Role of macrophyte interactions in seascape pattern formation and ecological resilience

[eng] Understanding ecosystems in a context of critical regime shifts is fundamental to ecology. In this thesis we study ecological interactions in seagrass meadows which determine the seascape configuration, the associated functions and ecosystem resilience. We focus on two fundamental properties of the seascape: (i) macrophytes biodiversity and (ii) fragmentation of the vegetation cover in the ecosystem. In chapter 4.1, we collect information on seagrass meadows worldwide —coordinates, monospecific or mixed status, species identity and shoot density. We detect that global interspecific coexistence is greater in tropical latitudes and that coexisting species follow three main patterns: (i) species that only form monospecific meadows, (ii) species that exhibit lower shoot density in conditions of mixed meadows, and (iii) species that exhibit the same range of monospecific and mixed shoot density. In the experiment described in Chapter 4.2, we analyze plant growth according to the level of intra- and interspecific coexistence in two species associations: Cymodocea nodosa – Zostera noltii in the Mediterranean and Cymodocea rotundata – Thalassia hemprichii in the Indo-Pacific Ocean. We observe a reduction in growth as levels of coexistence increase. Finally, in Chapter 4.3, we study interspecific coexistence mechanisms between C. nodosa and the macroalga Caulerpa prolifera, for which we assume an indirect interaction mediated by sulfides in the sediment, in Alfacs bay in the Ebro Delta (Catalunya). In mixed conditions, we detect greater sulfide accumulation in the sediment, which isn’t associated with greater sulfide intrusion into plant tissues. Conversely, in the mixed meadow, we detect unexpectedly higher metabolic activity which suggests a compensatory reoxidation of sulfides. We also detect that C. nodosa penetrates deeper the rhizosphere when coexisting with C. prolifera. These three studies allow us to conclude that competition is the dominant interaction among seagrass species, interplaying with species-specific mechanisms and the environmental conditions, to shape the interspecific coexistence in the ecosystem. In Chapter 4.4, we study the relationship between Posidonia oceanica fragmentation in Pollença bay (Mallorca), hydrodynamics, and community oxygen production. This relationship reveals a threshold beyond which fragmentation and water velocity favor seawater oxygenation. Nonetheless, across the wide range of fragmentation found in Pollença, oxygen production is expected to be strongly constrained by vegetation loss linked to higher fragmentation. In Chapter 4.5, 4.6 and 4.7, we study a highly fragmented pattern, called fairy rings, in P. oceanica in Pollença bay. We develop and validate a mechanistic model, based on the interaction between seagrass clonal growth and sulfide dynamics, which describes the formation of fairy rings. The results of this chapter suggest a regime shifts in the ecosystem, leading to vegetation collapse. Finally, in Chapter 4.7, we identify the genetic structure of fairy rings, which allows us to reject recolonization of the inner ring space by new genotypes. As a whole, the three chapters demonstrate the self-organization process underlying fairy ring formation and its implication for ecosystem resilience.