Influence of organic carbon cycling on the fate of persistent organic pollutants in marine environments

Persistent organic pollutants (POPs) are ubiquitously present in the aquatic environment. They are hydrophobic substances that sorb to organic carbon (OC), and thus their environmental fate is closely linked to OC fluxes and pools. In this project, we test the hypothesis that future changes in the OC cycle can influence POP flux from air to sediment and reduce the POP sink in Baltic Sea sediments. The hypothesis relies on the assumption that the OC sorption capacity is affected by the relative contribution of terrestrial OC as well as the trophic status (oligotrophic versus eutrophic) of the area. Four different coastal sites were sampled, to capture different carbon regimes in terms of nutrient status and level of terrestrial OC influence. Concentrations of POPs were analysed along high-resolution sediment porewater- bottom water interface profiles, in total sediment, suspended matter collected in sediment traps and plankton, in the water column and in air. Stable carbon isotope signatures (δ¹³C) showed that the sites are different in terms of the influence of terrestrial organic matter, and the sites differ in nutrient conditions.

Preliminary results demonstrate that in general, sediments (three sites analysed) act as a source of PAHs to overlying water, whereas sediment and water more often are in equilibrium for PCBs, although there are variations for individual compounds. At the high nutrient-low terrestrial site, which was sampled at two different seasons, both air and water concentrations were higher for PAHs and PCBs in the autumn compared to the summer, indicating the importance of air as source of these contaminants to the water column. Accordingly, air seems an important source of PAHs to the water column in the low terrestrial-low nutrient site, as concentrations in the water column are increasing towards the water surface, whereas this was not observed for PCBs at the same site. Preliminary results from two contrasting sites in the Gulf of Finland, both with high nutrient levels but with different relative contribution of terrestrial OC, demonstrate higher total sediment concentrations of PAHs in the sediment with more marine OC, which was not observed as clearly for PCBs. Data from the water column indicate that marine OC is more efficient in sorbing POPs as air and water concentrations were similar at both sites, even though the OC vertical export at the high terrestrial site was more than double compared to the low terrestrial site. The full data set, will allow for further evaluation of hypotheses on the links between OC cycling and contaminant fate in the Baltic Sea.