Seagrass meadows store sedimentary carbon and nitrogen and thus play an important role in climate change mitigation and nutrient retention globally. Here we reconstruct the impact of land-use and coastal land uplift on millennial–scale seagrass (Zostera marina) blue carbon and nitrogen accumulation in the Baltic Sea based on a multiproxy paleoreconstruction approach. We show that increased landscape clearance resulted in higher terrestrial runoff and increased accumulation of carbon and nitrogen, whereas land uplift led to hydrodynamically sheltered environments facilitating seagrass colonization, occurring 1700 and > 4000 years ago. The establishment of the seagrass meadows resulted in enhancement of the quality of carbon and nitrogen stocks with increased lignin contents. Seagrass establishment also likely supported export of organic matter to surrounding unvegetated areas and thereby contributed to carbon and nitrogen accumulation beyond habitat boundaries. The findings show that the long-term carbon and nitrogen accumulation rates and permanence are shaped by land cultivation and geomorphological changes and highlights the importance of seagrass conservation for maintaining thousands of years of sedimentary carbon and nitrogen storage.

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. 

The cumulative impact of anthropogenic pressures on coastal seas is important to consider for a strategic and sustainable management of marine ecosystems. We aim to demonstrate how, and to what extent, incorporating interactions among ecosystem components (species and habitats) and indirect effects of pressures through other ecosystem components can develop existing cumulative impact assessment (CIA) models. A Swedish case study area was selected to test a simplified version of the extended regional Symphony CIA model. Five pollution- and climate-driven pressures acting on three trophically connected ecosystem components, i.e. cod, herring and plankton species/organism groups, were used. In addition, we conducted a systematic review of the scientific literature to determine the impact weight scores for an advancement of the method. The results from the development of CIA models clearly indicate the importance of introducing ecosystem component interactions and indirect effects into CIA models. The total cumulative impact increased by 117 % in the test area, but even more importantly, the development of the model resulted in a spatially more detailed outcome with a greater spatial variability in the magnitude of the total cumulative impact. New areas were highlighted that are under pressure compared to the original model. Thus, the development of the model captures cumulative impacts that would otherwise be overlooked if ecosystem component interactions and indirect effects were ignored. These types of changes to CIA models are required to increase the predictive power and ecological relevance to accommodate solid holistic and ecosystem-based marine management.

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/m(2); mean TN: 320-570 g/m(2)), with enclosed bays having the highest percentage content of C-org and TN, but low sediment density (< 0.1 g cm(3)), 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 CO<inf>2</inf> 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.

Seagrass provides a crucial habitat for numerous marine species and serves as a vital food source for endangered species, like dugongs. While extensive research on restoration has been conducted on certain temperate and slow-growing climax seagrass species, limited attention has been given to tropical pioneer species. This study aimed to assess and compare two restoration methods for the pioneer seagrass Halodule uninervis and evaluate their potential for biodiversity recovery after planting. We conducted a field experiment at subtropical Inhaca Island, southern Mozambique, testing the efficiency of two planting methods (plugs and single shoots) and two planting densities (similar to 100 and similar to 300 shoots/m(2)). We monitored seagrass shoot density in two sites for 16 months, and benthic macrofauna density for 12 months. Results demonstrated that seagrass could grow in all combinations of planting methods and densities in both sites. Specifically, the single shoot method at the high-density treatment proved the most effective, resulting in approximately 1000 shoots/m(2) within a year. Faunal densities, primarily dominated by polychaetes followed by malacostraca, bivalves, and gastropods, indicated rapid colonization of the planted areas, especially in the high-density treatments. Our findings suggest that restoring H. uninervis is feasible using the two tested planting methods. This is particularly significant because H. uninervis is a preferred dugong food source, and its decline due to anthropogenic activities could be reversed through restoration efforts. Nonetheless, conserving existing seagrass should be the primary focus, and restoration approaches should be employed as a valuable tool for managing coastal areas.

In the face of climate and environmental changes, monitoring habitats is crucial to understand organism responses to stress and implement effective management strategies. This study focuses on seagrass distribution in lagoons of Mauritius, a small island state in the Indian Ocean, known for its rich biodiversity and extensive coastal seascapes. Seagrass meadows are vital ecosystems that provide climate change solutions, biodiversity support, and serve as nursery grounds for marine species. However, seagrasses are threatened globally by human activities such as unsustainable fishing, coastal development, and high nutrient levels. In Mauritius, seagrasses also face challenges from land conversion and tourist-induced removal. This research aimed to establish important baseline data for seagrass meadows across seven major lagoons of Mauritius. To achieve this, we conducted an ecological inventory based on ground-truthing, where data were collected using transect surveys to document seagrass species composition (diversity and cover), seagrass structural complexity (shoot density and leaf length) and spatial distribution of seagrass meadows. The field survey identified five seagrass species (Thalassodendron ciliatum, Halodule sp., Syringodium isoetifolium, Halophila ovalis, and Halophila stipulacea) occurring in mixed or monospecific seagrass stands (sometimes intermingling with macroalgae). Syringodium isoetifolium was generally the dominant species (occurring in all seven lagoons and at relatively high coverage), while the remaining four seagrass species occurred in high coverage in specific lagoons or relatively sparsely. The study emphasises the need for comprehensive spatial (and temporal) baseline data to monitor ecosystem shifts and track the effectiveness of management interventions.

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.