Understanding where blue carbon habitats occur and how they are affected by human activity contributes to effective management of natural carbon sinks. Here, we compiled geographical data for Sweden to map the distribution of coastal vegetated blue carbon (BC) habitats. The mapping effort focused on well-recognised (salt marshes and seagrass meadows) and emergent BC habitats (other rooted submerged macrophytes and forested wetlands). We also estimated the exposure to anthropogenic pressures on coastal BC habitats based on their proximity to land-based human activities, and subsequently, the portion of these BC habitats that were located within protected areas. The total area of BC habitats was estimated to around 1850 km², corresponding to ca. 35% of the Swedish coast. Seagrass meadows and other rooted submerged macrophytes were dominating, covering about 1500 km². Around 22% of the mapped BC habitats were expected to be exposed to high pressures from land-based human activities due to their location, while BC habitats within protected areas were often less exposed. This nationwide assessment of coastal vegetated BC habitats accentuates the need for strengthening conservation prioritisation to maximise the carbon storage potential of BC habitats.

Marine and salt marsh sediments contain large amounts of organic carbon (OC) and are therefore important in the global carbon cycle. Here, we collated previously published and unpublished measurements of sediment OC in marine and salt marsh sediments in European regional seas (EURO-CARBON; available at https://doi.org/10.5281/zenodo.14905489). To the extent possible the OC data were complemented by variables such as sediment porosity and dry bulk density. The EURO-CARBON dataset holds 61306 individual data entries of sediment OC content from different regions of European regional seas. Around three quarters (76%) were collected in coastal and deep sea bare sediments, 18% from salt marshes, 7% from seagrass habitats, and 0.03% from macroalgal habitats. For all habitats and sediment depth layers the OC content varied between <0.1 and 41.56 % (avg.: 2.47 ± 3.37 %; median: 1.39 %), with the content generally decreasing in the following sequence: salt marsh (5.01 ± 5.96 %; 3.03 %) > seagrass (2.37 ± 5.96 %; 3.03 %) > bare sediment (1.88 ± 2.03 %; 1.20 %). The EURO-CARBON dataset will serve as a basis for future work, and it will be an important resource for researchers, managers, and policymakers working towards protecting sediment OC pools.

Seagrass meadows are vital blue carbon habitats, with sedimentary organic carbon (OC) originating from both the seagrass itself and external sources. In this study, lipid biomarkers (n-alkanes), a well-known proxy for tracing OC sources, were used to indicate seagrass presence in sediment records and to correlate with sedimentary OC in cold-temperate seagrass (Zostera marina) sediments. We calculated a Zostera-ratio (seagrass/algae and terrestrial plants-ratio) using identified seagrass biomass n-alkanes (C15, C17, C19, C21, C23) as a fingerprint for seagrass-derived OC. Based on the presence or absence of seagrass plant remains in sediments, we confirmed an overall significant positive correlation (R2 = 0.49, with significant sites ranging from 0.66 to 0.81; p < 0.001) between the Zostera-ratio and OC in sediment profiles down to 2 m depth. The Zostera-ratio ranged from 0.0006 to 0.35 with higher values indicating seagrass plant material. The findings show that n-alkanes can serve as proxies for both seagrass presence and total OC levels in the sediment.

While marine seagrass habitats are acknowledged as sinks for carbon and nutrients, much less is known about sequestration in brackish-water vegetation. Here, we quantify the amount of organic carbon (C_org) and total nitrogen (TN) in shallow bay sediments (0–25 cm) in the brackish Baltic Sea and assess how it varies with morphometric isolation from the sea, catchment characteristics and abundance of brackish-water vegetation. The sedimentary C_org and TN content per surface area varied across the bay isolation gradient (mean C_org: 2500–4600 g/m²; mean TN: 320–570 g/m²), with enclosed bays having the highest percentage content of C_org and TN, but low sediment density (< 0.1 g cm³), while open bays had more compact sediment with lower percentage content of C_org and TN. The influence of catchment and vegetation characteristics on the sediment C_org and TN content was less clear, suggesting that coastal morphology affecting hydrodynamic exposure is an important determinant of C and TN accumulation in brackish-water bays. The results show that morphometrically isolated shallow coastal areas constitute significant sinks for carbon and nitrogen, which should be considered in management and in any regional estimates of blue carbon and nutrient sequestration functions.

Understanding the carbon sequestration potential of blue carbon ecosystems is important to inform climate policies and to guide restoration and protection efforts. Alkalinity generation is an often overlooked carbon sequestration mechanism, especially in seagrass meadows. Here, we quantified total alkalinity (TA) and dissolved inorganic carbon (DIC) fluxes in two cold-temperate Zostera marina seagrass meadows in Sweden using 24-h in-situ chamber incubations at the end of the high-productivity season. The seagrass meadows were similar net sources of TA (16 ± 45 mmol m⁻² d⁻¹ in Smalsund, 17 ± 16 mmol m⁻² d⁻¹ in Bökevik), whereas DIC fluxes were highly variable (34 ± 59 mmol m⁻² d⁻¹ in Smalsund, −43 ± 35 mmol m⁻² d⁻¹ in Bökevik). Fluxes followed a diurnal cycle consistent with photosynthesis-respiration cycles. As a result, seagrass meadows ameliorated ocean acidification locally during the day, but not during the night. The large CO₂ uptake provided higher buffering levels compared to mangroves and saltmarshes. The TA fluxes were comparable to those reported for Mediterranean and tropical seagrass meadows, but 16-times lower than in mangrove forests and 5-times lower than in saltmarshes. Alkalinity generation in these cold-temperate seagrasses exceeded soil organic carbon stocks accumulation by fourfold, potentially contributing to their carbon sequestration potential and warranting inclusion in seagrass meadow carbon budgets.

Coastal vegetated ecosystems are being increasingly recognized for their capacity to capture carbon, provide long-term biogenic storage, and alleviate nutrient pollution. To assess the ability of shallow, vegetated coastal bays to function as blue carbon and nutrient (nitrogen and phosphorus) sinks, we collected sediment cores in nine shallow enclosed bays (representative of the EU Habitats 1153 and 1154) in the archipelago areas of Sweden (Stockholm), Åland Island, and southwestern Finland. Our study showed strong uniformity of carbon and nutrient storage, substantial accumulation of carbon and nutrients, and minimal regional differences in carbon, nitrogen, and phosphorus sediment stocks. These findings are noteworthy given the large area (142 km²) the shallow enclosed bays cover and the multiple important ecosystem services they provide in the northern Baltic Sea seascape. An initial first-order estimate for the shallow bay ecosystems across the study region indicates that these ecosystems potentially store 84,000 to 430,000 t organic carbon over the top 25 cm sediment. The positive correlation between carbon and nitrogen stocks, and the potentially organically bound nature of phosphorus in sediment, suggests that climate regulation services can be managed in unison with nutrient management efforts. The findings, also considering the consistent pattern of slow sedimentation and accumulation, underscore the importance of protecting shallow coastal bays as carbon and nutrient sinks in the Baltic Sea region.

Assessing historical environmental conditions linked to habitat colonization is important for understanding long-term resilience and improving conservation and restoration efforts. Such information is lacking for the seagrass Zostera marina, an important foundation species across cold-temperate coastal areas of the Northern Hemisphere. Here, we reconstructed environmental conditions during the last 14,000 years from sediment cores in two eelgrass (Z. marina) meadows along the Swedish west coast, with the main aims to identify the time frame of seagrass colonization and describe subsequent biogeochemical changes following establishment. Based on vegetation proxies (lipid biomarkers), eelgrass colonization occurred about 2,000 years ago after geomorphological changes that resulted in a shallow, sheltered environment favoring seagrass growth. Seagrass establishment led to up to 20- and 24-fold increases in sedimentary carbon and nitrogen accumulation rates, respectively. This demonstrates the capacity of seagrasses as efficient ecosystem engineers and their role in global change mitigation and adaptation through CO₂ removal, and nutrient and sediment retention. By combining regional climate projections and landscape models, we assessed potential climate change effects on seagrass growth, productivity and distribution until 2100. These predictions showed that seagrass meadows are mostly at risk from increased sedimentation and hydrodynamic changes, while the impact from sea level rise alone might be of less importance in the studied area. This study showcases the positive feedback between seagrass colonization and environmental conditions, which holds promise for successful conservation and restoration efforts aimed at supporting climate change mitigation and adaptation, and the provision of several other crucial ecosystem services.

In the last three decades, quantitative approaches that rely on organism traits instead of taxonomy have advanced different fields of ecological research through establishing the mechanistic links between environmental drivers, functional traits, and ecosystem functions. A research subfield where trait-based approaches have been frequently used but poorly synthesized is the ecology of seagrasses; marine angiosperms that colonized the ocean 100M YA and today make up productive yet threatened coastal ecosystems globally. Here, we compiled a comprehensive trait-based response-effect framework (TBF) which builds on previous concepts and ideas, including the use of traits for the study of community assembly processes, from dispersal and response to abiotic and biotic factors, to ecosystem function and service provision. We then apply this framework to the global seagrass literature, using a systematic review to identify the strengths, gaps, and opportunities of the field. Seagrass trait research has mostly focused on the effect of environmental drivers on traits, i.e., “environmental filtering” (72%), whereas links between traits and functions are less common (26.9%). Despite the richness of trait-based data available, concepts related to TBFs are rare in the seagrass literature (15% of studies), including the relative importance of neutral and niche assembly processes, or the influence of trait dominance or complementarity in ecosystem function provision. These knowledge gaps indicate ample potential for further research, highlighting the need to understand the links between the unique traits of seagrasses and the ecosystem services they provide.

Seagrass meadows deliver a range of ecosystem services, where one of the more important is the capacity to store carbon and serve as sinks for atmospheric carbon dioxide. The capacity of seagrass meadows for carbon storage might, however, be modified and complicated by several factors; one important factor is the possible effects of calcification within the meadows. In tropical areas, seagrass meadows can contain high proportions of calcareous organisms, which through their calcification may cause release of CO₂. To study this aspect of the CO₂ balance within tropical seagrass systems, we investigated the air-water CO₂ flux in seagrass mesocosms with different plant community compositions, i.e. mixtures of seagrass and calcifying macroalgae, having similar overall photosynthetic oxygen evolution rates. The measured CO₂ fluxes changed both in rate and direction over the day and were significantly related to plant community composition. Downward fluxes of CO₂ were found only over vegetation with high proportion of seagrass and in the afternoon, whereas occurrence of calcifying algae appeared to reverse the flow. A partial least squares (PLS) regression model indicated that pH, pCO₂ and dissolved inorganic carbon (DIC) were the primary environmental variables predicting the CO₂ fluxes. Our findings show that algal calcification might partly counteract the carbon sequestration in seagrass meadows.

Beach wrack of marine macrophytes is a natural component of many beaches. To test if such wrack emits the potent greenhouse gas methane, field measurements were made at different seasons on beach wrack depositions of different ages, exposure, and distance from the water. Methane emissions varied greatly, from 0 to 176 mg CH₄-C m⁻² day⁻¹, with a clear positive correlation between emission and temperature. Dry wrack had lower emissions than wet. Using temperature data from 2016 to 2020, seasonal changes in fluxes were calculated for a natural wrack accumulation area. Such calculated average emissions were close to zero during winter, but peaked in summer, with very high emissions when daily temperatures exceeded 20 °C. We conclude that waterlogged beach wrack significantly contributes to greenhouse gas emissions and that emissions might drastically increase with increasing global temperatures. When beach wrack is collected into heaps away from the water, the emissions are however close to zero.