Field evidence of the resilience of the seagrass Posidonia oceanica to global warming effects

Sea surface temperature has risen globally in the last decades and in the Mediterranean Sea, a semi-enclosed basin considered a natural laboratory, the rate of increasing temperature seems to be even faster. As a consequence of the global warming, the occurrence of extreme climatic events, such as marine heat waves (MHW), has been increased recently. In the Mediterranean Sea, the endemic seagrass Posidonia oceanica (L.) Delile represents one of the most productive and valuable ecosystems. Meadows are vulnerable to climate change, and regression and declining growth rates after MHW for P. oceanica meadows have been already observed. The main aim of this thesis was to assess the response of P. oceanica to past, present, and future temperature conditions and MHWs at different functional levels (biochemical, physiological, morphological and molecular) in different phases of the seagrass life cycle. At this aim several mensurative and manipulative experiments were done in the field. Predicting the future responses of P. oceanica has become crucial to develop management, conservation and restoration strategies. The results showed how past MHW events negatively affected the yearly production of rhizome biomass, but they have also highlighted an overall resilience of P. oceanica to current and future MHWs. The importance of the local environmental conditions of the site where the plant lives in improving the ability of the plant to cope with single or multiple stressors has been described. Consequently, an acclimation period might be a good strategy for a successful restoration. Results also highlighted that, even though morphological and biochemical variables are good stress-indicators, molecular tools provide information for a more complete interpretation of the plant response. Overall, the findings are not discouraging about the performance of P. oceanica in the future climate change scenario, especially for those plants that live in very fluctuating thermal conditions.