Lipid compound classes display diverging hydrogen isotope responses in lakes along a nutrient gradient

Abstract

Compound specific hydrogen isotope ratios (2H/1H) of lipid biomarkers preserved in sediments are used as paleohydrologic proxies. However, several variables, including contributions from different source organisms and their growth rates, can influence 2H/1H fractionation between lipids and source water. Significant uncertainties remain about how these factors combine to produce the net 2H/1H signal exported to sediments.To assess the influence of phosphorus availability on 2H/1H ratios of lipids accumulating in lake sediments, we analyzed surface sediments and sediment traps from ten central Swiss lakes representing a wide range of trophic states. In agreement with results from laboratory cultures, 2H/1H fractionation for the diatom biomarker brassicasterol (24-methyl cholest-5,22-dien-3β-ol) increased in more productive lakes (0.6 ± 0.1‰ per μg/L total P in sediment traps and surface sediments). In contrast, 2H/1H fractionation for phytol, the isoprenoid side-chain moiety of chlorophyll, decreased with increasing total P (−0.4 ± 0.1‰ per μg/L total P in sediment traps), suggesting that different biochemical mechanisms are responsible for changes in 2H/1H fractionation for each type of isoprenoidal lipid. Opposing changes in 2H-fractionation for sterols and phytol cause their 2H/1H ratios to converge as total P increases. This response may be a new tracer for phytoplankton growth conditions and is not influenced by the source water isotope value.Interpreting the 2H/1H ratios of short to long chain (C14–C30) n-alkanoic acids and n-alkanols is complicated by likely contributions from heterotrophs and/or vascular plants. These values generally did not correlate with lake water isotopes, nor did their fractionation factors correlate with total P. For most lipids there was no significant difference between sediment trap and surface sediment 2H/1H ratios. However, n-C14–n-C18 fatty acids were 2H-enriched in the surface sediments, most likely due to degradation in the water column. Our results indicate that interpretations of short-chain fatty acid 2H/1H ratios as a water isotope signal likely require supporting information about ecological conditions and community structure, but that paired H isotope measurements of phytoplankton-derived sterols and phytol may be developed as a proxy for phytoplankton growth.