Abstract
Many lake ecosystems that have been severely disturbed by eutrophication, have also experienced largehuman efforts to restore“natural”conditions. However, the trajectories and the extent of recovery of these lakeecosystems are still poorly understood. In many shallow lakes, recovery was often delayed and counter-clock-wise hysteretic. Here, we study recovery and ecosystem trajectories in a large and deep lake using diatomremains in sediment cores and time series of phosphorus concentrations. We identified four periods of diatomcommunity change: slow change during early eutrophication, thereafter a short period of rapid change after the1950s, followed by community stability from the 1960s to the mid-1980s, andfinally a recovery phase until2010. Diatom community structure responded quickly and in a saturating way to increasing phosphorus con-centrations, but also fast to phosphorus decline. Hence, diatom community dynamics did not show counter-clockwise hysteresis but was characterized by a high degree of recovery and clock-wise hysteresis (CWH). Wesuggest that CWH in response to eutrophication and recovery is a typical and previously overlooked feature ofdeep lakes, which results from a more rapid change of average nutrient concentrations and thus productivity inthe epilimnion compared to average nutrient concentrations across the entire water column. Such nonlinearand hysteretic responses to changing nutrients need to be considered when analyzing the effects of otherstressors such as climate warming on ecosystem dynamics to prevent erroneous attribution of ecosystem changeto other stressors instead of nutrient change.