Increasing oceanic uptake of CO2 is predicted to drive ecological change as both a resource (i.e. CO2 enrichment on primary producers) and stressor (i.e. lower pH on consumers). We use the natural ecological complexity of a CO2 vent (i.e. a seagrass system) to assess the potential validity of conceptual models developed from laboratory and mesocosm research. Our observations suggest that the stressor-effect of CO2 enrichment combined with its resource-effect drives simplified food web structure of lower trophic diversity and shorter length. The transfer of CO2 enrichment from plants to herbivores through consumption (apparent resource-effect) was not compensated by predation, because carnivores failed to contain herbivore outbreaks. Instead, these higher-order consumers collapsed (apparent stressor-effect on carnivores) suggesting limited trophic propagation to predator populations. The dominance of primary producers and their lower-order consumers along with the loss of carnivores reflects the duality of intensifying ocean acidification acting both as resource-effect (i.e. bottom-up control) and stressor-effect (i.e. top-down control) to simplify community and trophic structure and function. This shifting balance between the propagation of resource enrichment and its consumption across trophic levels provides new insights into how the trophic dynamics might stabilize against or propagate future environmental change.

Carbon and nitrogen storage in exotic Halophila stipulacea were compared to that in native Posidonia oceanica and Cymodocea nodosa meadows and adjacent unvegetated sediments of the Eastern Mediterranean Sea and to that in native H. stipulacea of the Red Sea at sites with different biogeochemical conditions and level of human pressure. Exotic H. stipulacea possessed considerable storing capacity, with 2-fold higher C org stock (0.71 ± 0.05 kg m −2 in the top 20 cm of sediment) and burial (14.78 gC org m −2 y −1 ) than unvegetated areas and C. nodosa meadows and, surprisingly, comparable to P. oceanica . N (0.07 ± 0.01 kg m −2 ) and C inorg (14.06 ± 8.02 kg m −2 ) stocks were similar between H. stipulacea and C. nodosa or unvegetated sediments, but different to P. oceanica . C org and N stocks were higher in exotic than native H. stipulacea populations. Based on isotopic mixing model, organic material trapped in H. stipulacea sediments was mostly allochthonous (seagrass detritus 17% vs seston 67%). C org stock was similar between monospecific and invaded C. nodosa meadows by H. stipulacea . Higher stocks were measured in the higher human pressure site. H. stipulacea introduction may contribute in the increase of carbon sequestration in the Eastern Mediterranean.

Despite the wide knowledge about prevalent effects of ocean acidification on single species, the consequences on species interactions that may promote or prevent habitat shifts are still poorly understood. Using natural CO 2 vents, we investigated changes in a key tri-trophic chain embedded within all its natural complexity in seagrass systems. We found that seagrass habitats remain stable at vents despite the changes in their tri-trophic components. Under high pco 2 , the feeding of a key herbivore (sea urchin) on a less palatable seagrass and its associated epiphytes decreased, whereas the feeding on higher-palatable green algae increased. We also observed a doubled density of a predatory wrasse under acidified conditions. Bottom-up CO 2 effects interact with top-down control by predators to maintain the abundance of sea urchin populations under ambient and acidified conditions. The weakened urchin herbivory on a seagrass that was subjected to an intense fish herbivory at vents compensates the overall herbivory pressure on the habitat-forming seagrass. Overall plasticity of the studied system components may contribute to prevent habitat loss and to stabilize the system under acidified conditions. Thus, preserving the network of species interactions in seagrass ecosystems may help to minimize the impacts of ocean acidification in near-future oceans.www.nature.com/scientificreports www.nature.com/scientificreports/ due to eutrophication and/or the removal of predators that control herbivore populations by overfishing, either directly or indirectly via reduced control on small predators that feed on algae-removing mesograzers. We know comparatively less about changes in the strength of species interactions that may critically influence the persistence of seagrass ecosystems under ocean acidification.The Mediterranean endemic seagrass Posidonia oceanica forms complex systems with well-defined main trophic links. Two macroherbivores alone, the sea urchin Paracentrotus lividus and the sparid fish Sarpa salpa (commonly known as salema), may remove 50% of the annual seagrass productivity in shallow meadows 16 . In this study, we investigate mechanisms behind the change or stability of Posidonia habitats under ocean acidification. Particularly, we examined the interactions' strength under present (off-vent) and near-future OA conditions (CO 2 vents) of a tri-trophic food chain embedded within all its natural complexity. We studied multiple basal resources, one of the two main seagrass herbivores (sea urchin), and a territorial labrid fish that is known to predate on such herbivore 17,18 (Symphodus tinca, commonly known as peacock wrasse). Both consumers have restricted benthic home ranges, which ensures long-term exposure to high pCO 2 levels at the vent sites for them and their resources. Particularly, we investigated the strength of consumers' feeding by quantifying CNP stoichiometry, diet composition, trophic niche and position (stable isotope analysis (SIA)-based and diet-based), as well as the availability and pala...

Elasmobranchs are among the species most threatened by overfishing and a large body of evidence reports their decline around the world. As they are large predators occupying the highest levels of marine food webs, their removal can alter the trophic web dynamic through predatory release effects and trophic cascade. Suitable management of threatened shark species requires a good understanding of their behaviour and feeding ecology. In this study we provide one of the first assessments of the trophic ecology of the "vulnerable" smooth-hounds Mustelus mustelus and M. punctulatus in the central Mediterranean Sea, based on stomach contents and stable isotope analyses. ontogenetic diet changes were addressed by comparing the feeding habits of three groups of individuals: juveniles, maturing and adults. our results highlighted that the two species share a similar diet based mostly on the consumption of benthic crustaceans (e.g. hermit crabs). their trophic level increases during ontogeny, with adults increasing their consumption of large-sized crustaceans (e.g. Calappa granulata, Palinurus elephas), cephalopods (e.g. Octopus vulgaris) and fish (e.g. Trachurus trachurus). our results provide also evidence of ontogenetic shifts in diet for both species showing a progressive reduction of interspecific trophic overlap during growth. The results of this study contribute to improve the current knowledge on the trophic ecology of these two threatened sharks in the Strait of Sicily, thus providing a better understanding of their role in the food web. Elasmobranch species are commonly recognized to be important predators in the marine realm 1 and play a crucial role in regulating marine ecosystems 2,3. Greater awareness of the trophic ecology of sharks can provide important information about the role they play during their life cycles and improve understanding of marine communities' structure and functioning. Many decades of severe human impacts have led to a rapid decline of many elasmobranch species around the world 4,5 , exacerbated by their biological vulnerability (e.g. slow growth rate, low fecundity, and late age at maturity) 2,5. As a consequence, many shark species are now registered by the IUCN as threatened or endangered 6. It has been shown that the decline of elasmobranch species has had marked ecological consequences 7,8. The loss of predators may negatively alter the food chain, triggering new interactions among species and marine ecosystem degradation 9. Therefore, improved knowledge about the elasmobranch trophic ecology, including the prey consumed, trophic level, ontogenetic diet changes, especially in the Mediterranean species, can play a crucial role in the development of new fishery management strategies. In fact, resource partitioning is one of the main processes for the coexistence of species 10. Partitioning, which has been observed in several organisms 12−14 , can occur along the space, time, or feeding niche axes 11 , during the same or at different