Life history, population dynamics and conservation of underwater Mediterranean forests: insights from the long-lived alga Cystoseira zosteroides = Història de vida, ecologia de poblacions i conservació dels boscos submergits del Mediterrani: el cas de l'alga longeva Cystoseira zosteroides

Life history theory and population ecology have been especially neglectful of marine species, with most ecological principles developed from studies based on terrestrial species. This is especially true for macroalgae species, for which population dynamics and life history studies are still scarce. Given their fundamental role as habitat-forming species and primary producers in temperate seas worldwide, understanding the dynamics of macroalgae populations is fundamental, not only for their own conservation but also for their associated biodiversity and marine coastal ecosystems functioning. Therefore, the present dissertation aims to unravel some of the processes and mechanisms that shape the population dynamics of the deep-water, long-lived and habitat-forming macroalga, Cystoseira zosteroides. With this information, we aim to bolster our ability to predict the future of this species in a globally impacted world, as well as to develop management tools to improve their conservation status. The results obtained from our monitored populations show that C. zosteroides have slow population dynamics. By using comparative analyses, we observed contrasting life history strategies among macroalgae species. While other intertidal fucoid and kelp populations highly depend on reproductive processes and the growth of organisms, natural and undisturbed C. zosteroides populations are maintained by the high survival and long lifespan of adult individuals. We also demonstrated that the dispersal ability of brown macroalgae is very limited compared to other taxa, with kelps showing higher dispersal potential than fucoids. Our findings showed that disturbances highly influence C. zosteroides population dynamics. After an extreme storm and the impact of a ghost fishing net, their populations displayed a high increase in recruitment rates, suggesting a negative density-dependence effect of adults on early stages. We observed that in recruitment plates located inside well-developed adult canopies post-settlement survival is lower than outside the adult canopy This evidence that adult C. zosteroides individuals establish a ceiling for the development of recruits. Thus, density-dependence plays a key role regulating C. zosteroides population dynamics, triggering their recovery after major disturbances. In contrast to natural populations, after major mortality events, their recovery and dynamics highly depend on the reproductive process. This is particularly worrying given the limited effective dispersal of this species. Finally, despite the high ability of C. zosteroides to compensate morality pulses through density-dependence, their recovery can take decades, rendering their populations more vulnerable. Although isolated mortality events can be compensated, our demographic simulations showed that the combined effect of physical disturbances, compromise the viability of C. zosteroides populations. Furthermore, we demonstrated that warming has a high impact on early life stages of this species. Decreased early survival and settlement rates due to rising temperatures delay the recovery ability of C. zosteroides and increase the vulnerability of their populations. With this dissertation we evidence the key role of population ecology and life history into understanding the dynamics of species, stressing how these disciplines may help us to better comprehend the future of coastal marine ecosystems.