Mediterranean macroalgal forests under threat: The effects of ongoing climate change and design of restoration methods
Macroalgal forests represent some of the most productive and biodiverse habitats on Earth. In the Mediterranean Sea, species of the genus Cystoseira sensu lato (including Cystoseira C. Agardh, Ericaria Stackhouse and Gongolaria Boehmer) dominate the wellpreserved subtidal rocky habitats where they form complex macroalgal communities. However, since the end of the twentieth century, they have suffered important and widespread regressions across the Mediterranean Sea, mainly due to habitat destruction, changes in water quality and overgrazing by sea urchins. As a result, they are often replaced by simpler and less productive communities dominated by opportunistic and stress-resistant taxa that prevent Cystoseira s.l. settlement. Thus, the return to predisturbed conditions does not necessarily imply the natural recovery of Cystoseira s.l. stands, being active restoration the only feasible alternative to assist the recovery of these populations. Nowadays, climate change, especially gradual warming and acute marine heatwaves (MHWs), represents a global threat for macroalgal forests. However, local and regional factors and processes can modify climatic gradients and ultimately define responses of seaweed populations to warming. In this context, while no studies have reported evidence of warming impacts for Cystoseira s.l. forests, taking into account that Cystoseira s.l. populations have been historically impacted and the expected climate change scenarios for the Mediterranean Sea, it is of paramount importance to know how climate change can affect these species. Within the framework of this thesis two well-differentiated main objectives have been addressed. First, we aim at understanding how climate change may affect Cystoseira s.l. populations. Secondly, we design and experimentally test two new non-invasive restoration techniques for Cystoseira s.l. populations. In order to address the first goal, a manipulative thermo-tolerance experiment has been performed under controlled conditions to study the direct effects of temperature on Ericaria crinita at the individual-level. Moreover, in-situ observational ecological and environmental data have been analysed in order to elucidate how local factors may modulate between-population responses to MHWs. Finally, by means of another manipulative experiment under controlled conditions, we have characterized the thermo-tolerance of eight populations of Ericaria crinita coming from different thermal regimes. This information has then been combined with modelling approaches to forecast species response under a future climate scenario. Our results show that warming may severely impair the survival of Ericaria crinita, especially at early-life stages. Moreover, they suggest that local-scale environmental heterogeneity mediate population responses to MHWs highlighting the potential for local scale climatic refugia. Finally, we find intraspecific variability in the thermo-tolerance responses of Ericaria crinita, with populations from coldest areas being less tolerant. Our predictions show that such variability shapes species response to warming, which highlights its importance when forecasting species vulnerability under future climate change scenarios. Regarding the second objective, we have designed two restoration techniques based on obtaining new recruits (directly in situ and growing them ex-situ in laboratory conditions) from fertile branches of donor populations. We have also applied the designed techniques and we have established a mid-term monitoring program to assess the restoration success. Both techniques have resulted to be cost-effective to recover Gongolaria barbata populations after 6 years and with only one restoration action. In summary, our results show the potential effects of warming on populations of shallow Cystoseira s.l. species, pointing out the relevance of local environmental factors and processes ultimately defining the response of these populations to global trends of climate change. Moreover, we also offer new and promising tools to locally restore these habitats. Overall, our findings are relevant in order to inform local-scale management and conservation plans for safeguarding the persistence of these Mediterranean macroalgal forests. ; I have had the financial support of a 3-year fellowship of the University of Girona (IFUdG 2016), and a travel fellowship also financed by the University of Girona. The research conducted on this thesis has been supported by the European Union projects MERCES (No. 689518) and AFRIMED (No. EASME/EMFF/2017/ 1.2.1.12/S4/01/ SI2.789059), the Spanish Ministry ANIMA project (No. CGL2016-76341-R, MINECO/FEDER, UE) and the Intramural CSIC-PIE project (202030E180). ; Peer reviewed