Antonia Klatt, Cindy De Jonge, Daniel Nelson, Marta Reyes, Carsten Schubert, Nathalie Dubois, Nemiah Ladd

Reconstructions of past changes in algal community composition provide important context for future alterations in biogeochemical cycling. However, many existing phytoplankton proxies are indicative of individual algal groups and are not fully representative of the whole community. Here, we evaluated hydrogen isotope ratios of algal lipids (δ2HLipid) as a potential proxy for phytoplankton community composition. We sampled the water column of Rotsee, a small eutrophic lake in Switzerland, every second week from January 2019 to February 2020 and analyzed distributions and the relative offsets between δ2HLipid values (δ2HLipid1/Lipid2) from short-chain fatty acids, phytosterols and phytol. Comparing these data with phytoplankton cell counts, we found δ2HC16:0 Acid/Sterol and δ2HSterol/Phytol values reflect shifts in the eukaryotic algal community. To assess whether the selected phytoplankton groups were the main sources of the selected lipids, we further modeled algal δ2HLipid1/Lipid2 values based on δ2HC16:0 Acid, δ2HSterol and δ2HPhytol values from batch cultures of individual algal groups and their biovolume in Rotsee and evaluated the role of heterotrophy on δ2HLipid1/Lipid2 values using a model incorporating δ2HC16:0 Acid and δ2HSterol values from microzooplankton. Annually-integrated and amount-weighted δ2HLipid1/Lipid2 values measured in Rotsee were within 2 to 20 ‰ of the mean of modeled algal δ2HLipid1/Lipid2 values, demonstrating a strong link with the phytoplankton community composition, while δ2HLipid1/Lipid2 values including microzooplankton lipids had a larger offset. Additionally, cyanobacterial biovolume was positively correlated with the ratio of phytol and phytosterols (phytol:sterol ratio) as well as the ratio of unsaturated C18 and C16:0 fatty acids (C18:C16 ratio). Our results support the application of sedimentary δ2HLipid1/Lipid2 values in eutrophic lakes as a proxy for past phytoplankton community assemblages. Moreover, the calculation of sedimentary phytol:sterol and C18:C16 ratios provides an additional proxy for reconstructing cyanobacterial blooms.