Modelling impacts of climate change on mega-fauna habitats in the Mediterranean Sea

The Mediterranean Sea is a semi-enclosed basin that provides a variety of ecosystem services, including fisheries, climate regulation and recreational services. Nevertheless, it is subject to significant anthropogenic exploitation, and it is considered as one of the regions that is likely to be most affected by climate change. It also hosts an high biodiversity between marine megafauna and other species, which will be indeed threaten by the rising of sea temperature and the increasing of CO2. Research into the impacts of climate change on land and coastal regions is ongoing, but research into the impacts on pelagic waters is not yet complete. The initial focus of the present study was on the phytoplanktonic bloom that occurs in the North-Western Mediterranean during spring, which is subsequently followed by grazers and zooplankton. This bloom is indirectly linked to the presence of marine mammals in summer. Chlorophyll is the most effective method to study the phenology of this event, and it enabled the determination of the strength and duration of the bloom. A total of 24 years of chlorophyll data (1998-2022) were analysed, and it was found that significant negative trends were more pronounced in the last 6-7 years. In the context of increasing global concern regarding climate change, the monitoring of indicators such as phytoplankton is imperative for a comprehensive understanding of its impact on the marine ecosystem in its entirety. Cetaceans play a pivotal role in marine ecosystems as upper trophic species, facilitating the transfer of nutrients both vertically, from deep waters to the surface, and horizontally, thereby ensuring the maintenance of balanced and productive plankton communities. For these reasons, they have been designated as "ecosystem sentinels". It is an inevitable consequence of rising sea temperatures that marine mammals will undergo range shifts. Habitat modelling is the only resource that can be used to predict such shifts and to plan local action to mitigate the impacts. The high-emission scenarios RCP 8.5 were utilised to project the distribution of four marine mammal species in both the near and long term. The results demonstrated a clear shift between the three periods considered: 2008-2022, 2035-2055 and 2075-2095. While the core habitat appeared to expand over time, the degree of uncertainty increases as the study progresses and considers distant-future projections. The Mediterranean Sea is facing a number of threats and undergoing rapid changes. Rather than seeking an absolute truth, the objective was to enhance understanding, provide insights, and contribute to the ongoing scientific debate on how these ecosystems may respond to future environmental change.