Lipid Biomarkers of Suspended Particulate Organic Matter in Lake Bled (NW Slovenia)

The fatty acid, hydrocarbon and sterol composition were used to investigate the organic sources and compositional changes in particulate organic matter (POM) and trap material in the water column in the deepest part of Lake Bled (NW Slovenia) in October 2006. Fatty acids distribution of algal origin were abundant in particles and trap material in epilimnion, while in anoxic zone POM and trap material were enriched in bacterial fatty acids (e.g. 16:1n-7, 18:1n-7, 18:1n-9, iso-C14-C16 and anteiso-C15). The lowest 13C value of 51.7 parts per thousand was observed in 18:1n-7 FA in trap material and was the only FA which could be linked to methanotrophic bacteria. In addition Zooplankton left a marked imprint on particulate lipids and trap material at 12m by predominance of n-C18:0 over n-C16:0 FA, short-chain, even-carbon n-alkenols, the high proportion of cholest-5-en-3-ol (44.8% of total sterol concentration (TST)), cholesterol/phytosterol ratio of 0.49 and 15NPN values of 6.8 and 11.7 parts per thousand. It was shown that lacustrine phytoplankton biosynthesize 24-ethylcholest-5-en-3-ol which is often used as a marker of terrigenous organic matter. Phytoplankton represented an important source of cholest-5-en-3-ol and 24-methylcholest-5,22(E)-dien-3-ol, while 24-ethylcholesta-5,22E-dien-3-ol and 24-methylcholest-5-en-3-ol were of terrestrial origin. There was an evidence of microbial transformation of 5-stenols to 5(H)-stanols in POM and trap material in hypolimnion.