Quantifying ecological resilience in lakes – bridging theory and management

Abstract: Ecological resilience, defined as the capacity of an ecosystem to withstand disturbances without losing original structures, processes and functions, has remained elusive and challenging to quantify. In this thesis, two methods were used to assess the resilience of aquatic communities to different stressors and management practices. Time series modelling and discontinuity analysis were used to quantify scaling structures and dynamics, both relevant for ecological resilience. The aims of this work were to: (i) evaluate the performances of two time series modelling approaches: Asymmetric Eigenvector Maps (AEM) and Moran’s Eigenvector Maps (MEM), and determine which is best suited to study ecological resilience, (ii) assess the efficiency of liming as management technique to restore lakes to pre-acidified conditions, (iii) investigate if acidified lakes (degraded systems) are more vulnerable to environmental change compared to near-pristine circumneutral lakes, and (iv) investigate regional climatic effects on the resilience of shallow lakes using an experiment along a latitudinal gradient that manipulated nutrients and ecosystem size. AEM consistently outperformed MEM with differences in modelled scale-specific patterns. These results suggest that the outcomes are dependent on the modelling approach and that AEM might be better suited to assess resilience when temporal trends are relevant. Liming improved species richness and diversity, but resulted in communities that are neither typical of acidified nor of circumneutral lakes. The resilience attributes were similar across all lakes; however both limed and acidified lakes had higher proportions of species with stochastic dynamics. These findings suggest that liming is unable to break the feedbacks that maintain the system in an acidified state. Acidified and circumneutral lakes were found to be equally vulnerable to environmental change as both lake types had similar resilience characteristics. Similar resilience attributes were also observed in the experiment across countries, except when severe drought conditions occurred that apparently led to lower resilience.