The largest and endemic bivalve of the Mediterranean Sea, Pinna nobilis, is on the brink of extinction after a mass mortality event (MME) that has affected its populations since autumn 2016. Since then, different actions have been performed to improve the conservation status of P. nobilis. The monitoring of survivors in open coastal systems along the Spanish Mediterranean coast showed, after an 8-year period since the start of the MME (2017–2024), that the geographical distribution of the survivors in open sea is currently concentrated in a few regions, with focal points of specimen density in Cap de Creus (Catalonia) and Menorca (Balearic Islands). During the exhaustive monitoring of individuals of P. nobilis, the active participation of citizen science became decisive, locating almost half of the survivors. Most individuals were found in marine protected areas, mainly in Posidonia oceanica meadows in the upper 15 m. As a safety measure, several survivors were translocated to safer areas, while evaluation of the impact of the translocation showed no demonstrable effects. The knowledge acquired during these years has highlighted the necessity for collaborative monitoring, specifically to understand the current critical situation of P. nobilis and to implement effective conservation measures for this emblematic species.

Among the well-known greenhouse gases (GHGs), nitrous oxide (N2O) is the third most impactful, possessing a global warming potential approximately 300 times greater than that of carbon dioxide (CO2) over a century. The distribution of N2O in aquatic environments exhibits notable spatial and temporal variations, and emissions remain inadequately constrained and underrepresented in global N2O emission inventories, particularly for coastal zones. This study focuses on N2O levels and air–sea fluxes in the coastal waters of the Balearic Islands Archipelago in the Western Mediterranean basin. Data were gathered between 2018 and 2023 at three coastal monitoring stations: two on the densely populated island of Mallorca and the third in the well-preserved National Park of the Cabrera Archipelago. Seawater N2O concentrations varied from 6.5 to 9.9 nmol L−1, with no significant differences being detected across the sites. When these sink–source strengths are integrated on an annual basis, the Balearic Sea is close to equilibrium with atmospheric N2O, resulting in a neutral atmosphere–ocean exchange (0.1 ± 0.2 µmolm-2d-1). A consistent seasonal pattern was noted during the study period. Machine learning analysis indicated that seawater temperature was the primary factor influencing N2O concentrations, with lesser contributions from chlorophyll levels and salinity.

BackgroundSeagrass meadows are among the most abundant marine coastal ecosystems in the world. The wide variety of species, a worldwide distribution with overall high abundance, and especially their high productivity make them a plausible nature-based blue carbon solution to mitigate atmospheric CO2 levels. In the Mediterranean Basin, the endemic angiosperm Posidonia oceanica plays a remarkable role as a marine habitat provider in shallow waters through its vertical growth and as a carbon sink storing allochthonous carbon and biomass underneath the meadows.ObjectivesHere, we assess the capacity of a pristine meadow to oxygenate the water column in the coastal area of the Balearic Islands through an evaluation of the metabolic rates in the benthic compartment as well as the resulting oxygen concentrations in the pelagic compartment.MethodsGross primary production (GPP), respiration (R), and net community production (NCP) are determined from dissolved oxygen (DO) measurements using two different calculation methods: a model developed for this purpose is used for data obtained from water column sensors and benthic multiparametric sensors, whereas the mass balance of measured DO is used to calculate the metabolic rates inside benthic chambers.ResultsThe meadow at our study site was characterised as a net autotrophic ecosystem throughout the year. Oxygen productivity was significantly higher in the benthic compartment than in the water column and followed clear seasonal patterns, with enhanced productivity during spring.NoveltyThis work shows the key role of a healthy Posidonia oceanica ecosystem as a water column oxygenator by comparing primary production using three different sampling strategies. The potential of the seagrass as climate change mitigator and its importance for the Mediterranean coasts should be considered in future coastal planning strategies.

Extreme storms can trigger abrupt and often lasting changes in ecosystems by affecting foundational (habitat-forming) species. While the frequency and intensity of extreme events are projected to increase under climate change, its impacts on seagrass ecosystems remain poorly documented. In January 2020, the Spanish Mediterranean coast was hit by Storm Gloria, one of the most devastating recent climate events in terms of intensity and duration. We conducted rapid surveys of 42 Posidonia oceanica meadows across the region to evaluate the extent and type of impact (burial, unburial and uprooting). We investigated the significance of oceanographic (wave impact model), geomorphological (latitude, depth, exposure), and structural (patchiness) factors in predicting impact extent and intensity. The predominant impact of Storm Gloria was shoot unburial. More than half of the surveyed sites revealed recent unburial, with up to 40 cm of sediment removed, affecting over 50 % of the meadow. Burial, although less extensive, was still significant, with 10-80 % of meadow cover being buried under 7 cm of sediment, which is considered a survival threshold for P. oceanica. In addition, we observed evident signs of recently dead matte in some meadows and large amounts of detached drifting shoots on the sea bottom or accumulated as debris on the beaches. Crucially, exposed and patchy meadows were much more vulnerable to the overall impact than sheltered or continuous meadows. Given how slow P. oceanica is able to recover after disturbances, we state that it could take from decades to centuries for it to recoup its losses. Seagrass ecosystems play a vital role as coastal ecological infrastructure. Protecting vulnerable meadows from anthropogenic fragmentation is crucial for ensuring the resilience of these ecosystems in the face of the climate crisis.

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Warming as well as species introductions have increased over the past centuries, however a link between cause and effect of these two phenomena is still unclear. Here we use distribution records (1813-2023) to reconstruct the invasion histories of marine non-native macrophytes, macroalgae and seagrasses, in the Mediterranean Sea. We defined expansion as the maximum linear rate of spread (km year-1) and the accumulation of occupied grid cells (50 km2) over time and analyzed the relation between expansion rates and the species' thermal conditions at its native distribution range. Our database revealed a marked increase in the introductions and spread rates of non-native macrophytes in the Mediterranean Sea since the 1960s, notably intensifying after the 1990s. During the beginning of this century species velocity of invasion has increased to 26 ± 9 km2 year-1, with an acceleration in the velocity of invasion of tropical/subtropical species, exceeding those of temperate and cosmopolitan macrophytes. The highest spread rates since then were observed in macrophytes coming from native regions with minimum SSTs two to three degrees warmer than in the Mediterranean Sea. In addition, most non-native macrophytes in the Mediterranean (>80%) do not exceed the maximum temperature of their range of origin, whereas approximately half of the species are exposed to lower minimum SST in the Mediterranean than in their native range. This indicates that tropical/subtropical macrophytes might be able to expand as they are not limited by the colder Mediterranean SST due to the plasticity of their lower thermal limit. These results suggest that future warming will increase the thermal habitat available for thermophilic species in the Mediterranean Sea and continue to favor their expansion.

The critically endangered endemic bivalve Pinna nobilis from the Mediterranean Sea suffered a sudden population decline after a mass mortality event in early autumn 2016. Conservation efforts aimed at preventing extinction included safeguarding resistant individuals and implementing a breeding plan to contribute to the repopulation of the species. This study utilized a model combining Lagrangian dispersion and connectivity analyses to pinpoint optimal restocking sites in the Western Mediterranean. Our approach allowed to identify locations capable of sustaining and generating larvae for broader repopulation in key areas of the Western Mediterranean Sea prior to the mass mortality event. Six important repopulation locations from Murcia, Valencia and Balearic Islands were selected for reintroduction efforts. The results obtained in this study show how the network could be self-sufficient and able to self-replenish itself of recruits. Overall, our work can be used to direct the reintroduction of resistant animals in the Western Mediterranean Sea.

Methane (CH4) gas is the most important GHG after carbon dioxide, with open ocean areas acting as discreet CH4 sources and coastal regions as intense but variable CH4 sources to the atmosphere. Here, we report CH4 concentrations and air-sea fluxes in the coastal area of the Balearic Islands Archipelago (Western Mediterranean Basin). CH4 levels and related biogeochemical variables were measured in three coastal sampling sites between 2018 and 2021, with two located close to the densely populated island of Mallorca and one in a pristine area in the Cabrera Archipelago National Park. CH4 concentrations in seawater during the study period ranged from 2.7 to 10.9 nM, without significant differences between the sampling sites. Averaged estimated CH4 fluxes during the sampling period for the three stations oscillated between 0.2 and 9.7 μmol m-2 d-1 according to a seasonal pattern and in general all sites behaved as weak CH4 sources throughout the sampling period.

Limnology and Oceanography BulletinVolume 32, Issue 2 p. 84-84 Meeting Highlights It will be Worth the Wait: ASLO Aquatic Sciences Meeting 2023 in Palma de Mallorca Nona Sheila R. Agawin, Nona Sheila R. Agawin [email protected] orcid.org/0000-0001-5951-360X Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorIris E. Hendriks, Iris E. Hendriks orcid.org/0000-0002-2238-6018 Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), Esporles, SpainSearch for more papers by this authorEva Sintes, Eva Sintes orcid.org/0000-0002-7408-5647 Centro Oceanográfico de Baleares, IEO-CSIC, Palma, SpainSearch for more papers by this authorMaria Ll. Calleja, Maria Ll. Calleja orcid.org/0000-0002-5992-2013 Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorMaria Capa, Maria Capa orcid.org/0000-0002-5063-7961 Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorManuela Gertrudis García Márquez, Manuela Gertrudis García Márquez orcid.org/0000-0001-7698-5932 Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorLluis Gómez-Pujol, Lluis Gómez-Pujol orcid.org/0000-0002-6746-7604 Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorManuel Hidalgo, Manuel Hidalgo orcid.org/0000-0002-3494-9658 Centro Oceanográfico de Baleares, IEO-CSIC, Palma, SpainSearch for more papers by this authorHilmar Hinz, Hilmar Hinz orcid.org/0000-0003-4909-0089 Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), Esporles, SpainSearch for more papers by this authorNuria Marbá, Nuria Marbá orcid.org/0000-0002-8048-6789 Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), Esporles, SpainSearch for more papers by this authorElvira Mayol Alcover, Elvira Mayol Alcover orcid.org/0000-0002-6179-8269 Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), Esporles, SpainSearch for more papers by this author Nona Sheila R. Agawin, Nona Sheila R. Agawin [email protected] orcid.org/0000-0001-5951-360X Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorIris E. Hendriks, Iris E. Hendriks orcid.org/0000-0002-2238-6018 Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), Esporles, SpainSearch for more papers by this authorEva Sintes, Eva Sintes orcid.org/0000-0002-7408-5647 Centro Oceanográfico de Baleares, IEO-CSIC, Palma, SpainSearch for more papers by this authorMaria Ll. Calleja, Maria Ll. Calleja orcid.org/0000-0002-5992-2013 Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorMaria Capa, Maria Capa orcid.org/0000-0002-5063-7961 Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorManuela Gertrudis García Márquez, Manuela Gertrudis García Márquez orcid.org/0000-0001-7698-5932 Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorLluis Gómez-Pujol, Lluis Gómez-Pujol orcid.org/0000-0002-6746-7604 Department of Biology, University of the Balearic Islands, Palma, SpainSearch for more papers by this authorManuel Hidalgo, Manuel Hidalgo orcid.org/0000-0002-3494-9658 Centro Oceanográfico de Baleares, IEO-CSIC, Palma, SpainSearch for more papers by this authorHilmar Hinz, Hilmar Hinz orcid.org/0000-0003-4909-0089 Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), Esporles, SpainSearch for more papers by this authorNuria Marbá, Nuria Marbá orcid.org/0000-0002-8048-6789 Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), Esporles, SpainSearch for more papers by this authorElvira Mayol Alcover, Elvira Mayol Alcover orcid.org/0000-0002-6179-8269 Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), Esporles, SpainSearch for more papers by this author First published: 04 April 2023 https://doi.org/10.1002/lob.10565Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. Volume32, Issue2May 2023Pages 84-84 RelatedInformation

Seagrasses provide multiple ecosystem services and act as intense carbon sinks in coastal regions around the globe but are threatened by multiple anthropogenic pressures, leading to enhanced seagrass mortality that reflects in the spatial self-organization of the meadows. Spontaneous spatial vegetation patterns appear in such different ecosystems as drylands, peatlands, salt marshes, or seagrass meadows, and the mechanisms behind this phenomenon are still an open question in many cases. Here, we report on the formation of vegetation traveling pulses creating complex spatiotemporal patterns and rings in Mediterranean seagrass meadows. We show that these structures emerge due to an excitable behavior resulting from the coupled dynamics of vegetation and porewater hydrogen sulfide, toxic to seagrass, in the sediment. The resulting spatiotemporal patterns resemble those formed in other physical, chemical, and biological excitable media, but on a much larger scale. Based on theory, we derive a model that reproduces the observed seascapes and predicts the annihilation of these circular structures as they collide, a distinctive feature of excitable pulses. We show also that the patterns in field images and the empirically resolved radial profiles of vegetation density and sediment sulfide concentration across the structures are consistent with predictions from the theoretical model, which shows these structures to have diagnostic value, acting as a harbinger of the terminal state of the seagrass meadows prior to their collapse.

The increasing rates of CO2 due to anthropogenic activities are causing important potential climate threats for the Mediterranean Sea: ocean acidification and warming. In this region, two seagrass species, Posidonia oceanica and Cymodocea nodosa, can play a crucial role in climate change mitigation. Seagrasses can act as carbon sinks, buffer lowering pH values during the day and storing carbon in the sediment underneath their meadows. However, available data documenting these processes are scattered and collected using different methodologies, which makes its interpretation and generalization very challenging. In this study, we analyzed published and unpublished data (collected for this study) on seagrass community metabolism to compare two methodologies, benthic chambers and multiparametric sensors, and evaluate trends through time for these two species. Furthermore, we analyzed seasonal trends of both seagrass species' metabolic rates and their variation between the eastern and western Mediterranean basins. Most evaluated meadows, 80.9 %, were autotrophic. Calculated metabolic rates differ between methodologies, with multiparametric sensors estimating rates almost an order of magnitude higher, 143.22±28.21 (SE) mmol O2 m−2 d−1 for net community production (NCP) compared to an average of 18.75±3.80 (SE) mmol O2 m−2 d−1 for measurements with benthic chambers. However, sensors are not able to differentiate between habitats and only useful to assess seagrass metabolism at a broader community level, whereas benthic chambers are capable of evaluating rates at the species level and confirm that P. oceanica is more productive compared to C. nodosa. We found similar metabolic rates in the eastern and western Mediterranean regions for P. oceanica with the benthic-chamber technique and higher NCP in the west based on sensor measurements.

After the first successful virtual ASLO Aquatic Sciences Meeting (ASM) in 2021 where we transformed the scientific and social events into interactive media experiences due to the COVID19 pandemic, we are now hosting the ASLO ASM 2023 in-person with a new theme. The theme is resilience and recovery in aquatic systems inspired by the concept that, even facing disruptions and crises such as the COVID-19 pandemic, ASLO adapted and transformed the ASM 2021 Palma meeting to virtual with a positive attitude that in 2023 we would recover and meet in-person. We are constantly facing social, health, and economic crises, as well as human-influenced environmental changes in aquatic systems, but in this meeting, we will focus on positive aspects: resilience and recovery. The theme will be reflected in the plenary sessions with topics ranging from resilience in seafood dynamics to resilience and recovery in different freshwater and marine ecosystems. The logo is a nautilus shell-inspired figure which symbolizes growth, renewal, expansion, and order amidst chaos with images of the traditional mallorcan boat “Llaüt” and palm trees (Fig. 1). This new and meaningful theme and logo for the ASM 2023 is also reflected in the addition and renewal of members of the scientific committee to ensure that both freshwater and marine expertise are well represented to guarantee a successful meeting in Palma. If you have enjoyed the virtual ASM 2021 with all the virtual events, remember it was just a teaser for the face-to-face 2023 meeting in Palma, hope to see you there! In January 2023, the call for abstracts and registration opens, do not miss it! Nona Sheila R. Agawin, Department of Biology, University of the Balearic Islands, Palma, Spain; nona.agawin@uib.es

Anthropogenic impacts on marine ecosystems have led to a decline of biodiversity across the oceans, threatening invaluable ecosystem services on which we depend. Ecological temporal data to track changes in diversity are relatively rare, and the few long-term datasets that exist often only date back a few decades or less. Here, we use eDNA taken from dated sediment cores to investigate changes over approximately the last 100 years of metazoan communities in native (Cymodocea nodosa and Posidonia oceanica) and exotic (Halophila stipulacea) seagrass meadows within the eastern Mediterranean Sea, at two locations in Greece and two in Cyprus. Overall, metazoan communities showed a high turnover of taxa during the past century, where losses of individual taxa in a seagrass meadow were compensated by the arrival of new taxa, probably due to the arrival of exotic species introduced in the Mediterranean Sea from the Suez Canal or the Gibraltar Strait. Specifically, bony fishes (Class Actinopteri) and soft corals (Class Anthozoa) presented significantly higher richness in the past (before the 1980s) than in the most recent time periods (from 1980–2017) and some Cnidarian orders were solely found in the past, whereas sponges and Calanoids (Class Hexanauplia), an order of copepods, showed an increase in richness since the 1980s. Moreover, the Phyla Porifera, Nematoda and the Classes Staurozoa, Hydrozoa and Ophiuroidea were detected in P. oceanica meadows but not in C. nodosa and H. stipulacea, which led to P. oceanica meadows having twice the richness of other seagrasses. The greater richness resulted from the more complex habitat provided by P. oceanica. The combination of eDNA and sediment cores allowed us to reconstruct temporal patterns of metazoan community diversity and provides a novel approach to follow natural communities back in time in the absence of time series and baseline data. The ongoing loss of P. oceanica meadows, likely to be compounded with future warming, might lead to a major loss of biodiversity and the replacement by other seagrass species, whether native or exotic, does not compensate for the loss.

The decreasing seawater pH trend associated with increasing atmospheric carbon dioxide levels is an issue of concern due to possible negative consequences for marine organisms, especially calcifiers. Globally, coastal areas represent important transitional land-ocean zones with complex interactions between biological, physical and chemical processes. Here, we evaluated the pH variability at two sites in the coastal area of the Balearic Sea (Western Mediterranean). High resolution pH data along with temperature, salinity, and also dissolved oxygen were obtained with autonomous sensors from 2018 to 2021 in order to determine the temporal pH variability and the principal drivers involved. By using environmental datasets of temperature, salinity and dissolved oxygen, Recurrent Neural Networks were trained to predict pH and fill data gaps. Longer environmental time series (2012-2021) were used to obtain the pH trend using reconstructed data. The best predictions show a rate of [Formula: see text] pH units year[Formula: see text], which is in good agreement with other observations of pH rates in coastal areas. The methodology presented here opens the possibility to obtain pH trends when only limited pH observations are available, if other variables are accessible. Potentially, this could be a way to reliably fill the unavoidable gaps present in time series data provided by sensors.

Ocean acidification (OA) is a serious consequence of climate change with complex organism-to-ecosystem effects that have been observed through field observations but are mainly derived from experimental studies. Although OA trends and the resulting biological impacts are likely exacerbated in the semi-enclosed and highly populated Mediterranean Sea, some fundamental knowledge gaps still exist. These gaps are at tributed to both the uneven capacity for OA research that exists between Mediterranean countries, as well as to the subtle and long-term biological, physical and chemical interactions that define OA impacts. In this paper, we systematically analyzed the different aspects of OA research in the Mediterranean region based on two sources: the United Nation’s International Atomic Energy Agency’s (IAEA) Ocean Acidification International Coordination Center (OA-ICC) database, and an extensive survey. Our analysis shows that 1) there is an uneven geographic capacity in OA research, and illustrates that both the Algero-Provencal and Ionian sub-basins are currently the least studied Mediterranean areas, 2) the carbonate system is still poorly quantified in coastal zones, and long-term time-series are still sparse across the Mediterranean Sea, which is a challenge for studying its variability and assessing coastal OA trends, 3) the most studied groups of organisms are autotrophs (algae, phanerogams, phytoplankton), mollusks, and corals, while microbes, small mollusks (mainly pteropods), and sponges are among the least studied, 4) there is an overall paucity in socio-economic, paleontological, and modeling studies in the Mediterranean Sea, and 5) in spite of general resource availability and the agreement for improved and coordinated OA governance, there is a lack of consistent OA policies in the Mediterranean Sea. In addition to highlighting the current status, trends and gaps of OA research, this work also provides recommendations, based on both our literature assessment and a survey that targeted the Mediterranean OA scientific community. In light of the ongoing 2021-2030 United Nations Decade of Ocean Science for Sustainable Development, this work might provide a guideline to close gaps of knowledge in the Mediterranean OA research.Systematic Review Registrationhttps://www.oceandecade.org/

The state of the art of epidemic modelling in terrestrial ecosystems is the compartmental SIR model and its extensions from the now classical work of Kermack–Mackendrick. In contrast, epidemic modelling of marine ecosystems is a bit behind, and compartmental models have been introduced only recently. One of the reasons is that many epidemic processes in terrestrial ecosystems can be described through a contact process, while modelling marine epidemics is more subtle in many cases. Here we present a model describing disease outbreaks caused by parasites in bivalve populations. The SIRP model is a multicompartmental model with four compartments, three of which describe the different states of the host, susceptible (i.e. healthy), S, infected, I, and removed (dead), R, and one compartment for the parasite in the marine medium, P, written as a 4-dimensional dynamical system. Even if this is the simplest model one can write to describe this system, it is still too complicated for both direct analytical manipulation and direct comparison with experimental observations, as it depends on four parameters to be fitted. We show that it is possible to simplify the model, including a reduction to the standard SIR model if the parameters fulfil certain conditions. The model is validated with available data for the recent Mass Mortality Event of the noble pen shell Pinna nobilis, a disease caused by the parasite Haplosporidium pinnae, showing that the reduced SIR model is able to fit the data. So, we show that a model in which the species that suffers the epidemics (host) cannot move, and contagion occurs through parasites, can be reduced to the standard SIR model that represents epidemic transmission between mobile hosts. The fit indicates that the assumptions made to simplify the model are reasonable in practice, although it leads to an indeterminacy in three of the original parameters. This opens the possibility of performing direct experiments to be able to solve this question.

We investigated the effects of increasing seawater temperature and CO 2 concentration based on a high business‐as‐usual climate change scenario by year 2100 on the photosynthetic performance and productivity of Mediterranean seagrass Posidonia oceanica and alkaline phosphatase and N 2 ‐fixing activities of microbes associated with different plant parts during winter when the plants may be thermally more vulnerable. Our results suggest that elevated CO 2 and temperature benefit the overall photosynthetic performance of P. oceanica . Despite the benefits, the magnitude of respiration increased with elevated CO 2 resulting in a negative carbon balance for P. oceanica in winter. This trend is contradictory to the general notion of decreased respiration in plants with increasing CO 2 , and warrants future investigation on the mechanisms behind the opposite trend. Changes of alkaline phosphatase activities found here may not be a direct consequence of the different treatments, but indirectly, through changes in the demand for dissolved inorganic phosphorus for N 2 fixers. Of the several groups of N 2 fixers tested for nifH expression (a proxy for activity of nitrogenase, the enzyme required for N 2 fixation), only the unicellular N 2 ‐fixing cyanobacterial phylotypes, UCYNB and UCYNC, actively transcribed with a positive nifH transcription response of UCYNC to elevated CO 2 and temperature. Our results suggest that in future climate scenarios, the structure and diversity of N 2 microbial communities associated with seagrasses may change and high‐light the importance of investigating the responses of different groups individually in their natural habitat substrates.

Seagrass species play a critical role in the mitigation of climate change by acting as valuable carbon sinks and storage sites. Another important ecosystem service of this coastal vegetation is nutrient removal. However, coastal ecosystems are under increasing pressure of global warming and associated establishment of invasive species. To elucidate the respective contributions of seagrass species Posidonia oceanica and Cymodocea nodosa and the non-native macroalga Halimeda incrassata as primary producers and nutrient sinks in coastal habitats we conducted in-situ incubations in the North-western Mediterranean Sea. Measured metabolic activity and nutrient removal as well as calcification rates in these habitats over a 24 h period in spring and summer confirmed that the endemic seagrass P. oceanica represents a valuable ecosystem with high O2 production and considerable carbon capture. The documented regression of P. oceanica meadows with higher temperatures and decline in autotrophy as measured here causes concern for the continuity of ecosystem services rendered by this habitat throughout the Mediterranean Sea with progressing climate warming. In contrast, the enhanced performance of C. nodosa and the calcifying alga H. incrassata with increasing temperatures, under expected rates of future warming is uncertain to mitigate loss of productivity in case of a potential shift in marine vegetation. This could ultimately lead to a decline in ecosystem services, decreased carbon storage and mitigation of climate change. Furthermore, this study provides a first estimate for the growth rate of H. incrassata in the Mediterranean Sea, supporting evidence for the mechanism of its rapid extension.

The introduction and establishment of exotic species often result in significant changes in recipient communities and their associated ecosystem services. However, usually the magnitude and direction of the changes are difficult to quantify because there is no pre‐introduction data. Specifically, little is known about the effect of marine exotic macrophytes on organic carbon sequestration and storage. Here, we combine dating sediment cores (210Pb) with sediment eDNA fingerprinting to reconstruct the chronology of pre‐ and post‐arrival of the Red Sea seagrass Halophila stipulacea spreading into the Eastern Mediterranean native seagrass meadows. We then compare sediment organic carbon storage and burial rates before and after the arrival of H. stipulacea and between exotic (H. stipulacea) and native (C. nodosa and P. oceanica) meadows since the time of arrival following a Before‐After‐Control‐Impact (BACI) approach. This analysis revealed that H. stipulacea arrived at the areas of study in Limassol (Cyprus) and West Crete (Greece) in the 1930s and 1970s, respectively. Average sediment organic carbon after the arrival of H. stipulacea to the sites increased in the exotic meadows twofold, from 8.4 ± 2.5 g Corg m−2 year−1 to 14.7 ± 3.6 g Corg m−2 year−1, and, since then, burial rates in the exotic seagrass meadows were higher than in native ones of Cymodocea nodosa and Posidonia oceanica. Carbon isotopic data indicated a 50% increase of the seagrass contribution to the total sediment Corg pool since the arrival of H. stipulacea. Our results demonstrate that the invasion of H. stipulacea may play an important role in maintaining the blue carbon sink capacity in the future warmer Mediterranean Sea, by developing new carbon sinks in bare sediments and colonizing areas previously occupied by the colder thermal affinity P. oceanica.