Algal lipid distributions and hydrogen isotope ratios reflect phytoplankton community dynamics in Rotsee (Switzerland)

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 (ε2Lipid1/Lipid2) from
short-chain fatty acids, phytosterols and phytol. Comparing these data
with phytoplankton cell counts, we found ε2C16:0/sterol and
ε2sterol/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 ε2Lipid1/Lipid2 values
based on δ2HC16:0, δ2Hsterol and δ2Hphytol values from batch cultures of
individual algal groups and their biovolume in Rotsee and evaluated the
role of heterotrophy on ε2Lipid1/Lipid2 values using a model incorporating
δ2HC16:0 and δ2Hsterol values from microzooplankton. Annually-integrated
and amount-weighted ε2Lipid1/Lipid2 values measured in Rotsee were within
6 to 17 ‰ of the mean of modeled algal ε2Lipid1/Lipid2 values,
demonstrating a strong link with the phytoplankton community composition,
while ε2Lipid1/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 ε2Lipid1/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.