Persistent effect of ancient Romans on algal community in Murtensee (Switzerland)

Throughout recent decades, cultural eutrophication has threatened freshwater ecosystem functioning. However, anthropogenic activity has also heavily impacted aquatic environments earlier in human history. During the 2nd century BCE Aventicum (capital of the Roman province Helvetica), located at the southern border of Murtensee, was inhabited by ~20,000 people. This led to significant disturbances of the lake and its environment, which persisted for centuries, including increased soil erosion, cultural eutrophication, and major changes of the local vegetation. Here, we aim to investigate algal population dynamics in Murtensee in response to this known anthropogenic forcing during the Roman Period, by combining organic geochemical proxies with sedimentary ancient DNA (sedaDNA) analyses.

To analyse the effects on the algal community, a 15-meter-long sediment core, spanning > 8 kyr, was collected in April 2023. We present a record of the relative abundance of lipid biomarkers, which we used to reconstruct the ratio between eukaryotic algae and cyanobacteria and to provide insights about the diversity among eukaryotes. Our results reveal a prominent shift towards cyanobacteria during the Roman period, and a substantial decrease of specific phytosterols indicative of dinoflagellates and diatoms. We complement our analyses of biomarker distributions with compound-specific hydrogen isotope measurements of fatty acids, n-alkanols, and phytol, and use the offsets among these to further distinguish ecological changes in the lake. In addition to our organic geochemical data, we analysed sedaDNA using a metabarcoding approach targeting cyanobacteria. The results of these analyses indicate an increase in cyanobacterial diversity associated with the Roman eutrophication.

We analysed not only the immediate effects of Roman activities on the algal community but also investigated the long-term consequences of this disturbance over the following centuries. Although human pressure was reduced following the Roman period, the effect on the algal community composition was long-lasting, with persistent reduced diatom abundance for at least three centuries, and reduced dinoflagellate presence for over 1.5 kyr.

Our research emphasizes that even while ancient human activity was less pronounced compared to modern perturbations, it had a significant and persistent effect on the algal community. This suggests that centuries to millennia may be required for ecological recovery in the aftermath of a eutrophication disturbance.