Isotopic biogeochemistry of the lipids in recent sediments of Lake Bled (Slovenia) and Baldeggersee (Switzerland)

The distributions of neutral lipids and their stable carbon isotopic compositions were determined for recent sediments of two alpine lakes: Baldeggersee (Switzerland) and Lake Bled (Slovenia). The analysed lipids belong to structural groups involving linear lipids, steroids, hopanoids and other triterpenoids including tetrahymanol. The lipid biomarkers found in both sediments are quantitatively and qualitatively comparable and the source organisms are not obvious. Analysis of the stable carbon isotopic compositions of the biomarkers by irm-GC–MS allowed additional conclusions. For example, analysis of linear lipids shows that their sources have not changed significantly over the last 60 years for Baldeggersee whereas important variations are apparent during the last 50 years for Lake Bled. Similar isotopic compositions of sterols and stanols suggest that the source of stanols in the Recent sediments of both lakes is most probably an hydrogenation of the corresponding sterols. Marked differences in the isotopic composition of individual hopanoids suggest various sources for these compounds with a large contribution of methanotrophic bacteria, especially for Baldeggersee, and a significant change in the composition of the bacterial community of Lake Bled during the early 1980s.