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Global warming, bioinvasions, and parasitism affect single-species performances and species interactions, substantially impacting the structure and stability of marine ecosystems. In light of accelerated global change, the information derived from studies focusing on single species and single drivers is insufficient, calling for a multi-stressor approach under near-natural conditions. We investigated the effects of warming (+3°C) on the performance of a benthic community composed of native and invasive macroalgae, consumers and a trematode parasite in a mesocosm setting. We also assessed the effects of warming and parasitism on the survival and growth of gastropods and mussels and the thermal dependency of trematode performance. Our findings show that warming and grazing by infected gastropods had a large detrimental effect on the invasive macroalga growth. Furthermore, the single and interactive effects of parasitism and warming were detrimental to intermediate host survival and growth, especially to large mussels. Finally, cercarial emergence positively correlated to the natural peaks of summer temperatures, while infection intensity in mussels was higher in larger individuals. Our findings suggest that grazing and warming will be detrimental to the invasive macroalga, favoring the native alga. Moreover, parasitism will enhance grazing, especially in summer, when higher temperatures trigger parasite development. However, parasite-enhanced grazing may be buffered by higher mortality or a shift in the size of infected intermediate hosts under warming. Our findings demonstrate how complex effects of ocean warming can be on food webs and how they can be mediated by parasitism and, as a result, influence native and invasive macroalgae differently.

Seagrass detritus accumulates along the coasts forming beach-cast wracks. Being subject to environmental gradients and hydrodynamism, the size and composition of beach-cast wracks may vary in space and time. Also the composition may vary depending on both proximities to and productivity of the adjacent seagrass meadows. Despite their dynamic nature, seagrass beach-casts provide important ecosystem services, by contributing to the nutrient and carbon flow between marine and terrestrial systems, supporting beach biodiversity and trophic webs and protecting the coastline from erosion. However, the knowledge about beach-cast dynamics, ecological role and functions is still fragmented and deserves a greater effort. Here, we aimed to assess the seasonal dynamics of beach-casts in terms of mass, nutrient and carbon content, and composition. The study was conducted in November 2018 and February 2019 in a Sicilian beach (Capo Feto, TP), adjacent to large nearshore Posidonia oceanica meadows. Surficial and deep (0.5-2 m) seagrass beach-cast samples were collected through a handsaw along the seaward side of the wrack, at three equidistant sites (100 m). Beach-cast mass (kg DW m coastline-1) exhibited wide temporal fluctuations, significantly decreasing from November to February, as an effect of the winter sea storms. A higher mass was also evident in the deep layer than in the surficial one only in November, suggesting a higher compactness of deep layers in the highest beach-cast wracks. Accordingly, nitrogen and organic carbon content (kg DW m coastline-1) showed the same pattern revealing a key role of the beach-cast as a nutrient source between adjacent systems. Despite the high seasonal variability, beach cast composition was rather constant over time, showing an overall dominance of seagrass detritus, and an increase in sediment in the deep layers. This indicates a relevant input from the adjacent P. oceanica meadow and confirms the role of banquette as sediment trap.

Posidonia oceanica is an endemic Mediterranean seagrass that forms wide and dense meadows from the surface to depths of about 40 m. During fall and winter, due to aging and storms, leaves and rhizomes break away from the seagrass meadows and accumulate on the shores (from a few centimeters in the water to several meters inshore) forming conspicuous beach-casts of P. oceanica detritus. These structures have received increasing attention in the last decade for the potential ecosystem services they provide in protecting the coast from erosion, linking marine and terrestrial food chains and contributing to carbon and nutrients fluxes. However, seagrass beach-cast morphology is not easy to estimate because: i) beach-casts have a complex dynamics; ii) they can reach very large extensions; iii) their shapes are not comparable with standard geometric figures. The aim of this study was to investigate the morphology and dynamics of P. oceanica beach-casts in three beaches of Sicily (Capo Feto, Maragani and Scala dei Turchi) characterized by different productivity of the nearshore meadows. A remote sensing approach was used to assess the beachcast extension, morphology, volume and derivate parameters. In particular, very high resolution drone images, combined with GPS field data, were used to obtain a 3D-reconstruction of P. oceanica beach-casts. Generated ortophotos and digital elevation model were imported into geospatial analysis software to quantify volumes and their spatial and temporal evolutions. The 3D-model allowed to accurately estimate different levels of seagrass detritus accumulation. This approach enables to achieve a more detailed and complete understanding on the role of P. oceanica detritus beach-casts in coastal marine dynamics. This might also contribute to gain a better insight into the ecosystem services they provide.

Posidonia oceanica detritus accumulates in late summer-autumn along the coasts forming beach-cast seagrass wracks known as banquette. Although seagrass wracks are highly variable at both spatial and temporal scales, they have an important ecological role as they protect the coast from erosion, support beach biodiversity and trophic webs and supply nutrients to dune vegetation. Previous studies have focused mainly on the beach-cast morphological characterization and variability, as well as on associated biotic communities. However, few studies have focused on the quali-quantitative assessment of seagrass wracks. The aim of this study was to assess the spatial variability of the P.oceanica beach-cast wrack, in terms of biomass and composition, in relation to productivity of the nearshore meadows. We hypothesize that highly productive seagrass meadows sustain beach-cast characterised by a greater vegetal biomass to sediment ratios than low productivity meadows. This study was conducted in November 2018 in three Sicilian pocket-beaches with different levels of productivity of the nearshore meadows: high (Capo Feto, TP), medium (Maragani, AG) and low productivity (Scala dei Turchi, AG). At each site, samples of beach-cast wracks were collected and dry biomass (kg DW m-3) and the percentage composition of the main components (leaf detritus, fresh leaves, rhizomes, aegagropilae, sediment) were estimated. Preliminary results showed a high intra-site variability and no significant differences in biomass per unit volume among the beaches with different levels of seagrass productivity. However leaf detritus represented the main component of the wrack in the beach sustained by high seagrass productivity, followed by rhizomes, while sediment component increased in the other productivity levels. Next steps will relate biomass data to the total volume of beach-cast in order to estimate the global biomass accumulated on the beach. In addition, in order to evaluate the temporal dynamics of the P. oceanica beach-cast wrack, sampling will be repeated in the same sites every three months in 2019 and will allow to complete the preliminary picture here presented.

Posidonia oceanica meadows are among the most important marine coastal ecosystems in the Mediterranean Sea due to several roles they are able to play. They, in fact, support high levels of primary production, enhance sedimentation of suspended particles through their canopies, prevent resuspension of marine sediments by means of roots and rhizomes and counteract coast erosion. Part of the primary production of this seagrass is not transferred to herbivores either immediately to detritus pathway but it can accumulate on the shore in form of banquette, typical structures exerting a significant role in protection of beaches from erosion. To accurately measure the extension, morphology, volume and derivate parameters of these structures a remote sensing approach is often required, since very large banquettes can occur along the shore. The aim of this study was to investigate the architecture and dynamics of some banquettes observed along coasts of Sicily, by using a multitemporal analysis of remotely sensed images. In particular very high resolution drone images, combined with GPS field data, was used to obtain a 3D reconstruction of banquette morphology. The study was carried out along three pocket beaches: Capo Feto (TP), Maragani and Scala dei Turchi (AG). Generated ortophotos and digital elevation model were imported into geospatial analysis software to quantify volumes and their spatial and temporal evolutions. 3D approach allowed us to accurately estimate different levels of necromass accumulation.This approach will allow to achieve a more detailed and complete view on the role of P. oceanica detritus stocks within coastal marine dynamics. This might also contribute to gain a better insight into the ecosystem services they provide.

The biotic resistance hypothesis states that pristine communities, with high species richness and functional diversity, are less prone to biological invasions than species-poor areas either this is natural or it is a result of human activities. Complex communities with high levels of biodiversity, such as those hosted by Marine Protected Areas, should offer fewer niche opportunities (e.g. resources and space) to invasive species thereby reducing both their establishment possibilities and success (i.e. settlement and/or expansion). The present study aimed to evaluate the capacity of marine protected communities to provide a buffer against the establishment of one of the most invasive species in the Mediterranean Sea: the Percnidae crab Percnon gibbesi (H. Milne Edwards, 1853). Here, it was tested if predation provides a higher resistance to the invasion by P. gibbesi in protected native communities than in exploited ones. Specifically, this study aimed to compare (a) the abundance and diversity of predator assemblages of P. gibbesi, (b) the predation pressure on P. gibbesi and (c) the density of this species in protected and unprotected areas in the Mediterranean Sea. Tethering experiments were conducted in the field to assess the relative predation rate on two size classes of P. gibbesi, using long and short tethers, at two protected and two unprotected sites randomly chosen on the northern coast of Sicily (Southern Tyrrhenian Sea). The abundance and diversity of predators of P. gibbesi and the relative predation rate on tethered crabs were higher at protected than unprotected sites, independent of crab size and tether length; the density of P. gibbesi was significantly lower in protected than unprotected areas. The findings of this study, although restricted to a small spatial scale, support the existence of mechanisms for biotic resistance in Mediterranean protected native communities and provide experimental evidence that the restoration of predator assemblages confers better invasion resistance to marine protected areas against P. gibbesi invasion. More generally, the present study confirms that marine protected areas are an effective tool for ameliorating the impact of some biological invasions in the Mediterranean Sea.

Despite the field of invasion ecology has rapidly grown, the mechanisms behind the invasion success of alien species and the biotic resistance to invasions still need a thorough understanding. Comprehending the response of resident assemblages to biological invasions and the possible impacts produced by alien species introduction is, therefore, crucial to expand the current knowledge of the invasion process in the recipient systems. In this thesis different aspects of biological invasion process have been critically examined emphasizing the importance and complexity of the interactions between alien species and native biota, in the Mediterranean marine coastal ecosystems. Here, five different invaders were used as model organisms: two seaweed species, Caulerpa cylindracea and Caulerpa taxifolia var. distichophylla, the crab Percnon gibbesi and two rabbitfish species, Siganus rivulatus and Siganus luridus. Firstly, in Chapter 1 a general overview of the invasion processes was provided. The following two chapters were focused on the interactions between the generalist native herbivore, Paracentrotus lividus, and the two co-occurring alien seaweeds C. cylindracea and C. taxifolia var. distichophylla. Specifically, in Chapter 2 the feeding behaviour of P. lividus was investigated in order to predict the potential vulnerability of the two alien seaweeds (alone and intermingled) to sea urchin grazing and the possible biological control exerted by the native herbivore on the two seaweed populations. The results from feeding choice experiments showed that P. lividus is able to incorporate in its diet the two co-occuring alien seaweeds suggesting a potential capacity of the sea urchin to control their expansion in the invaded area. Chapter 3 aimed to investigate the potential physiological stress induced on P. lividus by the consumption of the two alien seaweeds, C. cylindracea and C. taxifolia var. distichophylla. Based on the presence of chemically distinct toxic metabolites in the two invaders, a synergistic effect of toxic metabolites on the sea urchin righting behaviour was predicted, when the two algae were consumed simultaneously. The findings showed a significant increase of P. lividus righting time as a consequence of the simultaneous consumption of the two alien seaweeds, suggesting that synergistic interactions among the two invaders can enhance their likelihood to survive. Reduction in sea urchin performance may have negative implications for their overall grazing activity and, ultimately, for their ability to control the expansion of alien seaweeds in the invaded area. More generally, it is well-known that native-alien species interactions may influence the invasion process favoring or hindering the establishment and spread of alien species. As predicted by theory, the success of alien species can be restricted by interspecific interactions, such as competition and predation, from resident species. Therefore, pristine communities, with high species richness and diversity, are less prone to invasions than those already degraded by human activities. In this regard, Marine Protected Areas (MPAs), supporting a greater number of species than unprotected areas, are expected to limit the successful establishment of undesirable invaders. In Chapter 4 it was tested if the high native diversity observed within MPAs can provide biotic resistance to the invasion of the alien crab P. gibbesi compared to fished areas. Results revealed a negative relationship between the diversity of native communities and the invasiveness of the alien crab. Specifically, it was shown that MPAs, as protected systems with high native species richness and diversity, may hinder the spread of the high invasive crab P. gibbesi. Finally, evidence from the Mediterranean Sea showed that the invasion of herbivorous fishes may induce phase shifts in the recipient communities. In the last chapter (Chapter 5) the relationship between the biomass of two alien herbivorous fishes (S. luridus and S. rivulatus) and the cover of barren was investigated in sublittoral rocky reefs across the Aegean Sea. Results showed that the biomass of the two alien fishes was positively correlated to the barren cover reinforcing the idea that these invaders are able to induce shifts from diverse and productive algal forest to the low-complex barren state, affecting the structure and composition of the shallow rocky reefs in the Mediterranean Sea. All findings of this thesis have interesting implications for the potential mechanisms of invasion process and contribute to the increase of knowledge on the invasion dynamics, reinforcing the idea that biotic interactions are among the main drivers affecting alien species invasion process.

Native generalist grazers can control the populations of non-indigenous invasive algae (NIIA). Here, it was found that the simultaneous consumption of two co-occurring NIIA, Caulerpa cylindracea and C. taxifolia var. distichophylla, hinders the grazing ability of the main Mediterranean herbivorous, the native sea urchin Paracentrotus lividus. The ingestion of any of the two NIIA alone did not produce any difference in sea urchin righting time with respect to usual algal diet. In contrast, the simultaneous consumption of both NIIA, which grow intermingled in nature and are consumed by P. lividus, retarded its righting behavior. Such result reveals substantial physiological stress in the sea urchin, which resulted in reduced motility and coordination. The reported findings reveal the potential of NIIA co-occurrence to escape the supposed control exerted by the main native generalist grazer in Mediterranean sublittoral communities, which in turn can be locked in an “invaded” state.