Submerged macrophytes as key players in aquatic ecosystems under global changea multiscale experimental approach

Resumen

Current global change is imposing alterations in the ecosystems worldwide through interactive changes in main environmental factors (e.g. temperature, nutrient concentration and ultraviolet radiation). Freshwater ecosystems are highly vulnerable to these changes and, specifically in the Mediterranean region, the situation is worse since the majority of them are shallow, exposed to environmental and anthropic disturbances. The meadows of submerged macrophytes, and particularly, charophytes, are a conspicuous element of these systems with a crucial role for their functioning. They provide habitat for both planktonic and benthic organisms and maintain water quality by limiting phytoplankton growth, reducing nutrient loading and preventing sediment resuspension. However, these meadows are declining critically due to current global change and this thesis addresses the performance of submerged macrophytes and the foreseeable impacts in the ecosystems they inhabit in the context of a changing world. The main aims were i) to investigate the specific and infraspecific responses of charophytes facing the interactive effects of global change-related factors, ii) to elucidate the propagation of these effects through the meadow-associated biological community, emphasizing the relevance of non-trophic relationships, and iii) to disentagle the role of charophytes in the functioning of Mediterranean shallow lakes facing the foreseeable changes and focusing on the sediment microbial community. These goals were addressed through microcosm experiments with a common garden approach with coastal and hig-mountain populations of two charophyte species, laboratory mesocosms simulating macrophyte-dominated shallow systems and field in-lagoon mesocosms with macrophytes meadows in a coastal ecosystem. We found both species- and population-specific patterns in the response of charophytes to concomitant environmental changes regarding growth, morphologic and metabolic variables. The coastal populations came up as those with the greatest phenotypic plasticity to overcome the expected environmental changes. On a community scale, through a network approach, a charophytes-zooplanktonic herbivores tandem emerged as crucially important for the structure of the aquatic community. Furthermore, contrasting configurations (phytoplankton and macrophyte-dominated) were achieved by subjecting the communities to ultraviolet radiation and warming scenarios, respectively. Transferring this approach to natural ecosystems allowed the emergence of different patterns of benthic-pelagic coupling between ponds and lakes. Finally, we assessed how charophytes meadows influence the sediment microbial community by favouring denitrification, thus, impacting on the functioning of aquatic ecosystems. This thesis has contributed to depict the complex puzzle of shallow freshwater ecosystems placing charophytes meadows as a central piece in their structure and functioning within the current global change context.