| 24 Aug 2026
Phytoplankton community responses to 20th century eutrophication of Swiss Plateau lakes: a proxy comparison from short sediment cores
Abstract. During the last century, temperate lakes have been strongly impacted by human eutrophication and climate change, causing shifts in phytoplankton communities and higher abundance of cyanobacteria. Paleolimnological reconstructions can extend the timeframe of observations beyond the extent of monitoring data and reveal long-term changes in aquatic communities. Existing paleolimnological studies often rely on proxies that are solely indicative of individual algal groups. As a result, reconstructing the overall algal community composition remains challenging. Here, we analyzed and cross-validated multiple proxies for phytoplankton community composition in sediment cores from three lakes on the Swiss Plateau that underwent extreme eutrophication in the 20th century. We measured the relative abundance of phytosterols and phytol (phytol:sterol index, PSI) and hydrogen isotope offsets between C16:0 fatty acid and phytol (δ2HC16:0 Acid/Phytol) and used these to detect past changes in the relative proportions of eukaryotic algae and cyanobacteria. We compared these new organic geochemical proxies with observational data and with gene amplicon sequence variants (ASVs) from plastid-derived 23S rRNA. PSI and δ2HC16:0 Acid/Phytol values were generally in good agreement with cyanobacterial 23S rRNA ASVs, and all proxies indicated increasing proportions of cyanobacteria in response to eutrophication. However, our study also revealed potential biases associated with each individual algal proxy. Specifically, δ2HC16:0 Acid/Phytol values were impacted by non-algal lipid sources, while the abundance of small coccoid cyanobacterial taxa was overestimated by 23S rRNA ASVs relative to their biovolume. Still, the combination of PSI and δ2HC16:0 Acid/Phytol values with other algal proxies is a promising tool for tracing the long-term relative abundance of cyanobacteria and could provide a comprehensive picture of changes in phytoplankton communities over millennial timescales.
General comments:
The manuscript by Klatt et al. has applied three different proxies (i.e., phytol:sterol index, PSI) and hydrogen isotope offsets between C16:0 fatty acid and phytol (δ2HC16:0 Acid/Phytol) and 23S metabarcoding to detect past changes in the relative proportions of eukaryotic algae and cyanobacteria from three different sediment core records. The new geochemical proxies seem very interesting and have great potential, but I think the authors could make their comparisons more quantitative.
Specific comments:
I believe the team has executed the lab work proficiently and interpreted the data cautiously, but I would have liked to see some statistical comparison of the data. I did not see any formal quantitative comparison of the different proxies as correlations or mixed model analyses where all the data could be pooled and data nested from the same core could be expressed as a random factor.
As a result, some statements in the text lack robust support. For example, line 505 “Regarding changing proportions of cyanobacteria and eukaryotic algae, PSI and δ2HC16:0 Acid/Phytol values recorded generally the same trends as cyanobacterial and eukaryotic algal plastid 23S rRNA ASVs.”
There are also places in the text where the authors report that the conditions are quite stable and yet their loess trend line is increasing. For example, on line 357 “In Greifensee, the PSI was relatively stable from the mid-1800s until ~ 1940” OR line 398 “δ2HC16:0 Acid/Phytol values fluctuated between ~ 140 ‰ and 200 ‰ from the late 1800s until ~ 1937 CE”