Abstract
Many European lakes have suffered oxygen declines from recent eutrophication raising the question as to which conditions are representative of their pre-impact state. Here we present a palaeoecological record revealing that deepwater oxygen in small lakes can be variable over long timescales and subject to human influence for millennia. Remains of chironomids and other invertebrates indicate that Bichelsee (Switzerland) shifted from a stable, oxygen-rich to a dynamic, oxygen-poor state before major human activity. Initially, increased human activity and clearcutting coincided with improved oxygen conditions for deepwater midge larvae. However, from the Roman Age onwards, similar disturbances were associated with low oxygen and reduced deepwater invertebrate populations. These results demonstrate that small European lakes can have multiple, contrasting pre-impact reference states, and that ecological effects of human activity, even their directionality, may shift over time. Furthermore, repeated cycles of impact and recovery can occur over millennia, complicating assumptions about their baseline conditions.
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Data availability
Data supporting the findings of this manuscript (chironomid and aquatic invertebrate counts, percentages, influxes, 210Pb and 137Cs measurements, radiocarbon dates) are available in the Supplementary Data associated with this manuscript and on Dryad at https://doi.org/10.5061/dryad.gb5mkkx47. Data published in previous studies used to draw the figures can be found in the Supplementary Data and Dryad (https://doi.org/10.5061/dryad.gb5mkkx47) or in the respective studies cited in the manuscript.
Code availability
The code used to calculate the CCAs is available in Dryad at https://doi.org/10.5061/dryad.gb5mkkx47.
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Acknowledgements
We would like to thank the owners of Bichelsee, the Aktiengesellschaft J. Kindlimann and the Genossenschaft Badi Bichelsee for supporting and approving the sediment sampling. Furthermore, we would like to thank Richard Niederreiter, Martin Niederreiter, Daniel Niederreiter and Günther Praunias from UWITEC for their help during the coring. Further thanks go to Roman Schawalder, Heinz Ehmann and the Amt für Umwelt (Kanton Thurgau) for the water chemistry measurements performed in August 2025 as well as René Thoma for providing the aerial photo of Bichelsee and Dominik Schmed for helping during the aquatic invertebrate sample preparation. We also thank the three reviewers for their valuable comments on the manuscript. This work was funded by the Walter-Enggist Fonds.
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Pierre Lapellegerie: Conceptualisation, Data Curation, Formal Analysis, Investigation, Visualisation, Writing – Original Draft, Writing – Review & Editing. Simona Breu: Investigation, Writing – Review & Editing. Lucia Wick: Conceptualisation, Investigation, Writing – Review & Editing. Fabian Rey: Conceptualisation, Investigation, Visualisation, Writing – Review & Editing. Ieva Grudzinska: Investigation, Writing – Review & Editing. Nathalie Dubois: Writing – Review & Editing. Soenke Szidat: Investigation, Writing – Review & Editing. Sandra Camara-Brugger: Writing – Review & Editing, Visualisation. Urs Leuzinger: Conceptualisation, Funding Acquisition, Writing – Review & Editing. Oliver Heiri: Conceptualisation, Formal Analysis, Funding Acquisition, Visualisation, Writing – Original Draft, Writing – Review & Editing.
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Lapellegerie, P., Breu, S., Wick, L. et al. Dynamic deepwater invertebrate populations challenge the concept of oxygen-rich reference conditions for European lakes.
Commun Earth Environ (2026). https://doi.org/10.1038/s43247-026-03284-7
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DOI: https://doi.org/10.1038/s43247-026-03284-7
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