Temporal trend studies of PCBs and DDTs in two annually laminated sediment cores from the offshore NW Baltic proper and the Stockholm archipelago demonstrate that studies of laminated sediments are useful tools for detecting time trends of HOCs. The concentration trends of PCBs and DDTs, on a dry weight basis, from the two cores show quite large similarities to trends reported in pelagic biota from the NW Baltic proper, although the reduction in concentrations from the 1970s and onwards is approximately half of the reductions seen in biota. The sediment accumulation trend for these substances was not coupled to the concentration trend in biota but is closely linked to climate-related erosion/resuspension dynamics. The gross accumulation rate in the archipelago area proved to be 8 times higher compared to that of the offshore NW Baltic proper (885 and 108 g m−2 water area yr−1, respectively). Since PCB and DDT concentrations were similar in offshore and coastal areas, and the burial rates of PCB and DDT were closely linked to gross accumulation rates, the sediment burial of contaminants in relation to water area was severalfold higher in the archipelago compared to the offshore area. The high burial of sPCB and sDDT in the archipelago indicated an import of contaminants from the open sea to the archipelago. Although eutrophication-induced scavenging of contaminants may have occurred in the Baltic Sea, natural processes have obscured evidence of this.
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Vol. 29 • No. 4