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The reduction of greenhouse gas emissions is essential to slow down the velocity of climate change. Likewise, the complementary implementation of other mitigation and adaptation strategies is equally important to increase environmental, social, and economic resilience. In coastal areas, one strategy is to focus conservation and management actions on foundation species of marine macrophytes, such as seagrasses and macroalgae. First, the capacity of these ecosystems to store carbon in the long term (blue carbon) makes them important carbon sinks at global scales. Second, their capacity to enhance organisms’ resilience, by minimizing environmental stress through the generation of chemical habitats, makes them important ecosystems to consider as local ocean acidification refugia. However, the complexity and high dynamism of biogeochemistry in coastal seawater, and the scarcity of comprehensive studies, have created uncertainty regarding the generality of potential benefits. Thus, the role of seagrass and macroalgal ecosystems as climate change refugia or hotspots of carbon sequestration is largely unresolved. This talk will show a synthesis of the latter research on these topics and will give a glance at how these concepts can also be applied to seaweed aquaculture as an effective measure to fight climate change in coastal areas

Deep-ocean seaweed dumping is not an ecological, economical, or ethical answer to climate-change mitigation via carbon “sequestration.” Without sound science and sufficient knowledge on impacts to these fragile ecosystems, it distracts from more rational and effective blue-carbon interventions. We call for a moratorium on sinking seaweeds to deep-ocean ecosystems until its efficacy is established, and there is robust, evidence-based assessment of its environmental, economic, and societal sustainability

Seagrass meadows are biodiversity hotspots for invertebrate species including decapods. Understanding the drivers of species abundance, richness and diversity of decapod assemblages is crucial for the conservation of such hotspots, but how drivers act across multiple spatial scales remains unexplored. Here we describe the decapod assemblages of Posidonia oceanica seagrass meadows and assess the influence of attributes from three increasing spatial scales (habitat, landscape, and geographical levels) on the assemblages’ structure and composition, as well as the variability partitioning among each one of these levels. Overall, geographical level attributes (i.e., inlet aperture, confinement) affected the most the decapod assemblages, while we only found a modest contribution from habitat (e.g., detritus biomass, sediment organic matter) and landscape attributes (e.g., fragmentation). We suggest that decapod assemblages are driven by the interaction of multiple processes occurring at different scales and other highly stochastic phenomena such as larval dispersion and recruitment

Global trends of ocean warming, deoxygenation, and acidification are not easily extrapolated to coastal environments. Local factors, including intricate hydrodynamics, high primary productivity, freshwater inputs, and pollution, can exacerbate or attenuate global trends and produce complex mosaics of physiologically stressful or favorable conditions for organisms. In the California Current System (CCS), coastal oceanographic monitoring programs document some of this complexity; however, data fragmentation and limited data availability constrain our understanding of when and where intersecting stressful temperatures, carbonate system conditions, and reduced oxygen availability manifest. Here, we undertake a large data synthesis to compile, format, and quality-control publicly available oceanographic data from the US West Coast to create an accessible database for coastal CCS climate risk mapping, available from the National Centers for Environmental Information (accession 0277984) at https://doi.org/10.25921/2vve-fh39 (Kennedy et al., 2023). With this synthesis, we combine publicly available observations and data contributed by the author team from synoptic oceanographic cruises, autonomous sensors, and shore samples with relevance to coastal ocean acidification and hypoxia (OAH) risk. This large-scale compilation includes 13.7 million observations from 66 sources and spans 1949 to 2020. Here, we discuss the quality and composition of the synthesized dataset, the spatial and temporal distribution of available data, and examples of potential analyses. This dataset will provide a valuable tool for scientists supporting policy- and management-relevant investigations including assessing regional and local climate risk, evaluating the efficacy and completeness of CCS monitoring efforts, and elucidating spatiotemporal scales of coastal oceanographic variability

Marine kelp forests cover 1/3 of our world's coastlines, are heralded as a nature-based solution to address socio-environmental issues, connect hundreds of millions of people with the ocean, and support a rich web of biodiversity throughout our oceans. But they are increasingly threatened with some areas reporting over 90% declines in kelp forest cover in living memory. Despite their importance and the threats they face, kelp forests are entirely absent from the international conservation dialogue. No international laws, policies, or targets focus on kelp forests and very few countries consider them in their national policy. The Kelp Forest Challenge addresses that gap. Together with 252 kelp experts, professionals, and citizens from 25 countries, the Kelp Forest Challenge was developed as a grassroots vision of what the world can achieve for kelp forest conservation. It is a global call to restore 1 million and protect 3 million hectares of kelp forests by 2040. This is a monumental challenge, that will require coordination across multiple levels of society and the mobilization of immense resources. Pledges may therefore include area for protection or restoration, enabling pledges which assist in conservation (funding, equipment, professional expertise, capacity building), or awareness-based pledges which increase awareness or education about kelp forests. Correspondingly, participants may be from government, scientific institutions, private sector, NGOs, community groups, or individuals. This challenge is the beginning of a 17-year mission to save our kelp forests and anyone and any organisation is invited to participate.

Marine kelp forests cover 1/3 of our world's coastlines, are heralded as a nature-based solution to address socio-environmental issues, connect hundreds of millions of people with the ocean, and support a rich web of biodiversity throughout our oceans. But they are increasingly threatened with some areas reporting over 90% declines in kelp forest cover in living memory. Despite their importance and the threats they face, kelp forests are entirely absent from the international conservation dialogue. No international laws, policies, or targets focus on kelp forests and very few countries consider them in their national policy. The Kelp Forest Challenge addresses that gap. Together with 252 kelp experts, professionals, and citizens from 25 countries, the Kelp Forest Challenge was developed as a grassroots vision of what the world can achieve for kelp forest conservation. It is a global call to restore 1 million and protect 3 million hectares of kelp forests by 2040. This is a monumental challenge, that will require coordination across multiple levels of society and the mobilization of immense resources. Pledges may therefore include area for protection or restoration, enabling pledges which assist in conservation (funding, equipment, professional expertise, capacity building), or awareness-based pledges which increase awareness or education about kelp forests. Correspondingly, participants may be from government, scientific institutions, private sector, NGOs, community groups, or individuals. This challenge is the beginning of a 17-year mission to save our kelp forests and anyone and any organisation is invited to participate.

[EN] In this May's "In Depth" we interview Aurora M Ricart, who has just landed at the ICM-CSIC to delve deeper into the effects of climate crisis on marine ecosystems, but specifically on underwater forests, [ES] En el "A fondo" de este mes de mayo entrevistamos a Aurora M Ricart, que acaba de aterrizar en el ICM-CSIC con el objetivo de estudiar el impacto de la crisis climática sobre los ecosistemas marinos, pero especialmente sobre los bosques marinos, [CAT] A l'”A fons” d'aquest mes de maig entrevistem l’Aurora M Ricart, que acaba d'aterrar a l'ICM-CSIC amb l’objectiu d’estudiar l'impacte de la crisi climàtica sobre els ecosistemes marins, però especialment sobre els boscos marins

The efficacy of marine protected areas (MPAs) may be reduced when climate change disrupts the ecosystems and human communities around which they are designed. The effects of ocean warming on MPA functioning have received attention but less is known about how multiple climatic stressors may influence MPAs efficacy. Using a novel dataset incorporating 8.8 million oceanographic observations, we assess exposure to potentially stressful temperatures, dissolved oxygen concentrations, and pH levels across the California MPA network. This dataset covers more than two-thirds of California’s 124 MPAs and multiple biogeographic domains. However, spatial-temporal and methodological patchiness constrains the extent to which systematic evaluation of exposure is possible across the network. Across a set of nine well-monitored MPAs, the most frequently observed combination of stressful conditions was hypoxic conditions (<140 umol/kg) co-occurring with low pH (<7.75). Conversely, MPAs exposed most frequently to anomalously warm conditions were less likely to experience hypoxia and low pH, although exposure to hypoxia varied throughout the 2014–2016 marine heatwaves. Finally, we found that the spatial patterns of exposure to hypoxia and low pH across the MPA network remained stable across years. This multiple stressor analysis both confirms and challenges prior hypotheses regarding MPA efficacy under global environmental change

Sinking vast amounts of seaweed in the deep ocean is currently being proposed as a promising ocean carbon dioxide removal strategy as well as a natural-based solution to mitigate climate change. Still, marketable carbon offsets through large-scale seaweed sinking in the deep ocean lack documentation and could involve unintended environmental and social consequences. Managing the risks requires a number of urgent actions.

The seagrass Posidonia oceanica is the main habitat-forming species of the coastal Mediterranean, providing millennial-scale ecosystem services including habitat provisioning, biodiversity maintenance, food security, coastal protection, and carbon sequestration. Meadows of this endemic seagrass species represent the largest carbon storage among seagrasses around the world, largely contributing to global blue carbon stocks. Yet, the slow growth of this temperate species and the extreme projected temperature and sea-level rise due to climate change increase the risk of reduction and loss of these services. Currently, there are knowledge gaps in its basin-wide spatially explicit extent and relevant accounting, therefore accurate and efficient mapping of its distribution and trajectories of change is needed. Here, we leveraged contemporary advances in Earth Observation—cloud computing, open satellite data, and machine learning—with field observations through a cloud-native geoprocessing framework to account the spatially explicit ecosystem extent of P. oceanica seagrass across its full bioregional scale. Employing 279,186 Sentinel-2 satellite images between 2015 and 2019, and a human-labeled training dataset of 62,928 pixels, we mapped 19,020 km2 of P. oceanica meadows up to 25 m of depth in 22 Mediterranean countries, across a total seabed area of 56,783 km2. Using 2,480 independent, field-based points, we observe an overall accuracy of 72%. We include and discuss global and region-specific seagrass blue carbon stocks using our bioregional seagrass extent estimate. As reference data collections, remote sensing technology and biophysical modelling improve and coalesce, such spatial ecosystem extent accounts could further support physical and monetary accounting of seagrass condition and ecosystem services, like blue carbon and coastal biodiversity. We envisage that effective policy uptake of these holistic seagrass accounts in national climate strategies and financing c

Long-term ecological studies are crucial to understand how and why natural ecosystems change over time and space. Through a revision of historical data and a comparison with current in situ field data, we contribute to the understanding of how the Mediterranean coastal seascape has changed in the last decades. Here we describe the large decrease of the main habitat-forming species Codium vermilara along the Catalan coast (NW Mediterranean). We have analyzed data on presence/absence, abundance and biomass. Since the 70s–80s, when the species reached its highest abundances, the species has totally disappeared from 45% of the revisited sites, and showed a decrease in 95% of its abundance and 97% of its biomass. Codium vermilara has also shown a reduction in its depth range, from 30 to the first 20 m depth. This study highlights the importance of having historical data to detect and describe changes in ecological systems.

The zooxanthellate scleractinian coral Oulastrea crispata, a widely distributed species across central Indo-Pacific nearshore marine habitats, has been first reported from the Mediterranean Sea (Corsica) in 2014. Here we report on two new sites for this species in the NW Mediterranean Sea and provide a general description of external morphological characters of the colonies and a detailed account of the cnidom to help future identifications. Living specimens may appear virtually identical to small colonies (~5 cm) of the Mediterranean zooxanthellate scleractinian Cladocora caespitosa. While this species shows long, ramified, independent corallites, with cylindrical calices, O. crispata has enlarged, cup-like calices, which can be joined by the coenosteum. It also shows clear differences among several groups of nematocysts, principally the presence in the filaments of large penicilli (p - mastigophore) of one type that is absent in C. caespitosa. Identifications based on underwater observations or even the analysis of photographs may easily lead to misleading identifications. We hypothesize that O. crispata may have gone unnoticed because of misidentifications as C. caespitosa. More detailed research is needed to get reliable maps of the actual distribution of this apparently non-indigenous species in the Mediterranean Sea.

This dataset presents the decapod species and abundance, along with the habitat (detached seagrass leaves, fine fraction of detritus, detrital macroalgae, organic matter in sediments, seagrass shoot density and height of unburried rhizomes), landscape (landscape configuration) and geographical attributes (inlet aperture and confinement) in Posidonia oceanica seagrass meadows in 5 localities accross the NW Mediterranean sea. We found that geographical level attributes (i.e., inlet aperture, confinement) affected the most the decapod assemblages, while we only found a modest contribution from habitat (e.g., detritus biomass, sediment organic matter) and landscape attributes (e.g. fragmentation). We suggest that decapod assemblages are driven by the interaction of multiple processes occurring at different scales and other highly stochastic phenomena such as larval dispersion and recruitment

"Blue carbon” ecosystems, which include tidal marshes, mangrove forests, and seagrass meadows, have large stocks of organic carbon (Corg) in their soils. These carbon stocks are vulnerable to decomposition and – if degraded – can be released to the atmosphere in the form of CO2. We present a framework to help assess the relative risk of CO2 emissions from degraded soils, thereby supporting inclusion of soil Corg into blue carbon projects and establishing a means to prioritize management for their carbon values. Assessing the risk of CO2 emissions after various kinds of disturbances can be accomplished through knowledge of both the size of the soil Corg stock at a site and the likelihood that the soil Corg will decompose to CO2

Biotic and abiotic factors influence the accumulation of organic carbon (Corg) in seagrass ecosystems.We surveyed Posidonia sinuosa meadows growing in different water depths to assess the variability in the sources, stocks and accumulation rates of Corg. We show that over the last 500 years, P. sinuosa meadows closer to the upper limit of distribution (at 2-4m depth) accumulated 3- to 4-fold higher Corg stocks (averaging 6.3 kgCorg m-2) at 3- to 4-fold higher rates (12.8 gCorg m-2 yr-1/ compared to meadows closer to the deep limits of distribution (at 6-8m depth; 1.8 kg Corg m-2 and 3.6 g Corg m-2 yr-1/. In shallower meadows, Corg stocks were mostly derived from seagrass detritus (88% in average) compared to meadows closer to the deep limit of distribution (45% on average). In addition, soil accumulation rates and fine-grained sediment content (< 0.125 mm) in shallower meadows (2.0mmyr-1 and 9 %, respectively) were approximately 2-fold higher than in deeper meadows (1.2mmyr-1 and 5 %, respectively). The Corg stocks and accumulation rates accumulated over the last 500 years in bare sediments (0.6 kgCorg m-2 and 1.2 g Corg m-2 yr-1/ were 3- to 11-fold lower than in P. sinuosa meadows, while fine-grained sediment content (1 %) and seagrass detritus contribution to the Corg pool (20 %) were 8- and 3-fold lower than in Posidonia meadows, respectively. The patterns found support the hypothesis that Corg storage in seagrass soils is influenced by interactions of biological (e.g., meadow productivity, cover and density), chemical (e.g., recalcitrance of Corg stocks) and physical (e.g., hydrodynamic energy and soil accumulation rates) factors within the meadow. We conclude that there is a need to improve global estimates of seagrass carbon storage accounting for biogeochemical factors driving variability within habitats.

The database compiles published data (in Serrano et al. 2015, Biogeosciences, doi:10.5194/bgd-12-18913-2015) on biogeochemical characteristics (age, density, organic carbon, inorganic carbon, stable carbon isotopes and sediment grain size) of sediments underneath seagrass meadows (Posidonia sinuosa) along a bathymetric gradient (from 2 to 8 m) and adjacent un-vegetated patches in Garden Island (Perth, Western Australia). The dataset compiles data on biogeochemical sediment characteristics (including organic and inorganic carbon stocks and burial rates) for a total of 5 cores (4 cores from seagrass meadows and 1 core from adjacent un-vegetated sediments). Enquiries about the dataset may be sent to Oscar Serrano o.serranogras@ecu.edu.au.