Aquatic microorganisms are a (potential) provider of long-chain fatty acids to lake sediments in the temperate climate zone

Annually varved lake sediments are great environmental archives giving unique insights in past environmental and climate settings and changes. They can also be used as an archive of past status and changes in erosion dynamics of the lakes catchment. In an earlier publication, Lavrieux et al. (2019), we used the compound-specific stable carbon isotopes of long-chain fatty acids (LC-FAs) to track back changes in the erosion dynamics and associated land-use at the catchment scale at Lake Baldegg catchment, Central Switzerland. In the 130-years old varved lake sediment core from Lake Baldegg we discriminated grassland, arable land and forest soils using the d13C composition of LC-FAs C26:0 and C28:0, but for the sediments older than 1940 we were not able to explain the isotopic signal by todays terrestrial sources. The isotopic signal of the LC-FAs C24:0 and C26:0 in the lake sediment was significantly depleted compared to the terrestrial input signal. To explain the latter, we checked hypothetical additional sources of the assumed terrestrial long-chain fatty acids like (1) the influence of historical peatlands, (2) former reed grass areas around the lake, and (3) in-situ LC-FA production by aquatic organisms. Consistent with our results, van Bree et al. (2018) and Volkman et al. (1988, 1998) already showed alternative production of long chain fatty acids by aquatic organisms, but in rather extreme environments, like polar or equatorial regions. While we could not find historical peatlands or former reed grass areas as a likely source to our sediments, algal biomass from Lake baldegg shows C24:0 FA, in small quantities, but depleted compared to the terrestrial catchment sources. With these results we can create a potential scenario which would explain the different isotopic deviations of the LC-FAs in the lake sediments before 1940. This indicates at least the potential source of depleted LC-FAs 90 years ago, when the lake status was severely eutrophic. The proof of significant aquatic contribution of LC-FAs to lacustrine sediments in Swiss lakes would be an important finding also regarding the common use of assumed terrestrial biomarkers in lake sediments for climate reconstruction. References: Lavrieux, M., Birkholz, A., Meusburger, K., Wiesenberg, G. L. B., Gilli, A., Stamm, C., and Alewell, C.: Plants or bacteria? 130 years of mixed imprints in Lake Baldegg sediments (Switzerland), as revealed by compound-specific isotope analysis (CSIA) and biomarker analysis, Biogeosciences, 16, 21312146, https://doi.org/10.5194/bg-16-2131-2019, 2019. van Bree, L. G. J., Peterse, F., van der Meer, M. T. J., Middelburg, J.J., Negash, A. M. D., De Crop, W., Cocquyt, C., Wieringa, J.J., Verschuren, D., Sinninghe Damst, J.S., Seasonal variability in the abundance and stable carbon-isotopic composition of lipid biomarkers in suspended particulate matter from a stratified equatorial lake (Lake Chala, Kenya/Tanzania): Implications for the sedimentary record, Quat. Sc. Rev., 192. 208-224, 2018. Volkman, J.K., Burton, H.R., Everitt, D.A. and Allen, D.I.: Pigment and lipid compositions of algal and bacterial communities in Ace Lake, Vestfold Hills, Antarctica, Hydrobiologia, 165, 41-57, 1988. Volkman, J.K., Barrett, S.M., Blackburn, S.I., Mansour, M.P., Sikes, E.L. and Gelin, F.: Microalgal biomarkers: A review of recent research developments, Org. Geochem., 29, 1163-1179, 1998.