Algal lipid distributions and hydrogen isotope ratios reflect phytoplankton community dynamics

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 (δ 2 H Lipid ) 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 δ 2 H Lipid values (δ 2 H Lipid1/Lipid2 ) from short-chain fatty acids, phytosterols and phytol. Comparing these data with phytoplankton cell counts, we found that δ 2 H C16:0 Acid/Sterol and δ 2 H Sterol/Phytol values reflect shifts within the eukaryotic algal community. To assess whether the selected phytoplankton groups were the main sources of the selected lipids, we further modeled algal δ 2 H Lipid1/Lipid2 values based on δ 2 H C16:0 Acid , δ 2 H Sterol and δ 2 H Phytol values from batch cultures of individual algal groups and their biovolume in Rotsee and evaluated the role of heterotrophy on δ 2 H Lipid1/Lipid2 values using a model incorporating δ 2 H C16:0 Acid and δ 2 H Sterol values from microzooplankton. Annually-integrated and amount-weighted δ 2 H Lipid1/Lipid2 values measured in Rotsee were within 2 to 20 ‰ of the mean of modeled algal δ 2 H Lipid1/Lipid2 values, demonstrating a strong link with the phytoplankton community composition, while δ 2 H Lipid1/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 δ 2 H Lipid1/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.