On the Borderline of Dissolved and Particulate Organic Matter: Partitioning and Bioavailability of Polycyclic Aromatic Hydrocarbons

The functionality of dissolved organic matter (DOM) was studied by assessing the availability of polycyclic aromatic hydrocarbons (PAHs) spiked in pore water samples separated from sediments by water extraction and centrifugation with or without subsequent filtration. The purpose was to compare the effects of traditionally defined DOM (0.45-μm cut off) and larger colloidal material present in the separated pore water samples on the partitioning and bioavailability of PAHs. The tested PAHs included phenanthrene (Phe), fluoranthene (Flu), pyrene (Pyr) and benzo[a]pyrene (BaP). Bioavailability of the selected PAHs was tested with two ecologically different organisms: pelagic filter feeder Daphnia magna and sediment-dwelling deposit feeder Lumbriculus variegatus. Sorption to DOM (i.e. in filtered samples) was clearly higher for BaP than for the other PAH. This was also reflected in significantly reduced bioavailability for both model organisms in the filtered samples compared to DOM-free conditions. For the other PAHs the sorption was significant only in the unfiltered samples indicating the importance of larger colloidal material. Thus, the bioavailability of PAHs was also more effectively reduced by the colloidal material. This holds true for both the model organisms, indicating that the ecological differences i.e. filter feeder vs. deposit feeder do not affect in this respect. It appears that considering only traditionally defined DOM, material that may be present in environmental samples and is important for the speciation and bioavailability of contaminants is ignored.