Sediment trap sampling of settling particulate matter (SPM) was carried out in remote coastal and offshore stations in the Bothnian Bay (BB) and the Bothnian Sea (BS) located in the northern Baltic Sea. This was done to investigate occurrence, composition profiles, sedimentation, and spatial differences of PCBs, HCHs, hexachlorobenzene, DDTs, chlordane compounds, and dieldrin. The contamination load at the coastal stations was compared to that of surface bottom sediments. At one coastal station, sediment samples were collected in two different seasons (autumn and spring) in order to compare seasonal variations. All compound groups were found in the SPM and sediment samples analyzed. The contaminant concentrations in SPM at the coastal stations ranged, on dry weight basis, from 0.41 to 8.6 ng/g for chlordanes and HCHs and at the offshore stations from 3.2 to 31 ng/g for hexachlorobenzene and PCBs. The levels in SPM at the offshore stations were 10 times higher (on the average) than the coastal stations, likely because of an increased carbon content in the offshore SPM. The profiles of compounds in SPM reflect the corresponding sediment profile, and only small differences in concentrations and profile of compounds between autumn and spring surface sediments were observed. A comparison between the BS and the BB for offshore and coastal stations showed that the concentrations of compounds were similar although the sedimentation of contaminants, estimated by down fluxes collected in the sediment traps per unit of time, was 3-5 times higher in the BS than the BB. The total annual sedimentation volume of PCBs was approximately 1.1 ton/yr in the BS and 0.4 ton/yr in the BB. An extrapolation to total sedimentation of PCBs during 1 year in the whole Baltic Sea resulted in an approximate value of 7 ton/yr.

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