Origin and composition of surface slicks in lakes of differing trophic status1

The chemical and biological composition of freshwater microlayers was followed in surface slicks of eutrophic, oligotrophic, and dystrophic lakes and compared with conditions in the subsurface water. The slicks were sampled with a revolving cylinder coated with hydrophilic Teflon which collected a layer that ranged in thickness from 23 to 48 µm when collected at surface pressures of 3–8 mN m−1. The levels of organic and inorganic residues in the slicks were generally higher than those in the subsurface water. POC was enriched to a significantly greater extent in the dystrophic lake than in the three other lakes, but no relationship between trophic status and DOC enrichment was noted. Dissolved lipids were enriched by factors of 2.6–5.2 but represented only a minor fraction of the DOC content (~ 5%) of the microlayer. The DOC: DON ratio, color, UV absorbance, and fatty acid composition of the slicks suggested that most of the microlayer components were derived allochthonously. Combining the results of chemical and biological observations suggests that olizotrophic lakes may be partly covered by a thin layer of water with eutrophic properties.