The role of seagrass meadows as Blue Carbon ecosystems and nutrient and pollutant traps: supporting factors and threats

Seagrass meadows are aquatic angiosperms that are common in estuaries and coastal waters of all continents except Antarctica and provide various ecosystem services, including the promotion of biodiversity and fisheries, coastal protection, support for tourism, recreation, education and research. These underwater habitats are also referred to as Blue Carbon ecosystems, as they act as carbon sinks, storing organic carbon from senescent seagrass biomass and allochtnonous organic matter in their underlying sediment, for centuries to millennia. Additionally, seagrass meadows function as filtering habitats, particularly for nutrients like nitrogen, thereby playing a crucial role in the mitigation of water eutrophication. Furthermore, recent studies suggested that seagrasses act as a filter for pollutants of emerging interest (microplastics). The storage of carbon, nutrients and pollutants in seagrass sediments appears to be very heterogeneous, even when considering the same species. For this reason, recent studies have focused on the interaction between the characteristics of seagrasses and the environmental factors that may influence their storage and filtering capacity. Nevertheless, knowledge gaps persist regarding the factors that influence seagrass storage capacity of carbon and nutrients as well as their role as traps for microplastics. Unfortunately, seagrass meadows are being lost globally, due to diverse anthropogenic factors like habitat destruction, pollution, non-native species introduction and climate change. The loss of seagrass meadows exposes sediments to remineralization and physical disturbance, potentially leading to the release of stored carbon, nutrients and pollutants. For this reason, marine habitat restoration has become a pivotal management strategy to counter seagrass ecosystems decline and enhance resilience. However, effective monitoring programs and knowledge of the areas subjected to restoration are essential to evaluate the outcomes of these projects. This study aims to advance our understanding of seagrass ecosystems as Blue Carbon systems and nutrient/pollutant filters, by exploring their interactions with environmental factors in coastal settings. The research also seeks to contribute valuable insights to the science of seagrass restoration, urgently needed to address the overarching decline of these important ecosystems. The first chapter addresses the question of how the interaction between seagrass traits and environmental factors influences carbon and nitrogen storage, looking at the most important species of the Mediterranean, namely Posidonia oceanica (L.) Delile and Cymodocea nodosa Ucria Acherson. The results show that higher stocks are found in areas where higher seagrass biomass, a higher proportion of mud in the sediment and lower hydrodynamics interact to promote the deposition and retention of carbon and nutrients, while higher hydrodynamics appear to lead to higher stocks of inorganic carbon in the sediments. The second chapter deals with microplastic pollution in seagrass ecosystems, with the aim to gain insights into the effects of environmental factors on the occurrence and spread of these pollutants in seagrass sediment, water and biota. In addition, this chapter examines how the trophic ecology of the fish species that characterize the seagrass fauna influences the probability of ingestion. Microplastics were found in the sediment as well as in the water and in allfish species examined. The results confirm the role of seagrass beds as filter of pollutants, and the amount of microplastics in their sediment is significantly higher than in the water column. Hydrodynamics and geomorphology appear to influence the abundance of microplastics in the environment. Fish individuals that ingested microplastics showed a narrower isotopic range, suggesting that a more selective feeding strategy based on less variable resources may increase the likelihood of microplastics ingestion, but further studies are needed to generalise this pattern. The third chapter evaluates the historical and current environmental conditions of a P. oceanica transplantation site and reconstructs the change in geochemical factors and pollutant inputs throughout the past century in the area, comparing three P. oceanica habitats: a natural meadow, a meadow trasplanted onto dead matte, and dead matte. The results indicate that there was a decrease in particle trapping capacity in the dead matte, possibly associated with a slight sediment erosion, while the transplanted meadow showed stability in comparison with the natural meadow, suggesting a positive outcome of previous reforestation efforts.Overall, the results of the present thesis expand our knowledge on the ecological functioning of seagrass ecosystems. Accurate knowledge on how abiotic factors interact with seagrass traits in driving carbon and nutrient storage and pollutant distribution can help in the management of restoration and conservation projects. Restoration by transplanting P. oceanica onto dead matte seems a promising method to promote the carbon sequestration potential and recovery of damaged meadows.The importance of this research goes beyond purely scientific interest, as promoting and protecting the ecological services of seagrass ecosystems are beneficial measures for the environment and society. Policymakers, stakeholders and the public should work together by promoting sustainable management practices to protect the ecological services of seagrass ecosystems and create resilient coastal environments, which is a step towards a sustainable future for generations to come.