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Francesco Polazzo defended his thesis on the effects of anthropogenic stressors on the stability of freshwater communities and ecosystems

Francesco Polazzo, researcher at the IMDEA Water Institute, has defended his doctoral thesis at the University of Alcalá.

The thesis, framed within the Doctoral Program in Hydrology and Water Resources Management, has the title "Multiple stressor effects on community stability" and has been directed by Dr. Andreu Rico, associated researcher at IMDEA Water. This doctor's degree is granted by the University of Alcalá and the Rey Juan Carlos University.

Humans are posing increasing pressure upon natural ecosystems, threatening their diversity, stability and functioning across the planet. In addition, a large number of anthropogenic stressors, such as eutrophication, chemical pollution and extreme events, act simultaneously. In this thesis, new approaches to understand the individual and combined effects of anthropogenic stressors on the stability of freshwater communities and ecosystems were evaluated and implemented. This was done by first reviewing the available literature on the combined effects of heat waves and chemical pollutants, highlighting knowledge gaps and suggesting efficient ways to integrate extreme events into the assessment of the environmental impact of multiple stressors. This thesis also relied on large outdoor mesocosm experiments to investigate the effects of multiple stressors on freshwater communities and ecosystems. In this work, the effects of three agricultural stressors (two pesticides and nutrient pollution) were analysed at the population, community and ecosystem levels, and their impacts on ecosystem stability. Moreover, this thesis also studied the dimensionality between parameters describing this stability. Furthermore, the potential of food webs to elucidate the mechanisms driving compositional dissimilarity of aquatic communities after the application of a point disturbance was explored, and the mechanisms influencing the emergence of interactions between stressors in the long term were assessed. Finally, this work analysed the effects of extreme events, such as heat waves, on the complexity of freshwater ecosystems using topological and quantitative food web metrics. In that sense, changes in food web complexity were linked to the four components that define the stability of the system, and that characterise the functional, structural and energy flow stability among the species that make up the food web. Overall, this thesis has proposed a general approach to elucidate the effects of global change on ecosystem stability, providing useful information for preserving ecosystem structure and functionality under changing environmental conditions.

This doctoral thesis is part of the ECORISK 2050 project, funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 813124.