Coastal oceans are particularly affected by rapid and extreme environmental changes with dramatic consequences for the entire ecosystem. Seagrasses are key ecosystem engineering or foundation species supporting diverse and productive ecosystems along the coastline that are particularly susceptible to fast environmental changes. In this context, the analysis of phenotypic plasticity could reveal important insights into seagrasses persistence, as it represents an individual property that allows species’ phenotypes to accommodate and react to fast environmental changes and stress. Many studies have provided different definitions of plasticity and related processes (acclimation and adaptation) resulting in a variety of associated terminology. Here, we review different ways to define phenotypic plasticity with particular reference to seagrass responses to single and multiple stressors. We relate plasticity to the shape of reaction norms, resulting from genotype by environment interactions, and examine its role in the presence of environmental shifts. The potential role of genetic and epigenetic changes in underlying seagrasses plasticity in face of environmental changes is also discussed. Different approaches aimed to assess local acclimation and adaptation in seagrasses are explored, explaining strengths and weaknesses based on the main results obtained from the most recent literature. We conclude that the implemented experimental approaches, whether performed with controlled or field experiments, provide new insights to explore the basis of plasticity in seagrasses. However, an improvement of molecular analysis and the application of multi‐factorial experiments are required to better explore genetic and epigenetic adjustments to rapid environmental shifts. These considerations revealed the potential for selecting the best phenotypes to promote assisted evolution with fundamental implications on restoration and preservation efforts.

Seagrass meadows are disappearing at rates comparable to those reported for mangroves, coral reefs, and tropical rainforests. One of the main causes of their decline is the so-called cultural eutrophication, i.e., the input of abnormal amounts of nutrients derived from human activities. Besides the impact of eutrophication at a local scale, the occurrence of additional stress factors such as global sea warming may create synergisms in detriment of seagrass meadows’ health. In the present study, we aimed to evaluate if plants undergoing chronic cultural eutrophication and plants growing in relatively pristine waters are more (or less) sensitive to heat stress, nutrient load and the combination of both stressors. To address this question, a mesocosm experiment was conducted using Posidonia oceanica collected from two environments with different nutrients load history. Plants were exposed in controlled conditions to high nutrient concentrations, increased temperature and their combination for 5 weeks, to assess the effect of the single stressors and their interaction. Our results revealed that plants experiencing chronic cultural eutrophication (EU) are more sensitive to further exposure to multiple stressors than plants growing in oligotrophic habitats (OL). OL and EU plants showed different morphological traits and physiological performances, which corroborates the role of local pressures in activating different strategies in response to global environmental changes. EU-plants appeared to be weaker during the treatments, showing the greatest percentage of mortality, particularly under increased temperature. Temperature and nutrient treatments showed opposite effects when tested individually and an offset response when combined. The activation of physiological strategies with high energetic expenses to cope with excess of nutrients and other stressors, could affect plants present and future persistence, particularly under eutrophic conditions. Our results represent a step forward in understanding the complex interactions that occur in natural environments. Moreover, unraveling intraspecific strategies and the role of local acclimation/adaptation in response to multiple stressors could be crucial for seagrass conservation strategies under a climate change scenario.

Seagrasses are marine flowering plants providing key ecological services and functions in coasts and estuaries across the globe. Increased environmental changes fueled by human activities are affecting their existence, compromising natural habitats and ecosystems’ biodiversity and functioning. In this context, restoration of disturbed seagrass environments has become a worldwide priority to reverse ecosystem degradation and to recover ecosystem functionality and associated services. Despite the proven importance of genetic research to perform successful restoration projects, this aspect has often been overlooked in seagrass restoration. Here, we aimed to provide a comprehensive perspective of genetic aspects related to seagrass restoration. To this end, we first reviewed the importance of studying the genetic diversity and population structure of target seagrass populations; then, we discussed the pros and cons of different approaches used to restore and/or reinforce degraded populations. In general, the collection of genetic information and the development of connectivity maps are critical steps for any seagrass restoration activity. Traditionally, the selection of donor population preferred the use of local gene pools, thought to be the best adapted to current conditions. However, in the face of rapid ocean changes, alternative approaches such as the use of climate-adjusted or admixture genotypes might provide more sustainable options to secure the survival of restored meadows. Also, we discussed different transplantation strategies applied in seagrasses and emphasized the importance of long-term seagrass monitoring in restoration. The newly developed information on epigenetics as well as the application of assisted evolution strategies were also explored. Finally, a view of legal and ethical issues related to national and international restoration management is included, highlighting improvements and potential new directions to integrate with the genetic assessment. We concluded that a good restoration effort should incorporate: (1) a good understanding of the genetic structure of both donors and populations being restored; (2) the analysis of local environmental conditions and disturbances that affect the site to be restored; (3) the analysis of local adaptation constraints influencing the performances of donor populations and native plants; (4) the integration of distribution/connectivity maps with genetic information and environmental factors relative to the target seagrass populations; (5) the planning of long-term monitoring programs to assess the performance of the restored populations. The inclusion of epigenetic knowledge and the development of assisted evolution programs are strongly hoped for the future.

Ocean acidification and marine heatwaves are key drivers of marine ecosystem changes that can interact with one another and influence marine organisms. Seagrasses, including the long-lived Posidonia oceanica that is endemic to the Mediterranean Sea, are widely distributed along coastal habitats, forming highly valuable underwater meadows. The germination and survival of the early life stages of P. oceanica are strongly affected by environmental changes. To assess the impact of warming and acidification on its future, we conducted a multifactorial experiment in which P. oceanica seedlings were grown under ocean acidification conditions for 6 months and then exposed to a seawater warming event. Seedling performance was investigated by analysing photo-physiology, antioxidant capacity, energetic metabolism, and transcriptomic profiles. A weighted gene correlation network analysis was used to integrate phenotypic plant traits with transcriptomic results to identify central genes involved in plant responses to ocean acidification and temperature exposure. Results demonstrated that prolonged ocean acidification exposure enhances P. oceanica seedling resilience to marine heatwaves. Specifically, seedlings regulated their antioxidant systems and transcriptomic machinery to better cope with thermal stress. Under current CO2 concentrations, elevated temperatures induced stress in P. oceanica seedlings, impacting photosynthesis and respiration. However, ocean acidification could mitigate the impact of warming in the future, enhancing the resilience to global stressors of P. oceanica.

Aim: Seed dispersal plays a key role in shaping the distribution and genetic complexity of seagrass populations and affects their resilience capacity under disturbance. The endemic seagrass Posidonia oceanica is a key component of Mediterranean coastal ecosystems, but knowledge about movement ecology in this species is limited, especially regarding seed movement pathways and dispersal potential.Location: Western coast of Sicily (central Mediterranean).Methods: Beach‐cast fruits of the Mediterranean seagrass P. oceanica were collected from nine localities along the Western coast of Sicily, along with adult shoots from eight putative donor meadows. We determined pair‐wise genetic differentiation between established meadows and seed cohorts. Genetic assignment tests were used to infer the most likely meadow of origin of individual seeds and were complemented with forward and backward Lagrangian simulations of dispersal.Results: A significant genetic differentiation was found between seed pools and the most‐likely meadow of origin. The genetic assignment confirmed that seeds from the same cohort originated from multiple meadows and emphasised the presence of long‐distance‐dispersal (LDD) events (up to hundreds of km). Genetic connectivity appeared to be greater than that predicted by oceanographic simulations, which may reflect the longer temporal scales on which gene flow is shaped, in contrast to contemporary dispersal patterns. Lagrangian simulations highlighted that fruits were physically capable of dispersing beyond the study area and that the north Tunisian coast could be a key source of propagules for the populations studied.Main Conclusions: Our study represents a significant step forward in the understanding of P. oceanica movement ecology and could guide meadows' conservation and restoration actions. Our findings are significant in a broader context outside of the research area and could be the basis of similar studies in other regions, especially considering the increasing number of fruiting events recorded across the Mediterranean likely associated with ocean warming.