Paleoecological evidence for a multi-trophic regime shift in a perialpine lake (Lake Joux, Switzerland)

Freshwater ecosystems are under new and increasing threats from anthropogenic change. Our ability to detect and predict consequences of environmental perturbations on ecosystem function and water quality is limited by the lack of empirical data over broad temporal and spatial scales. Paleo-ecological records present a unique opportunity to broaden our understanding of past lake dynamics and ecological transitions over decadal to millennial timescales. We aimed to test the occurrence of regime shifts, and to track changes throughout the lake food web beyond the typical instrumental era, using both “traditional” paleo-ecological proxies (e.g. microzoo-plankton and -benthos, photosynthetic pigments) and more recently developed molecular genetic methods (sedimentary DNA). The history of human settlement and land-use practices around the deep perialpine lake Lake Joux, (Jura mountains, Switzerland), has been well documented since the Medieval period. We found paleo-ecological evidence for an abrupt and unprecedented food-web reorganization in the second half of the 20th century following several centuries of relatively stable communities and despite growing human pressure. Time-varying autoregression computed using dynamic linear modelling identified this transition, triggered by the onset of rapid cultural eutrophication in the 1950s, as a true regime shift. Since this time, despite decades of re-oligotrophication, lacustrine biotic communities of Lake Joux have not returned pre-disturbance composition, most likely because of other confounding factors such as climate warming that may prevent the lake from returning to an earlier equilibrium state. Our paleo-ecological reconstruction further reveals that cladocerans responded earlier to disturbance, which is highly relevant for lake monitoring and management strategies. We highlight the pertinence of multi-trophic level assessments to detect and characterise regime shifts dynamics.