oa Algal Lipid Distributions and Hyrdogen Isotope Ratios as Proxy for Past Phytoplankton Community Dynamics

Reconstructions of past phytoplankton community dynamics provide important information for predicting future changes of algal communities. However, many existing proxies for individual algal groups might not be fully representative of the whole community.

Here, we present a new proxy for phytoplankton communities based on the distribution and relative offsets between hydrogen isotope ratios (d2HLipid1/Lipid2) of short-chain fatty acids, phytosterols, and phytol. We sampled the water column of Swiss lake Rotsee, biweekly from January 2019 to February 2020 and compared lipid distributions and d2HLipid1/Lipid2 values with phytoplankton cell counts. We found that cyanobacterial biovolume was positively correlated with the ratio between phytol and phytosterols (phytol:sterol ratio) and that d2HC16:0 Acid/Sterol and d2HSterol/Phytol values reflect changes in the eukaryotic algal community. We further modeled d2HLipid1/Lipid2 values based on d2HLipid values from batch cultures of individual algal groups and their biovolume in Rotsee. Measured annually-integrated and amount-weighted d2HLipid1/Lipid2 values were within 2 to 20 ‰ of the mean of modeled d2HLipid1/Lipid2 values.

We applied our proxy to a 14 m long sediment core from Rotsee spanning the last 13ka BP. Preliminary phytol:sterol ratios indicate high cyanobacterial abundance during the Younger Dryas (YD) followed by a strong decrease during the YD – Holocene transition.