Abstract. Ocean alkalinity enhancement (OAE) has emerged as a promising carbon dioxide removal strategy aimed at increasing seawater alkalinity and enhancing long-term oceanic carbon uptake. However, concerns remain regarding the potential ecological impacts of large-scale alkalinity manipulation on marine planktonic communities, particularly phytoplankton, which play a central role in marine biogeochemical cycles and primary production. This review synthesizes the current experimental evidence on phytoplankton responses to different OAE approaches, including hydroxide-based, mineral-based, and bicarbonate-based treatments. We compare early conceptual predictions with recent laboratory, microcosm, mesocosm, and modelling studies, highlighting how the field has evolved from theoretical risk assessment toward increasingly mechanistic and experimentally grounded investigations. Current evidence generally indicates that phytoplankton communities exhibit a relatively high tolerance to moderate CO2-equilibrated OAE scenarios, with limited effects on biomass, productivity, and community composition. More pronounced physiological and ecological responses are observed under unequilibrated or mineral dissolution treatments, where rapid pH shifts, trace metal release, and nutrient interactions may alter species-specific performance and community dynamics. Responses vary substantially across taxonomic groups and environmental contexts, reflecting the importance of carbonate chemistry, nutrient availability, and trace metal sensitivity in shaping OAE outcomes. Overall, existing studies suggest that phytoplankton responses to OAE are more nuanced and context-dependent than initially hypothesized, although substantial uncertainties remain regarding long-term ecosystem restructuring and large-scale biogeochemical feedback.

High-resolution marine geophysical surveys conducted in 2004 revealed submarine morphologies offshore Cape Licosa (Tyrrhenian Sea, Italy), whose origin remains debated for years. Remote and direct data collected within the CORSUB project between 2024 and 2025 identified a field of subcircular to polygonal mounded structures distributed between approximately 75 m and 90 m of water depth along a ridge. These bedforms were analyzed using multibeam bathymetry and backscatter data, side-scan sonar, sub-bottom profiling, and video observations. CHIRP profiles indicate that these morphologies occur above a rugged acoustic basement attributed to the Miocene Cilento Flysch, which is draped by a thin, discontinuous Holocene sedimentary cover. Morphometric analysis delineated 565 discrete features with average dimensions of 8.6 & times; 6.1 & times; 0.4 m and a prevailing northeast to southeast orientation. They exhibit a distinctive high-reflectivity ring surrounding a lower-reflectivity core, producing a regular beehive-like seabed texture. Videos document a wavy-profiled seascape characterized by coarse-grained biogenic sediments, including abundant boxwork rhodoliths, and localized encrustations of coralline algae and bivalves on rocky outcrops. The data suggest that an inherited, complex paleo-topography of the Flysch substrate formed during subaerial exposure of the ridge at the Last Glacial Maximum. Holocene sedimentation draped this surface, involving both sediment accumulation driven by intense hydrodynamic activity and benthic colonization of sparse rocky substrates, thereby preserving this complexity and resulting in a wavy seafloor profile. These findings highlight the control of paleo-topography and post-glacial sedimentary dynamics in shaping mesophotic seabed morphology and distinctive seabed landforms along Mediterranean continental shelves.

Miocene hardgrounds of the circum-Mediterranean region are widely interpreted as records of upwelling of nutrient-rich water, commonly associated with phosphatization along platform margins. On the Latium–Abruzzi Platform, the Sirente Mt. and Tornimparte successions document two distinct phosphatic intervals: in situ authigenic hardgrounds and reworked phosphate-rich deposits, respectively. At Tornimparte, phosphorite beds are interpreted as the product of gravity flows triggered by internal-wave breaking along a density-stratified outer ramp. Erosion of a phosphatization zone generated lithoclasts that were transported basinward and redeposited chaotically, as supported by taphonomic evidence and CT-scan analyses of macrofossil-bearing blocks. The in situ deposits of Sirente Mt. represent fossil rhodolith beds, dominated by boxwork morphotypes, and composed chiefly of coralline algae of the genus Mesophyllum, with subordinate Lithothamnion, encrusting bryozoans, acervulinids, and serpulids. Their loose internal structure, multispecific encrustations, and fine sediment infill indicate low-energy conditions and relative substrate stability. Taxonomic composition, absence of shallow-water coral taxa, and dominance of thin hapalidialean thalli suggest development on a deep tropical oligophotic ramp (ca. 70–100 m). Growth-rate contrasts between slow-growing corallines and faster bryozoans, together with alternating algal- and bryozoan-dominated layers observed in rhodoliths, imply multiannual fluctuations in nutrient supply rather than simple seasonal forcing. Upwelling-driven nutrient enrichment likely enhanced phytoplankton productivity, turbidity, and pCO₂, periodically favoring suspension feeders over coralline algae in a context of intense space competition. These cyclical ecological shifts recorded in rhodoliths may reflect interannual to decadal variability in wind-driven upwelling intensity, consistent with broader Tortonian paleoceanographic perturbations. Collectively, sedimentological, compositional, and taphonomic evidence supports a dynamic outer-ramp system modulated by recurrent upwelling pulses and possible internal-wave processes.

Coralligenous bioconstructions are biogenic calcareous structures characterized by low accretion rate and high sensitivity to natural and anthropogenic impacts. Assessing their ecological quality and health status requires non-destructive approaches. In the project "FISR 2019_CRESCIBLUREEF", a ROV-based coring device has been developed to collect samples while ensuring minimal impact on the ecological and structural integrity of bioconstructions and overcoming scuba diving depth and safety constraints. This study evaluates whether fragments retrieved by the device are representative of whole coralligenous build-ups ("tal-quale") for geobiological characterization at the microscale. Representativeness is assessed through (i) preservation of microfacies textures and framework integrity in thin sections and (ii) relative abundances of skeletal and non-skeletal carbonate components quantified by point-counting. The results suggested that a coring device represents a powerful tool for obtaining representative bioconstruction samples at least in terms of the relationships and distribution between different carbonate components. This innovative approach opens new frontiers in the study of bioconstructed habitats, allowing the collection of samples suitable for qualitative and quantitative analyses while preserving ecosystem integrity. It therefore represents a step toward in sustainable marine research, with great potential for monitoring, conservation, and management of benthic habitats.

Although Coralligenous is considered a priority habitat for conservation within European directives largely considering its complex 3D structure promoting spatial and ecological heterogeneity and related high biodiversity, detailed information on species richness and composition is still scant, at least for some taxonomic groups, including bryozoans, notwithstanding their ecological relevance within the habitat. Focusing on the bryozoan community associated with six build-ups sampled between 33 and 37 m depth offshore Marzamemi (SE Sicily, SW Ionian Sea) this study contributes to improve knowledge on coralligenous bryozoan biodiversity, from a locality in the still understudied eastern Mediterranean. A total of 127 species were recorded, raising to 145 and 257 the coralligenous bryozoan diversity currently known for Marzamemi and the entire Mediterranean, respectively, the latter representing a high proportion (44%) of the overall Mediterranean bryozoan diversity (588). Heterogeneity was detected at small spatial scale, among build-ups in the Marzamemi area, and at the Mediterranean scale through comparison with the scant existing literature, which mostly focused on the western basin. Facies differentiation and related ecological heterogeneity, rather than geographic distance, are considered the main drivers. Finally, our results emphasise the critical importance of microscopic and SEM examination of specimens for species detection.

The Macaronesian archipelagos host exceptionally well-preserved coastal sedimentary deposits formed during the warmest period of the Last Interglacial episode, the Marine Isotope Substage 5e (MIS 5e). Numerous MIS 5e fossiliferous outcrops occur, scattered across several islands of the Canary Archipelago. Among these is San Juanito, a small outcrop located in the eastern sector of Punta del Hidalgo (northeast Tenerife Island), where MIS 5e sediments are distributed over an area of approximately 480 m2. A multidisciplinary study was conducted, aiming to: (i) determine the age of the fossiliferous sediments; (ii) define the stratigraphic relationships between the sedimentary deposit and the underlying/overlying volcanic sequences; (iii) assess the taxonomic richness and the functional palaeobiodiversity of this palaeosite; and (iv) provide a comprehensive palaeoecological reconstruction of the MIS 5e environment. Based on two key ecostratigraphic indicator species for the Canarian MIS 5e, the San Juanito sequence is here assigned to the Last Interglacial. Qualitative sampling yielded forty mollusc taxa, including three gastropods that represent new records—Alvania johannae Moolenbeek & Hoenselaar, 1998, Krachia tiara (Monterosato, 1874), and Barleeia unifasciata (Montagu, 1803)—bringing the current MIS 5e checklist for the Canary Islands to 202 gastropods and 80 bivalves. The highly cemented matrix of the San Juanito deposits prevented the collection of standardized 1 kg bulk sediment samples. Nevertheless, we strongly recommend adopting this quantitative approach in future studies of suitable MIS 5e outcrops across the archipelago. The faunal assemblage indicates that the San Juanito region was dominated by rocky shores during the MIS 5e, much like today. This paleoenvironmental reconstruction is based on the high frequency of species associated with hard substrates—including echinoids, vermetids, fissurellids, and patellids—and the overwhelming dominance (95%) of epifaunal gastropods.

During latest Miocene times the Mediterranean Sea main connection with the global ocean was through the Atlantic, just as it is today. This configuration could have likely resulted in a pronounced longitudinal environmental gradient, with warm and highly oligotrophic waters in the east, and cooler, less oligotrophic conditions in the west (a scenario resembling modern conditions). This setting provides a natural laboratory to test the effectiveness of quantitative microfacies analysis in tracking palaeoenvironmental gradients. Two approaches were compared: the presence/absence of carbonate producers assessed on the basis of literature data and a quantitative skeletal assemblage analysis of reef sites spanning the Western, Central, and Eastern Mediterranean. While the presence/absence approach only offers a broad environmental overview, the quantitative analysis better differentiates between the eastern and western settings. The eastern sector displays a higher relevance of symbiont-bearing foraminifera, suggesting warmer conditions. In contrast, the western sector is comparatively richer in heterotrophic organisms, likely reflecting a higher nutrient availability and cooler water. These results support the reliability of quantitative microfacies analysis and provide a framework for comparing reef-bearing, carbonate systems across the Mediterranean within climatic homogeneous time intervals. Both approaches suggest that these carbonate factories represent subtropical, modern-like, biotic assemblages adapted to the cooler, oligotrophic, and likely semi-restricted conditions that were prevalent prior to the Messinian Salinity Crisis.

Ocean alkalinity enhancement (OAE) is emerging as a promising carbon dioxide removal (CDR) strategy, yet its ecological implications for benthic communities remain poorly understood. This study presents the first experimental evaluation of pH-equilibrated ocean alkalinization (pHeqOA), a technique designed to produce bicarbonate-enriched seawater without increasing pH, and investigates whether increasing alkalinity levels affect the recruitment, growth and biomass of macro-benthic calcifiers (MBCs), as well as whether excessive alkalinization can trigger abiotic carbonate precipitation with negative ecological consequences. A mesocosm experiment was conducted in the port area of La Spezia (Liguria, Italy) using five alkalinization levels (Control, Low, Medium, High, and Oversaturated), and limestone settlement plates were deployed to quantify recruitment, growth, surface cover, and biomass of MBCs. Colonization was dominated by the polychaete Serpula sp. Abundance and growth-related metrics did not differ significantly among the Control, Low, Medium, and High treatments, with maximum values generally observed under Medium and High alkalinization level. In contrast, the Oversaturated treatment exhibited negligible recruitment and growth, driven by extensive abiotic calcium carbonate precipitation on settlement surfaces. These results indicate that alkalinity enhancement doesn't affect the performance of MBCs when maintained below a system-specific saturation threshold, whereas excessive alkalinization suppresses settlement and growth. Overall, pHeqOA appears ecologically compatible with local macro-benthic communities within an optimal alkalinization range, underscoring the importance of defining site-specific saturation thresholds that balance carbon removal efficiency with ecological integrity when implementing OAE technologies.

Miocene hardgrounds of the circum-Mediterranean region are widely interpreted as records of nutrient-rich upwelling, commonly associated with phosphatization along platform margins. On the Latium–Abruzzi Platform, the Sirente Mt and Tornimparte successions document two distinct phosphogenic settings: in situ authigenic hardgrounds and reworked phosphate-rich deposits. At Tornimparte, phosphorite beds are interpreted as the product of gravity flows triggered by internal-wave breaking along a density-stratified outer ramp. Erosion of a phosphatization zone generated lithoclasts that were transported basinward and redeposited chaotically, as supported by taphonomic evidence and CT-scan analyses of macrofossil-bearing blocks. Rhodolith assemblages from Sirente Mt are dominated by boxwork morphotypes composed chiefly of Mesophyllum, with subordinate Lithothamnion, encrusting bryozoans, acervulinids, and serpulids. Their loose internal structure, multispecific encrustations, and fine sediment infill indicate low-energy conditions and relative substrate stability. Taxonomic composition, absence of shallow-water coral taxa, and dominance of thin hapalidialean thalli suggest development on a deep tropical oligophotic ramp (ca. 70–100 m). Growth-rate contrasts between slow-growing corallines and faster bryozoans, together with alternating algal- and bryozoan-dominated layers, imply multiannual fluctuations in nutrient supply rather than simple seasonal forcing. Upwelling-driven nutrient enrichment likely enhanced phytoplankton productivity, turbidity, and pCO2, periodically favoring suspension feeders over coralline algae in a context of intense space competition. These cyclical ecological shifts may reflect interannual to decadal variability in wind-driven upwelling intensity, consistent with broader Tortonian paleoceanographic perturbations. Collectively, sedimentological, compositional, and taphonomic evidence supports a dynamic outer-ramp system modulated by recurrent upwelling pulses and internal-wave processes.

The Mediterranean Sea is a vast, semi-enclosed basin characterised by high biodiversity and a unique mix of Atlantic, endemic, and non-indigenous tropical species. This complexity also stems from its ongoing “retropicalisation” driven by climate change alongside human-altered connections to the Red Sea and accidental introductions. Interestingly, these modern shifts represent an inverted parallel to the Late Miocene. Back then, climate cooling and restriction of oceanic connections degraded the Mediterranean's tropical environment, paving the way for the establishment of temperate conditions during the Pliocene. Rather than a direct analogue the Late Miocene record provide a critical benchmark for understanding how marine ecosystems react to forced biotic reorganisation in a semi-enclosed basin. Across the Western, Central, and Eastern Mediterranean, this special issue compiles case studies that reconstruct the environmental evolution of the basin during this major transition. The resulting picture is not one of sudden collapse, but rather a prolonged, spatially heterogeneous, ecological reorganisation. Tropical ecosystems progressively adapted and displayed remarkable resilience before ultimately, though not at the same time, succumbing to the dramatic shifts of the Messinian Salinity Crisis. This new evidence provides an unprecedentedly detailed baseline for understanding tropical ecosystem collapse driven by coupled climatic change and altered basin connectivity.

Coralligenous build-ups in the Ionian Sea off southeastern Sicily, Italy, first discovered about 20 years ago, were recently studied as part of the Italian FISR project 'CresciBluReef'. Coralligenous build-ups are a priority habitat in the Mediterranean Sea, yet their origin and species richness are still poorly known. In the studied area, calcareous algae constitute the primary framework of the concretions, and serpulid worms and other invertebrates contribute secondarily. Live and dead serpulid associations were sampled from four build-ups at a depth of 33-37 m. Thirty-five living species of Serpulidae were detected (12 Serpulinae, 15 Filograninae and 8 Spirorbinae), some of which were frequent and abundant. This greatly outnumbers the 7 species previously found at a single build-up in the same area, and the 10 species reported from nearby coralligenous concretions. All but two of the species were already known from the Mediterranean Sea, although eight of the species (three of which are still under description) are reported for the first time from the coralligenous biocoenosis. Our findings highlight that: (i) the serpulid species richness of the studied build-ups is higher than that known in other Mediterranean areas; (ii) the serpulid associations at the four build-ups are comparable in composition and structure, except for a slight reduction in numbers of individuals with depth; and (iii) at each build-up, serpulids are more abundant in the top part than in the bottom part of the concretions.

The occurrence and distribution of mesophotic autogenic habitat engineers are critical for the effective management and conservation of marine resources, as well as for regulating human activities. Recent deep-sea explorations in the Mediterranean Sea have revealed the occurrence of algal reefs (coralligenous), alongside notable contributions from invertebrates in creating reefs. As part of the CRESCIBLUREEF project, we investigated a sector offshore Marzamemi (Ionian Sea), at depths between 30 and 100 m, using integrated acoustic surveys and underwater videos. This study presents a detailed characterization of the acoustic facies associated with a dense population of Dendrophyllia ramea, a vulnerable arborescent scleractinian coral included in the cold-water coral group and recognized as habitat-former. Found between 65 and 90 m of depth in a muddy-sandy sedimentary environment, D. ramea forms small reef-like banks with patchily distributed colonies ranging in size from less than 10 cm to over 50 cm, generally on buried biogenic hard substrates, occupying an area of approximately 0.42 km2. D. ramea functions as an autogenic habitat engineer, supporting rich epibenthic assemblages including abundant Neopycnodonte cochlear at the base of the colonies. However, evidence of marine litter, particularly abandoned ropes entangling the corals, indicates anthropogenic impacts, likely from fishing activities. By identifying the acoustic signature of D. ramea and validating it with in situ observations, this study successfully maps previously undocumented mesophotic coral banks in the Marzamemi area. Our findings underscore the ecological relevance and vulnerability of D. ramea habitats, highlighting the need for their inclusion in marine spatial planning and conservation strategies.

The dissolution of CO2 in seawater as bicarbonate ions (HCO₃⁻) offers a promising alternative to geological storage, provided the process ensures long-term stability and avoids harming marine ecosystems. Storing CO2 in the form of bicarbonate ions could remain effective for geologic timescales, potentially up to 10,000 years [1–3]. This approach involves treating natural seawater by mixing it with pre-equilibrated seawater solutions produced from the reaction of CO2 with Ca(OH)2, adjusted to maintain the same pH as seawater. Recent research [4] has shown that the resulting bicarbonate-rich solution is stable, but concerns persist regarding its potential environmental impacts. While alkalinity itself does not directly affect marine biology, its increase significantly alters pH and the concentrations of key ions and molecules, such as those in the carbonate system, which can directly influence biological processes. The extent of modifications to seawater carbonate chemistry depends on the amount of alkalinity added per unit volume and the rate at which this volume mixes with surrounding waters. The rate at which perturbed seawater equilibrates with the atmosphere is also a critical factor. Seagrasses, marine angiosperms that evolved from terrestrial plants and returned to the sea during the Cretaceous period (approximately 140 to 100 million years ago), play a vital role in marine ecosystems. Seagrass meadows are among the most productive ecosystems on Earth, with an average primary productivity ranging from 394 to 1200 g C m⁻² y⁻¹. These meadows provide numerous essential ecosystem services. Seagrasses are thought to benefit from ocean acidification, as they can utilize both CO₂ and HCO₃⁻ for photosynthesis, although they have a higher affinity for CO₂ and are often carbon-limited [6–7]. Additionally, evidence from natural volcanic CO₂ vents at Ischia, Panarea Islands, and Basiluzzo Islet—where conditions of natural acidification occur—indicates a correlation between increased dissolved inorganic carbon (DIC) and enhanced net primary production [8]. Building on existing literature, this analysis will explore the potential co-benefits of increased bicarbonate concentrations for seagrasses, aiming to assess how these benefits could enhance seagrass health and growth. It will also evaluate the opportunity to integrate this technology with Nature-Based Solutions, such as seagrass restoration, to maximize ecosystem resilience and climate mitigation efforts.References[1] Renforth & Henderson. (2017). Assessing Ocean Alkalinity for Carbon Sequestration. Rev. Geophys. [2] Middelburg et al. (2020). Ocean Alkalinity, Buffering and Biogeochemical Processes. Rev. Geophys. [3] Eisaman et al. (2023). Assessing the Technical Aspects of Ocean-Alkalinity-Enhancement Approaches. State Planet, 2-oae2023, 1–29.[4] Varliero et al. (2024). Assessing the Limit of CO2 Storage in Seawater as Bicarbonate-Enriched Solutions. Molecules. 29, 4069.[5] Duarte et al. (2005). Major role of marine vegetation on the oceanic carbon cycle. Biogeosciences 2, 1–8.[6] Invers et al. (2001). Inorganic carbon sources for seagrass photosynthesis: an experimental evaluation of bicarbonate use in species inhabiting temperate waters, J. Exp. Mar. Biol. Ecol., 265, 203–217, 2001.[7] Koch et al. (2013). Climate change and ocean acidification effects on seagrasses and marine macroalgae, Glob. Change Biol., 19, 103–132.[8] Guilini et al. (2017). Response of Posidonia oceanica seagrass and its epibiont communities to ocean acidification. PLoS ONE 12 (8): e018153

Calcifying red algae foster unique and rich biological communities and are important component of the global C cycle. Rhodolith beds are globally distributed biodiversity hotspots that are engineered by free-living calcifying red algae, and maerl beds are a type of rhodolith bed typically characterized by free-living, twig-like coralline algae with a branched growth form. Phymatolithon calcareum, along with the more recently described Phymatolithon lusitanicum is considered a major component of maerl beds in Europe. Here, we explore the morphology of the vegetative thallus of P. calcareum and P. lusitanicum. Our aim is to identify statistically valuable morphological features that can be used to differentiate these two species of algae that are macroscopically very similar, frequently sterile, and share the same habitat. Morphological features have historically failed to aid in distinguishing P. lusitanicum from P. calcareum. Our observations of seasonal growth patterns, the arrangement of perithallial cells in filaments, and the size of cell fusions provide noteworthy advances in our ability to use morphological features to identify the different species. Additionally, ultrastructural characteristics appear to be a reliable distinguishing feature between the two Phymatolithon species.

Ocean acidification (OA) due to anthropogenic CO2 emissions has significantly altered ocean chemistry since the industrial era. Ocean alkalinity enhancement (OAE) is an innovative strategy to mitigate excess CO2, with ocean liming (OL) serving as a potential carbon dioxide removal (CDR) method, through the spreading of Ca(OH)2 (slaked lime) at the ocean surface. This study examined the ecological effects of OL on a natural zooplankton community from the ultraoligotrophic Eastern Mediterranean Sea during a 14-day mesocosm experiment conducted in spring-summer. We investigated how varying concentrations of slaked lime (low: 0.00074 g L-1, high: 0.0067 g L-1) affected zooplankton community structure, abundance, composition and species diversity The experiment revealed complex temporal dynamics in the zooplankton community across all treatments. Tintinnina and Copepoda dominated the community (80-100 %), with nauplii significantly outnumbering copepodites and adults by an order of magnitude. Redundancy Analysis and correlations showed that pH, temperature and nutrient availability were crucial factors shaping the zooplankton community structure. These environmental variables explained more of the community variation than the lime treatments themselves, highlighting the importance of considering multiple ecological factors when assessing OAE impacts. The overall community response to OL treatments was subtle, suggesting potential resilience of the zooplankton community to short-term alkalinity enhancement. This research provides insights into the ecological implications of OL as a potential OA mitigation strategy. It emphasizes the need for longer-term studies to fully understand the cascading effects on marine food webs and ecosystem functioning. Future research should focus on the interplay between OL, nutrient dynamics, and trophic interactions to better predict the ecological consequences of large-scale OAE implementation.

The exploration of the Linosa Island shelf (Sicily Channel, Mediterranean Sea) by remotely operated vehicle surveys revealed a previously undescribed morphotype of coralligenous algal reef. These biogenic solid substrates are characterized by planar to conical shapes, ranging from single to multilayered structures, with an elevation of 20-30 cm and a concave to convex arrangement. Such coralligenous assemblages primarily cover the seafloor at depths between 80 and 100 m, developing on a sedimentary substrate rich in biogenic components, particularly abundant rhodoliths. We document the role of crustose coralline algae as autogenic engineers, in transforming mobile biogenic sediments into stable substrates by algal reef formation, as theorized by benthic bionomics for the corallige`ne de plateau.

Understanding Earth's changing climate is a crucial challenge. However, the available time series of direct measurements are often insufficient to fully capture climatic processes that unfold over centuries and millennia. Combining history and geology can fill this gap. Focusing on rainfall and flood events, this study proposes a multidisciplinary approach to integrate the sedimentary and meteorological records of the Magra River (Northern Italy), using historical data as a bridge between the two datasets. A pristine record of near shore river-mouth deposits, covering the last thousand years, is analysed, interpreting sand layers as flood events. The results are merged with a coherent historical record of floods and river activity spanning six centuries and instrumental measurements spanning two centuries. The timing of the deposition of the sand layers interpreted as flood events is reasonably consistent with rainfall data and historical records, testifying to the reliability of the river mouth sedimentary record as a proxy for river discharge. The complete dataset and the comparison with other basins of the northwestern Mediterranean clearly indicate common floods during the 1150–1200, 1350–1400, 1550–1590, 1720–1820, and 1950–1970 intervals, all periods characterised by predominantly negative phases of the North Atlantic Oscillation.

Current efforts to reduce CO2 emissions are being insufficient to decrease its atmospheric concentration and to avoid exceeding the warming threshold in the Paris agreement. Although reducing emissions remains essential, additional tools to limit global warming are being actively searched. These include methods to reduce the concentration of atmospheric CO2 by capturing it from the air (the so-called Negative Emissions Technologies, NET). Ocean Alkalinity Enhancement (OAE) is a potentially viable NET that consists on the addition of alkaline substances, including slaked lime (calcium hydroxide), to the ocean, which enhances the ocean’s capture of atmospheric CO2 and raises the pH of the seawater, thus countering ocean acidification. Beyond technological challenges to cost-effective OAE methods, a rigorous assessment of potential ecological and geochemical impacts is necessary. Ocean liming on the wake of ships is proposed as one of the most efficient ways for OAE. The discharge of slaked lime as a side activity of maritime traffic avoids the need of dedicated boats thus increasing the efficiency of OAE by reducing the amount of CO2 emitted to perform this technique. Nevertheless, this procedure can cause local pH peaks, which may have temporary and local effects on the pelagic ecosystem, e.g. by selecting less sensitive plankton species and promoting the growth of calcifiers, thus shifting the phytoplankton composition and the functioning of the whole plankton community. The impact of OAE on the structure and functioning of plankton communities is however poorly known. Here we present results of the impact on phytoplankton biomass and plankton community metabolism (photosynthesis and respiration) of repeated additions of slaked lime (Ca(OH)2) during two mesocosm experiments in two contrasting coastal environments: the highly productive upwelling system of the Ría de Vigo (NW Spain) and the ultraoligotrophic eastern Mediterranean in Crete (Greece). The same experimental design was conducted at the CIM-ECIMAT (University of Vigo) and CRETACOSMOS (Hellenic Centre for Marine Research) facilities. Nine mesocosms were filled with natural coastal seawater. Three served as control, and Ca(OH)2 slurry additions were repeated on days 1,3,5 (Vigo) and 1,3,5,7,9,11 (Crete) to simulate the chronic disturbance expected from repeated discharges from ships. Two different concentrations of calcium hydroxide were used, with three replicates each. pH, O2, salinity, and temperature were recorded with a ten-minutes frequency. Size-fractionated chlorophyll a (0.2-2, 2-20, >20 μm) results indicate a dose-dependent effect on the phytoplankton community, with a differential response depending on the phytoplankton size-fraction. Gross primary production (GPP), community respiration (CR) and net community production (NCP) were determined from in vitro changes in O2 concentration after 24 h light and dark incubations. Preliminary results indicate that the trophic functioning of the plankton community was impacted only by the high slurry addition treatment (H), and more notably in the eutrophic ecosystem of the Ría de Vigo. The response, however, was similar in both experiments, with GPP decreasing to a greater extent than CR, which caused a reduction of NCP in the H with respect to the L and control mesocosms.

The coralligenous algal reefs are priority ecosystems of the Circalittoral zone of the Mediterranean Sea shelves. Characterized by decimeter- to meter-high build-ups, these reefs create a complex environment hosting high biodiversity. Despite their common occurrence, the roles of mollusks within coralligenous structures have been overlooked until recently. To address this gap, the CRESCIBLUREEF project investigated samples of coralligenous build-ups collected from diverse settings at similar depths (33.5- 37.2 m) along the SE Sicilian shelf (Italy). In the studied samples, we identified 158 mollusk species –mostly autochthonous– including some previously undocumented for this habitat. Our study underscores the importance of using appropriate sampling techniques in studying mollusk biodiversity and the pivotal role algal reefs play in supporting a diverse array of mollusk species, spanning not only those associated with hard substrates but also epiphytic, cryptic, and infaunal species. Despite the samples being sourced at similar depths, multivariate statistical analysis based on mollusk abundance indicated a distinction between coralligenous morphologies in the living assemblage. Overall, our findings contribute to improving the knowledge on mollusks associated with coralligenous reefs and emphasize the valuable role of mollusks as sensitive indicators of environmental conditions, confirming their vital importance in conservation science and benthic ecology.

Throughout the past decades, the rise of atmospheric carbon dioxide (CO2) levels has been one of the most important global issues. Higher CO2 concentrations contribute to a strengthened greenhouse effect, resulting in elevated temperatures and a severe acidification of the oceans. Recently, the Intergovernmental Panel on Climate Change (IPCC) highlighted the need to develop CO2 removal approaches, as an essential support to mitigate the ongoing climate change. To this purpose, Negative Emission Technologies (NETs) are capable of extracting CO2 from the atmosphere, keeping it stored in geological reservoirs for long periods. Among NETs, Limenet s.r.l. is proposing a pH equilibrated Ocean Alkalinity Enhancement (OAE) process which involves the permanent storage of carbon dioxide in seawater in the form of bicarbonates using calcium hydroxide and releasing a carbon enriched solution at the same pH of natural seawater. The life cycle assessment conducted on this process demonstrated that the advantages of CO2 capture and storage outweigh the greenhouse gas emissions produced by the entire process. Although this technology is economically promising and the chemical analysis has shown that CO2 stored in the form of bicarbonates in the seawater is quite stable, it’s necessary to assess any possible impact of the pH equilibrated OAE on marine organisms. In light of this, the aims of this project are: 1) To assess the short-term response of the biota after the treatment. 2) To study the effects of a prolonged exposure to the treated water on planktonic and benthic communities through mesocosms experimentation. All experiments are conducted in the Gulf of La Spezia (North-West Italy).