Climate change is causing an increase in the frequency and intensity of marine heatwaves (MHWs) and mass mortality events (MMEs) of marine organisms are one of their main ecological impacts. Here, we show that during the 2015-2019 period, the Mediterranean Sea has experienced exceptional thermal conditions resulting in the onset of five consecutive years of widespread MMEs across the basin. These MMEs affected thousands of kilometers of coastline from the surface to 45 m, across a range of marine habitats and taxa (50 taxa across 8 phyla). Significant relationships were found between the incidence of MMEs and the heat exposure associated with MHWs observed both at the surface and across depths. Our findings reveal that the Mediterranean Sea is experiencing an acceleration of the ecological impacts of MHWs which poses an unprecedented threat to its ecosystems' health and functioning.

1. Understanding the combined effects of global and local stressors is crucial for conservation and management, yet challenging due to the different scales at which these stressors operate. Here, we examine the effects of one of the most pervasive threats to marine biodiversity, ocean warming, on the early life stages of the habitat-forming macroalga Cystoseira zosteroides, its long-term consequences for population resilience, and its combined effect with physical stressors.2. First, we performed a controlled laboratory experiment exploring the impacts of warming on early life stages. Settlement and survival of germlings were measured at 16°C (control), 20°C, and 24°C, and both processes were affected by increased temperatures. Then, we integrated this information into stochastic, density-dependent integral projection models.3. Recovery time after a major disturbance significantly increased in warmer scenarios. The stochastic population growth rate (λ s ) was not strongly affected by warming alone, as high adult survival compensated for thermal-induced recruitment failure. Nevertheless, warming coupled with recurrent physical disturbances had a strong impact on λ s and population viability. 4.Synthesis. The impact of warming effects on early stages may significantly decrease the natural ability of habitat-forming algae to rebound after major disturbances. These findings highlight that, in a global warming context, populations of deep-water macroalgae will become more vulnerable to further disturbances, and stress the need to incorporate abiotic interactions into demographic models. K E Y W O R D Sclimate change, demography, human impacts, population ecology, quasi-extinction, recovery, seaweeds, stress interactions 1130 | Journal of Ecology CAPDEVILA Et AL.

Canopy-forming algae play a key role in temperate coastal ecosystems sustaining complex habitats that provide food and refuge for rich associated biotic communities. These macroalgae are in decline in many coastal areas, where overgrazing by herbivores can lead to the loss of these highly structured and diverse habitats toward less complex sea urchin barren grounds. Once established, low productive barren grounds are considered stable states maintained by several positive feedback mechanisms that prevent the recovery of marine forests. To revert this global decline, restoration efforts and measures are being encouraged by EU regulations and local actions. Here, we tested the success of active revegetation techniques as a tool to promote functional and productive Treptacantha elegans forests in sea urchin barren grounds under different restoration strategies (active, and combined active with passive strategies). Active revegetation was performed in 6 barren grounds, 3 located inside a Mediterranean No-Take marine reserve (active and passive strategy) and 3 outside (active strategy alone), following a three-step protocol: (1) sea urchin population eradication, (2) seeding with Treptacantha elegans, and (3) enhancement of T. elegans recruitment. Revegetation success was assessed 1 year later in the six barren grounds, but was only achieved after combining active with passive restoration strategies. Our results encourage revegetation of barren grounds to shift from less productive habitats to complex T. elegans forests, highlight the potential of the combined passive and active restoration strategies, as well as the important role of marine reserves not only in conservation but also in ecological restoration.

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Canopy-forming macroalgae play a crucial role in coastal primary production and nutrient cycling, providing food, shelter, nurseries, and habitat for many vertebrate and invertebrate species. However, macroalgal forests are in decline in various places and natural recovery is almost impossible when populations become locally extinct. Hence, active restoration emerges as the most promising strategy to rebuild disappeared forests. In this regard, significant efforts have been made by several EU institutions to research new restoration tools for shallow and mesophotic reef habitats (e.g., MERCES EU project, AFRIMED, and ROCPOP-life) and effective techniques have subsequently been proposed to promote self-sustaining populations. Recent research indicates that macroalgal forest recovery requires a broad spectrum of measures, ranging from mitigating human impacts to restoring the most degraded populations and habitats, and that the viability of large restoration actions is compromised by ongoing human pressures (e.g., pollution, overgrazing, and climate change). We propose a roadmap for Mediterranean macroalgal restoration to assist researchers and stakeholders in decision-making, considering the most effective methods in terms of cost and cost-effectiveness, and taking background environmental conditions and potential threats into account. Last, the challenges currently faced by the restoration of rocky coastal ecosystems under changing climate conditions are also discussed.