Effect of depositional regimes on polycyclic aromatic hydrocarbons in Lake Bled (NW Slovenia) sediments

Polycyclic aromatic hydrocarbons (PAH) were measured in sediment cores from two contrasting depositional regimes in urban Lake Bled (NW Slovenia). The concentration of PAH in surface sediments, mostly derived from combustion, was higher in Zaka Bay where bottom waters are permanently oxic, than in the western basin where bottom waters oscillate between suboxic and anoxic (1930 vs. 1150 ng PAH g−1 dry weight (dw), respectively). This pattern is the reverse of what might be expected if PAH were to preferentially degrade under oxic conditions. Two PAH from non-combustion sources, perylene and retene, are also present in the sediments. Post-depositional diagenetic formation of perylene appeared to occur only in anoxic sediments. Differences in the source of sedimentary organic matter (autochthonous vs. allochthonous) is probably not a major factor in perylene formation, since the two locations have markedly different sources of organic matter but quite similar concentrations of perylene in deeper sediments, ranging from 250 to 350 ng g−1 dw. Thus, redox conditions may play a role in perylene generation. On the other hand, redox conditions in general can not be the only or even the major factor affecting the fate of sedimentary PAH since uniform compositions and depth distributions for pyrolytic PAH were observed in both cores, in spite of the two locations differ in redox, mixing and biota conditions. Thus, degradative loss of PAH is unlikely in Lake Bled sediments.