SPATIAL VARIABILITY AND ADAPTATION OF MEDITERRANEAN CORALLIGENOUS REEFS IN THE CURRENT AND GLOBAL WARMING SCENARIO

Variability in temperature explains much of the spatial and temporal patterns we observe in the distribution and abundance of species worldwide. Taking advantage of the natural differences in temperature conditions between different biogeographical regions in Sardinia (Italy) the present Doctoral Dissertation aims to estimate the spatial variability, and adaptation of an iconic habitat of the Mediterranean Sea, the coralligenous reefs, both under current conditions and into global warming scenario, focusing in particular on one of the most important bio-constructor of the coralligenous habitat, the crustose coralline algae Lithophyllum stictiforme. The main questions that triggered my research hypothesis are: i) Does the structure of coralligenous assemblages change spatially in different biogeographic areas of Sardinia? ii) Is the spatial variation of the structure of coralligenous assemblages consistent through depth? iii) Is it possible to relate and thus predict differences in habitat structure based on thermal environment? iii) What is the response of one of the main builders of this habitat, the coralline algae L. stictiforme, to warming? In order to answer these questions, four studies, three descriptive and one manipulative, were conducted.In Chapters 1 and 2, the spatial variability of the deep coralligenous assemblages in different biogeographical areas around Sardinia and the vertical distribution of the coralligenous cliff in three different biogeographical areas of the Mediterranean Sea was evaluated, respectively. The observed variations seem largely related to biogeographic patterns rather than spatial distance and the results support the hypothesis that coralligenous assemblages, unaffected by local anthropogenic disturbance, may be relevantly different in structure and indicate that the lack of iconic species such as gorgonians and bryozoans, could merely be the result of biogeographic models probably related to the thermal environment.In Chapter 3, the knowledge of the subtidal climatology has been improved to be able to identify temperature descriptors that can be useful predictors in the structure of the coralligenous community and evaluate the relationship between the subtidal temperature and the change in the community structure. The aims were twofold: i) to evaluate if such heating descriptors can be useful predictors of the coralligenous reef community structure, ii) to identify the structure and the taxa of the community associated with the current heating events, so to provide tools to draw the trajectories of change of this community due to future warming scenarios. All the heating descriptors selected influenced several coralligenous response variables so that they all might be considered useful predictors for climate change investigations on the coralligenous reef.Finally, in Chapter 4, manipulative experiments on L. stictiforme were performed for the first time in the field to examine the temperature effects on the performance of the algae. Two experiments were done: in the first, algae were cross transplanted from a cold site to a warmer one and vice versa using two different depths, while in the second, transplants were done from 34 m to 15 m of depth within the same site to evaluate the influence of the thermocline, which is predicted to be increasingly deeper and persistent due to global warming. The main goal was to understand if L. stictiforme is adaptable to different thermal environments and identify the changes in thallus performance due to the different temperatures. An increase in temperature positively affected the alga responses, highlighting an unexpected adaptability of this crustose coralline alga and providing useful information to forecast the effects of warming and thermocline deepening on this habitat bio-constructor and to draw up future guidelines for restoration efforts of the coralligenous habitat.