Seasonal sedimentation of organic matter and contaminants in the Gulf of Finland

The levels of organic contaminants were analyzed in the accumulating material of the Gulf of Finland from April to October in 1996. Settling particles were collected at two locations: GF2 (open sea), and Schepelev (Neva Bay). The contaminants included in the study were: PCBs (7 congeners), DDT and its metabolites, HCHs, chlordanes, trans-nonachlor, and HCB. Sedimentation of total oil and total organic halogens (EOX) was also quantified. The average fluxes of the chemicals were calculated, and the total sedimentation to the whole Gulf of Finland accumulation areas were extrapolated from the flux values obtained for one open sea station. The sedimentation of total particulate carbon, nitrogen and phosphorus was also measured. The contribution of phytoplankton carbon (PPC) and other biogenic material to the total settled carbon was estimated. The sedimentation of the compounds was most intense during the times of increased dry matter accumulation. During the spring, diatoms were dominant in the settled material and phytoplankton comprised about 78% of total settled carbon. During the autumn, the effect of resuspension seemed important. The results from the trap at Schepelev indicated smaller fluxes of PCBs and DDTs than in the open Gulf, but the PCB concentration was high. With the exception of DDTs (90%), the Gulf of Finland was calculated to receive 1–9% of the reported total input of different contaminants to the Baltic Sea.