Aquatic biomonitors and biomarkers reveal temperature and environmental changes during the Thirty Years‘ War: A case study from Bad Waldsee, Germany

The Thirty Years' War (1618-1648) had significant consequences for both human populations and natural ecosystems in Europe. To gain a better understanding of its impact on the local aquatic environment, we analyzed a sediment core from Stadtsee, a lake located in the city of Bad Waldsee, Upper Swabia, using a variety of aquatic biotic and abiotic proxies, including bioindicators, biomarkers, and sedimentological data. Through this multi-proxy approach, we infer changes in aquatic communities and their environment during this tumultuous period. Our findings indicate that this war had a marked impact on the lake ecosystem, as evidenced by changes in diatom assemblages, with declining diatom concentrations and progressively decreasing contributions of hypereutrophic species. These changes suggest a lowering in nutrient supply, possibly due to a reduced load of human and animal waste. This is supported by decreasing productivity (low Si/Ti) and declining δ¹³C_org values. It appears that the war led to a lake recovery and an improvement in water quality, as suggested by the presence of three bryozoans Plumatella species and the absence of Cristatella mucedo, indicating reduced nutrient levels and a well-oxygenated environment, in agreement with high Mn/Fe ratios. Additionally, the presence of chironomids, such as Corynoneura arctica-type and C. intersectus-type, point to a concomitant decline in temperature. This is consistent with preliminary results obtained by the lipid paleothermometer HDI₂₆. Our study thus highlights the valuable insights provided by bioindicators, biomarkers and sediment geochemistry in a concerted approach about water quality and the health of the Stadtsee ecosystem during the Thirty Years' War.