1. Seagrasses constitute a key coastal habitat world-wide, but are exposed to multiple perturbations. Understanding elements affecting seagrass resistance to disturbances is critical for conservation. Distinct biogeographical scenarios are intrinsically linked with varying ecological and evolution backgrounds shaped across millennia.2. We addressed whether the resistance (change in shoot abundances) and performance (change in leaf morphology and growth) of the seagrass Cymodocea nodosa to a local stressor, light reduction, varied across three regions (Southeast Iberia, the Balearic Sea, and the Canaries) within the temperate northern Atlantic realm.We hypothesized that distinct biogeographical scenarios, in terms of distinct ecological/environmental conditions and genetic diversity of meadows, would affect seagrass resistance and performance, with flow-on effects on associated epifauna. The same experiments, in terms of shading intensities, timing and duration, were replicated at three seagrass meadows within each region.3. Results demonstrated inter-regional variation in the resistance and performance of C. nodosa. Under moderate and high shading, shoot abundance was abruptly decreased, relative to controls, in the Canaries with concurrent, but less accentuated, changes in leaf morphology and no changes in growth. In the other two regions, however, moderate and high shading had a negligible effect on shoot abundance, leaf morphology, and growth. Shading had no overall effect over the total abundance and assemblage structure of epifauna; these faunal attributes, however, varied between regions. Low seagrass resistance at the Canaries is linked with the peripheral distribution of the species there, favouring isolation and decreased genetic diversity.

Aim: It has been predicted that there should be concordance between biogeographical and phylogeographical processes structuring multi-species regional assemblages.We hypothesise that oceanographic barriers in the marine environment affect concomitantly the distribution and the connectivity of the marine biota, thus producing congruent biogeographical and phylogeographical structures. We also predict that macro-and meio-eukaryotes will be differentially affected by hydrological features. Location:The Atlanto-Mediterranean transition along the E Iberian coast marked by the Almeria-Oran Front (AOF) and the Ibiza Channel hydrological discontinuities. Taxon: Eukaryotes.Methods: A new analytical framework based in the metabarcoding of community DNA and a hypervariable marker is presented. This framework allows the simultaneous detection of multispecies biogeographical and phylogeographical structures.Shallow hard-bottom communities were sampled at 12 sites over the littoral zone and community-DNA metabarcoding was performed using the cytochrome c oxidase I marker. The resulting dataset was analysed at several levels: beta diversity of Molecular Operational Taxonomic Units (MOTUs) as surrogate for species, and Exact Sequence Variants as surrogate for haplotypes. We also assessed genetic differentiation within MOTUs (metaphylogeography). Analyses were performed for the combined dataset and separately for macro-and meio-eukaryotes.Results: Both hydrological discontinuities had a detectable effect, more marked at all levels for the AOF than for the Ibiza Channel. The MOTU dataset provided more clear-cut patterns than the ESVs. The metaphylogeographical approach provided the highest resolution in terms of differentiating localities and identifying geographical barriers. The separate analyses of macro-and meio-eukaryotes showed a higher differentiation of the latter, both in terms of beta diversity and genetic differentiation.Main Conclusions: Metabarcoding coupled with metaphylogeography allowed the characterisation of the heterogeneity in community composition and population genetic structure along the Atlanto-Mediterranean transition, coherent with known

Against the accelerating pace of worldwide species extinction, reliable biodiversity assessments are critical, both as baselines and to track potential declines. DNA metabarcoding techniques allow for fast and comprehensive assessment of biodiversity in both terrestrial and marine habitats. However, these methods need to be adapted and standardised for each ecosystem in order to be effective. Seagrass meadows are among the most diverse marine habitats and are irreplaceable in terms of the ecosystem services they provide, yet metabarcoding has never been implemented for these systems. In this study, we developed and tested a protocol for metabarcoding the eukaryotic community of meadows of the iconic species, Posidonia oceanica L. (Delile). This seagrass is the main habitat-forming species in Mediterranean coastal waters and is known for its high diversity due to the structural complexity of its canopy and rhizome structures. This habitat is experiencing a range-wide retreat, and there is an urgent need for fast and efficient methods for its biomonitoring and detection of early changes. Our proposed method involves direct sampling of the community, collecting and processing the leaves and rhizome strata separately. To test the utility of the method in distinguishing between different meadow conditions, we sampled two distinct meadows that differ in their prevailing wind and surge conditions, and a nearby rocky reef for comparison. We then adapted a method and pipeline for COI metabarcoding using generalist primers that target the eukaryote diversity present. We detected a high diversity in the two meadows analysed (3,350 molecular operational taxonomic units, dominated by Metazoa and Archaeplastida) and a clear differentiation of the seagrass samples from those of the nearby rocky reefs. The leaves and rhizomes harboured clearly distinct assemblages, and differences were also detected between the two meadows sampled. This new tool has the potential to deliver big biodiversity data for seagrass habitats in a fast and efficient way, which is crucial for the implementation of protection and management measures for this key coastal habitat.