• anthropogenic disturbance
• climate change
• coastal systems
• habitat-formers
• macroalgal forests
• restoration

Climate change and increased human activities threaten coastal biodiversity, particularly affecting key species like habitat-forming macroalgae. As habitat formers provide refuge to sensitive species in harsh environments, their presence is crucial in stressed ecosystems. To date, there have been few empirical assessments of: i) the spatial and temporal consistency of the ability of habitat-forming macroalgae to modulate physical conditions and associated diversity; ii) how current and predicted environmental changes may impact the functioning of such systems; iii) how these responses may be modulated by interactions with other biological components. Therefore, the first objective of this thesis was to investigate the extent to which intertidal macroalgal canopies buffer environmental conditions and their effect on the associated diversity (Chapter 2). Through a distributed experiment along the Atlantic and Mediterranean coast, physical variables and diversity were compared between intertidal areas with or without canopy-forming macroalgae, in warm and cold periods. Th results showed the ability of macroalgae to ameliorate environmental conditions, with effects on diversity varying between basins, likely depending on dominant species life traits and regional environmental factors.
A generalized decline of macroalgal forests has been documented across the Mediterranean basin due to local and global stressors, causing their fragmentation and replacement by alternative states characterized by lower structural complexity. As a second aim, the thesis investigated the functioning of subtidal benthic communities in areas with different levels of anthropogenic pressure. Through an in situ incubation experiment, I compared the functioning of benthic communities between pristine and urban areas ( Chapter 3). Through light and dark incubation, community net primary production and respiration were estimated. Contrary to initial expectations, the communities from pristine and urban environments exhibited comparable net primary production and similar structure, suggesting the potential ecological value of both macroalgal canopies and alternative habitats on the mainland coast of Tuscany. Subsequently, through a mesocosm experiment, I assessed the impact of an extreme warming event on the functioning of the habitat-forming macroalga Ericaria brachycarpa and its associated community (Chapter 4). Rock with E. brachycarpa and associated organisms were exposed Control (23 °C) and MHW (30.5 °C, for 5 consecutive days) conditions. Results indicated low primary productivity and reduced carbon uptake in communities exposed to MHW. The shift from autotrophic to heterotrophic metabolic balance of communities in response to thermal stress suggests potential alterations in community functioning under future climatic regimes.
Macroalgal forest degradation has prompted increasing attention to strategies for their conservation and restoration. However, the success of such efforts is often hindered by unsuitable abiotic and biotic conditions during post-settlement phases. In this context, I evaluated crucial factors for the recruitment of habitat-forming macroalgae (Chapter 5). Specifically, I assessed the ability of Ericaria amentacea zygotes to settle on substrates pre-colonized by epilithic microbial assemblages from sites exposed to different human pressures. Bare panels were deployed at pristine and urban sites to allow bacterial biofilm colonization. Then, in the laboratory, E. amentacea zygotes were released on the pre-colonized surfaces to assess germling survival. Higher zygote settlement rates were observed on surfaces from pristine than urban sites, potentially influenced by variations in biofilm composition revealed by DNA metabarcoding analysis. These findings highlight the importance of assessing target substrates’ suitability before implementing restoration strategies in coastal habitats. This research enhances understanding of the role of canopy-forming macroalgae as providers of biogenic habitats with implications for the conservation and restoration strategies of key species and habitats under the ongoing climate change and increase of human pressures in coastal areas, a main focus of current ecological research worldwide and national and European environmental directives.