Hydrogen isotopes from lipid biomarkers record eutrophication induced changes in algal community assemblages

Phytoplankton are an important biogeochemical force, collectively impacting nutrient cycling as well as atmospheric and aquatic chemistry. However, it remains challenging to reconstruct changes in algal productivity and community assembly throughout the geologic past. Here, we lay the foundation for a sensitive proxy of past algal ecology based on compound-specific hydrogen isotope compositions (δ²H values) of common algal lipids. While such measurements have previously been used as indicators of water hydrogen isotope ratios, our results from laboratory cultures and experimental ponds demonstrate that changes in the δ²H values of ubiquitous lipids such as palmitic acid associated with taxonomic changes are an order of magnitude greater than those caused by hydrologic change. These results indicate that δ²H values of algal lipids, and the relative offset of these values among different compounds, can be used to reconstruct past changes in algal community assemblages, including those driven by changes in nutrient supply.

We apply this approach to a ~180 year sedimentary record from Lake Greifen, a lake in the central Swiss Plateau that underwent well-documented eutrophication and partial recovery in the second half of the 20^th century. As total phosphorus concentrations in the lake increased from > 100 mg/L to ~ 500 mg/L in the 1950s-1970s, palmitic acid d²H values increased by 40 ‰ and phytol δ²H values by 20 ‰;  δ²H values of both compounds subsequently declined with total P following maximum values in the early 1980s. During this entire time interval, mean annual precipitation δ²H values fluctuated within a ~10 ‰ range and are not correlated with the changes in lipid δ²H values. Additionally, the decline in lipid δ²H values is correlated with declining relative abundance of green algae as the eutrophication pressure on Lake Greifen receded in the past decades. This correlation matches the prediction from our culturing and mesocosm results, where green algae produced exceptionally ²H-enriched fatty acids, and indicates that lipid δ²H can be applied to reconstruct nutrient induced shifts in algal populations.