Marine Chemistry 
Nordseezustand 2003 
19 
edly higher mercury levels in the area southeast of Helgoland, concentrations were 
again on the order of the 1998 levels. However, more recent analyses of sediment 
cores from the area showed that the sediment layer with elevated mercury levels was 
merely covered by a 3 cm surface layer with lower mercury concentrations. 
Radioactive Substances 
The concentrations of most artificial radio nuclides in sea water were hardly higher 
than those measured in the open Atlantic surface water. Despite the generally low lev 
els, emissions from the nuclear reprocessing plants in Sellafield and La Hague were 
reflected both in the concentrations and activity ratios of the transuranic elements. Re 
suspended 137 Cs from sediment of the Irish Sea is the second strongest source of ra 
dioactive substances in the North Sea, besides direct emissions from the nuclear re 
processing plants in Sellafield and La Hague. However, the highest 137 Cs activity 
levels in the North Sea are still attributable to the nuclear accident at Tchernobyl, due 
to Baltic Sea surface water flowing into the Skagerrak. 
The observed concentrations of artificial radio nuclides were so low that a relevant ra 
diation exposure through enrichment in the food chain is to be expected neither for 
biota nor the human population.