Coastal ecosystems are highly threatened worldwide by multiple anthropogenic stressors that act at a range of spatial scales, from local to the global, and adversely affect their ecological functions and associated biodiversity. Global warming is one of the most pervasive stressors, and the assessment of how the species (or other levels of biological organization) react to it is an urgent need in a rapidly warming world. Moreover, thermal stress rarely acts in isolation from other stressors. The potential synergies among global warming and local stressors is of particular concern in what it regards foundation species, such as the case of seagrasses, due to their crucial role in maintaining the structure and function of coastal ecosystems. The main objective of this PhD thesis is to improve the knowledge of how warming alone and in combination with different local factors can affect seagrasses. This research has been conducted based upon mesocosm experiments submitted to different temperatures (and, in some cases, to other agents), and plants responses measured from biochemical to population levels. The results obtained are considered an approach to what may occur in the real world, always acknowledging the limitations of our methodology. Chapter 1 revealed different tolerance to warming among the two main Mediterranean seagrass species, Cymodocea nodosa and Posidonia oceanica. C. nodosa tolerates temperature increases much better than P. oceanica probably due to its life story (opportunistic), habitat (from confined waters to the open sea) and biogeographical affinity (tropical and subtropical). This will potentially cause changes in the distribution area of these two species in the Mediterranean under a future scenario of warming. As shown in Chapter 2, an increase in nutrient concentration in water does not modify the response of C. nodosa to warming. However, the increase of organic matter in sediment clearly worsens, synergistically in some plant traits, the effects of warming, entailing a hazardous combination for plant survival. P. oceanica, in turn, is severely affected by conditions of high nutrient content and high temperatures (Chapter 3), again displaying synergistic effects, and confirming not only a thermal sensitivity in this species greater than in C. nodosa, but also a greater vulnerability to the exacerbation of thermal effects by other local stressors. Finally, the interactive effects of warming and salinity (Chapter 4) in an estuarine seagrass species, Halophila ovalis, in southwestern Australia resulted beneficial for plant survival, as the negative effect of warming was buffered by concomitant salinity increases. Overall, this research highlights the complexity of global warming effects in at least two aspects. Firstly, the multiplicity of biological levels at which those effects act and, secondly, the importance of studying not only isolated effects of temperature increases but also their joint effect with other stressors. Advances in these two directions will yield more realistic predictions concerning global warming and seagrass ecosystems and help to develop management policies to protect seagrass ecosystems in a changing world.

Programa de doctorat: Ecología, Ciències Ambientals i Fisiologia Vegetal