Dynamic deepwater invertebrate populations challenge the concept of oxygen-rich reference conditions for European lakes

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.