuaries; mercury; methylmercury; speciation
Dissolved and particulate mercury and methylmercury concentrations were determined in the Southern Bight of the North Sea and the Scheldt estuary in the period 1991–1999. Mercury and methylmercury concentrations are higher before 1995 than after 1995, especially in the fluvial part.The North Sea: In the offshore stations, dissolved Hg concentrations are generally higher in winter than in summer while the reverse is true for particulate Hg KD values (KD=the concentration of particulate Hg (HgP in pmol kg-1) divided by the concentration of dissolved Hg (HgD in pmol l-1)) range from 100,000 to 1000,000 l kg-1. Dissolved methylmercury concentrations vary from 0.05 to 0.25 pmol l-1 in summer and from d.l. to 0.23 pmol l-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 KD ranges from 9,000 to 219,000 l kg-1.The Scheldt estuary: In winter, dissolved Hg concentrations are elevated in the upper estuary, decrease exponentially in the low salinity range followed by a very slow decrease towards the mouth. In summer, they are low in the fluvial part, increase in the low salinity range or in the mid-estuary and sometimes show an increase in the lower estuary. Particulate Hg concentrations do not show any seasonal trend.Dissolved MMHg concentrations are much lower in winter, when maximum concentrations are found in the upper estuary, than in summer. In summer, the MMHg 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 increase 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 observed. The ratio of dissolved MMHg to dissolved Hg (cruise averages between 1.3% and 20%), is higher than the ratio of particulate MMHg to particulate Hg (cruise averages of 0.27–0.90%). The KD 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 pattern. Dissolved MMHg ranges from 0.1 to 0.39 pmol l-1 and particulate MMHg from 3.1 to 43.5 pmol g-1. The MMHg concentrations are comparable to those found in the estuary and no seasonal variations could be observed.