Influence of leaf traits, hydrodynamics and global change on seagrass-grazer interactions

Abstract

Seagrass-grazer interactions play a fundamental role in ecological processes by regulating structure and functioning of plant communities. Within biotic connections, herbivory is a crucial process because it involves the matter and energy transference from primary producers to higher trophic levels, affecting the physical structure of the community and ecosystem productivity. Contrary to previous beliefs, new insights suggest that grazing may have an important impact on seagrass communities and that their grazers may be key to understand seagrass food webs. Thus, the contrasting response found in previous research may indicate that the susceptibility of seagrass species to be grazed may have a temporal and spatial component and even within a community, a preferential consumption of some species among others may exist depending on the palatability of their tissues. Therefore, the assessment of the strength of the seagrass-grazers links and those mechanisms that regulate this relationship, it is of crucial importance to increase our understanding on community and ecosystem structure and dynamics. In addition, it is well known that seagrasses are living under local and global threats, and consequently, there is a pressing need to include the global change factors (eutrophication, acidification and warming) into the current experimental approaches, in order to forecast the future responses of this fundamental community in the changing world.

This PhD Thesis is structured in 6 chapters to answer particular questions about seagrass-grazer interactions. In the first two chapters, the importance of hydrodynamics in the seagrass-grazer interactions was studied, and it demonstrated how this abiotic factor acts as a top-down regulator in the community. Moreover, seagrass acclimation to hydrodynamics modulates intraspecific seagrass traits, and then seagrass palatability, influencing the selection of leaves by consumers (chapters 2 and 3). On the other hand, eutrophication and the main global change factors (i.e. warming and acidification) influenced positively Cymodocea nodosa palatability through the alteration of leaf traits and by promoting the growth of epiphytes (chapters 4 and 5). In the last chapter, we looked at the seagrass responses to biomass loss promoted by herbivory, noting how the colonization states influences on the compensation responses to herbivory in the fast-growing tropical species such as Halodule wrightii. The research presented in this PhD Thesis contributes to identify changes on seagrass palatability under different environmental conditions and global change factors. Particularly, this research displays how hydrodynamics modulate feeding behaviour of consumers and how indirect effects of eutrophication and global change enhance seagrass consumption by herbivores. In addition, this Thesis shows how seagrass colonization states affect to compensatory responses derived by herbivore damage.

The importance of considering biotic processes in conservation and management plans is emphasized in order to enhance seagrass ecosystems resilience and to identify solutions to support the maintenance of key ecosystem services provided by seagrass meadows to our planet.