This study presents a step-by-step statistical analysis for tracing dioxins sources that have contributed to levels in Baltic herring during the last decades. The study is based on the concentrations of the 17 toxic (2,3,7,8-substituted) dioxin congeners in herring and sediment from the Baltic Sea, and it evaluates how the impacts of the sources may have changed during the studied periods, i.e. 1990–2009 in the Bothnian Bay, 1979–2009 in the Bothnian Sea and 1988–2009 in the Baltic Proper. The modelling technique used (PMF) extracted three dioxin patterns in herring that could be used to obtain source patterns after applying transformation factors. The transformed patterns were compared to real dioxin source patterns available through previous measurement and modelling studies. The identified sources included tetrachlorophenol (TCP), pentachlorophenol/atmospheric background (PCP/AB) and emissions from thermal activities.

The results indicate that the thermal source type has been the major contributor of dioxins to Baltic herring during the pre-and post-2000 periods (72% and 59%, respectively). Its impact appears, however, to have declined by 19% in the Bothnian Bay, by 67% in the Bothnian Sea, and by 48% in the Baltic Proper (TEQ-basis). On the other hand, the relative importance of TCP and PCP/AB appear to have increased over time, from 1.4% and 1.5% to 19% and 6.6% in the Bothnian Bay, from 3.3% and 7.2% to 12% and 10% in the Bothnian Sea, and from 8.9% and <1% to 33% and 13% in the Baltic Proper. Comparisons using absolute values (pg TEQ g-1 lipid weight) indicate an increase of the TCP source by five times in the Bothnian Bay from the pre-2000 to the post-2000 period, a slight increase in the Bothnian Sea, and more than a doubling of the levels in the Baltic Proper. The agreement between the trends in the three sub-basins is a good indication for an increased impact of the TCP source during recent years (post-2000). Corresponding analysis for the PCP/AB source type, indicate slightly decreased TEQ levels from the PCP/AB source type in the Bothnian Sea (by ~50%), more than twice as high in the Bothnian Bay, and more than triplicated in the Baltic Proper. While the declining trends of the thermal source type encourages continuing management efforts for air emissions, the apparent increase of TCP and PCP/AB call for more attention to such sources in the Baltic Sea. As the use of technical products containing TCP and PCP have been banned/restricted since the 1970s and 1980s, more focus on contaminated sites may be required in the mitigation actions of such sources.