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Canopy-forming brown algae support highly productive ecosystems whose decline has been attributed to the interplay of several anthropogenic disturbances. Climate change could have disruptive effects on the biology of these species, but the role of temperature in the development of early life stages is poorly understood. The aim of this study was to assess the response of Ericaria giacconei, a winter-reproducing Southern–Mediterranean endemic species, to thermal stress by testing five temperatures (12, 15, 18, 24, and 28°C) on adults and early stages. Chlorophyll a fluorescence of adult plants was measured at 0, 24, 72, and 120 h on nine fronds in each of the three aquaria per treatment. To assess egg release, zygote settlement, and embryo growth rate, approximately 1,200 receptacles were cultured on six Petri dishes per temperature treatment, and 10 random subsections of 2 ×2 mm were examined in three Petri dishes at 0, 20, 44, and 92 h after fertilization. Adult plants showed a plastic physiological response, and thermal stress had no significant effect on PSII efficiency. Embryos fully developed only at 12 and 15°C. Mortality increased at 18 and 24°C, and no zygotes survived at 28°C. In a scenario of further increasing temperatures, the effects of warming could affect the recruitment of E. giacconei and increase its vulnerability to further stresses. These effects on the survival of early stages, which are the bottleneck for the long-term survival of the species, should be taken into account in conservation and restoration measures to maintain canopy-forming macroalgal populations and associated biodiversity and ecosystem services.

In coastal ecosystems, large brown macroalgae are important foundation species, growing on rocky shores from temperate to polar regions. They form marine forests, also known as kelp forests, which are among the most productive coastal marine ecosystems in the world. Kelps are of high ecological and economic value, however, climate change poses a great threat to these ecosystems. Warming has vast implications on marine forests and their global biogeographic patterns, but also other drivers affect kelps and strong interactive effects are observed. The aim of this study was to gain a deeper understanding of the inter- and intraspecific acclimation processes of kelp species to abiotic conditions along large spatial and environmental gradients. The thermal tolerance of the broadly distributed kelp Saccharina latissima towards marine summer heatwaves across latitudes and its biochemical and morphological variability across its entire distribution range in Europe was investigated. Seasonal and inter-annual differences in the susceptibility of S. latissima to marine heatwaves in summer were revealed and the impact of potential interactions of marine heatwaves in summer paired with hyposalinity or enhanced nutrient availability was analyzed. The combined effect of different temperatures and salinities on physiological and biochemical response variables was also investigated in young Laminaria solidungula sporophytes. In conclusion, this thesis presents important information on the functional variability of two kelp species with different distributional ranges. Both studied species exhibit different sensitivity towards drivers related to environmental change. Furthermore, intraspecific variation must not be underestimated. The observed changes in the kelp populations of Europe, thus, cannot exclusively be ascribed to temperature variation but to an interplay of various abiotic and biotic factors. These findings contribute further knowledge to support future approaches to the conservation of marine forests.

1. The worldwide decline of marine forests, due to human impacts and climate change, emphasizes the need to develop and implement effective and sustainable solutions to restore these endangered habitats and to re-establish the services they provide. 2. In this study, the ex situ restoration of Treptacantha barbata, a Mediterranean subtidal habitat-forming species of brown seaweed, was for the first time implemented in a marine protected area in the Adriatic Sea. Two restoration efforts were performed in 2019. The first one was started in winter, after a marine heatwave that triggered early fertility, the second one in spring, when the species usually reproduces. 3. This study aimed to evaluate: 1) the disruptive effects of a thermal anomaly on the reproductive biology and performance in culture of T. barbata; and 2) the impact of the grazing pressure on juveniles after the outplanting. 4. The first cultivation was more productive than the second one, in terms of zygote release and germling growth. To mitigate the low efficiency of the second culture and to avoid prolonged highly-demanding maintenance in the mesocosms, the cultivation period was extended outdoors using a structure suspended in the water column. 5. The modular frames conceived for outplanting T. barbata proved to be effective because of their easy operability and low cost. Controlling for herbivorous fish had significant positive effects on both juvenile survival and growth. 6. The outcomes highlighted that an unpredictable climatic event and fish grazing were major threats that impaired the restoration process of T. barbata. These stressors should be considered when developing plans to implement effective large-scale restoration of canopy-forming macroalgae.