PCB levels in laminated coastal sediments of the Baltic Sea along gradients of eutrophication revealed by stable isotopes (delta N-15, delta C-13)

A systematic investigation of PCB in coastal sediments of the Baltic Sea wa s initiated in 1996 by retrieving dated subsurface layers (1992-1994) from 88 sites in 19 sheltered archipelago areas, where laminated sediments were found to be common. PCBs7 (HELCOM's standard sum of 7 congeners, approximat e to 1/6 of total PCB) was quantified, as well as the abundance and isotopi c composition of nutrients (N, C), in order to trace effects of eutrophicat ion and potential influences on the turnover of PCB. In most of the areas, the median of PCBs7/C was within 70 to 120 ng g(-1) C. This is similar to r ecent values in regional offshore sediments, and thus consistent with a rap id exchange between coastal and open waters. Elevated PCBs7 levels up to ar ound 1000 ng g(-1) C were found within 50 km from Stockholm. Here, but not in other areas, PCBs7/C increased linearly with the degree of eutrophicatio n. This was evident from the isotopic composition of sediment N and C, refl ecting local eutrophication as well as its link to freshwater input. In con trast, concentrations or ratios of nutrients in sediments were poor indicat ors of eutrophication. Our results demonstrate that the Stockholm watershed still is a source of both PCB and excess nutrients to the Baltic Sea, but do not support any enrichment or dilution of PCBs7/C ratios caused by eutro phication.