Biodiversity-ecosystem functioning relationships in marine habitats: Insights from case studies with natural biodiversity variation

This work examines biodiversity-ecosystem functioning relationships in coastal marine habitats by linking habitat structure, associated biodiversity and benthic metabolism. It focuses on natural systems where biodiversity and environmental conditions vary in space and time, rather than on simplified experimental gradients. The first part develops and applies a multivariate framework to quantify seagrass structural complexity as a latent ecological property. Using Cymodocea nodosa meadows as a case study, the thesis shows that shoot density alone does not adequately represent habitat complexity, because canopy traits, biomass allocation, epiphytes and below-ground structures vary independently and nonlinearly. A Structural Complexity Composite Index was therefore used to describe meadow architecture and assess its relationship with macroalgal and macrofaunal assemblages. Results suggest that complexity is more informative for community composition and species turnover than for local alpha diversity, highlighting the value of heterogeneous meadow configurations.The second part shifts from biodiversity patterns to ecosystem functioning, using aquatic eddy covariance to measure benthic oxygen fluxes under natural field conditions. In mixed Cymodocea nodosa-Caulerpa prolifera habitats and naturally acidified systems at Vulcano Island, metabolism was interpreted through species identity, biomass distribution, sediment biochemistry, hydrodynamic footprint and seasonal context. The findings suggest that ecosystem metabolism cannot be attributed to biomass alone, but emerges from interacting autotrophic, heterotrophic, sedimentary and physical processes. Overall, the thesis contributes an integrated, cautious framework for studying BEF relationships in complex marine habitats, emphasizing that biodiversity effects depend on structure, traits, species identity, sediments, and environmental forcing.