The fast changes in climate are driving global efforts to reduce greenhouse emissions and offset those that cannot be avoided. Interest in vegetated coastal ecosystems, known as Blue Carbon Ecosystems (BCEs), has grown rapidly due to their potential contribution to global carbon sequestration. Spain and Portugal host two of the main BCEs types; seagrass meadows and tidal marshes. To date, no comprehensive national assessment of BCEs carbon stocks has been conducted for Spain and Portugal. We have assessed the magnitude of the carbon sink associated with them across the entire Iberian Peninsula and insular Spanish territories and the potential CO2 emission resulting from their degradation. The BCEs in the studied area are estimated to store 95 Tg CO2-eq in the biomass and top meter of soil, equivalent to about 25% of the CO2 emissions of Spain and Portugal in 2022. The average rate of accumulation of organic carbon to the soil stock was estimated at 0.15 Tg CO2-eq y−1, equivalent to 0.04% of the annual anthropogenic CO2eq emissions of these two countries (in 2022). Additionally, the loss of BCEs in Spain and Portugal over the last century may have released 11–27 Tg CO2-eq, whereas we predicted that 1.3–5.6 Tg CO2-eq will be released over the next 30 years. Which underscores the urge to increase conservation and restoration efforts. This study provides the first comprehensive Spanish and Portuguese national blue carbon inventory for its inclusion in NDCs, providing baseline data for the implementation of blue carbon offsetting projects.

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.

Seagrass meadows provide multiple ecosystem services, yet the magnitude of these services relative to unvegetated sediments and their variability across seagrass genera and regions are largely unexplored. Here, we present a global meta-analysis of three ecosystem services, organic carbon sequestration and pollution mitigation, and benthic biodiversity enhancement; synthesizing 755, 339 and 401 paired comparisons between seagrasses and unvegetated sediment, respectively. Using random-effects models and meta-regression, we assessed both the magnitude of seagrass ecosystem service enhancement and its variability across genera, tidal regimes, bioregions, and geomorphological settings. Overall, seagrass meadows increased organic carbon contents in sediments, although colonising and small seagrass species showed no significant effect on organic carbon accumulation, suggesting limited contribution to carbon offsetting efforts. Beyond serving as nutrient sinks, our analysis suggests that seagrass meadows may act as nutrient sources in oligotrophic areas, potentially playing a regulatory role in nutrient cycling. Although seagrass meadows promote microplastic and metal accumulation, the scarcity of studies with unvegetated samples as control limits the assessment of the magnitude and variability of this service. Seagrass meadows strongly enhance infaunal abundance and biodiversity, however the lack of a significant effect size on microfauna diversity highlights that both vegetated and unvegetated sediments support their own community assemblages, compositions, and functions. This meta-analysis confirms that seagrass meadows make variable contributions to these three ecosystem services, showing that seagrass ecosystem service provision is not uniform and depends strongly on biological traits and environmental contexts. Context-specific, service-specific approaches are therefore needed for ecosystem service accounting, management and restoration planning, and to avoid overgeneralisation from a small subset of well-studied seagrass taxa.

Current knowledge of long-term mercury (Hg) deposition is predominantly based on studies from the Northern Hemisphere, leading to a geographical bias in the comprehension of the global Hg cycle. Aiming to contribute to fill this knowledge gap, our study presents a high-resolution Hg record of a seagrass Posidonia australis sedimentary archive encompassing the last 3300 years in the Waychinicup estuary (Western Australia, WA). This setting is an ideal site for studying the natural Hg cycle, as it is located in the southwest of the state, outside the prevailing wind patterns that transport emissions from major Hg sources. Our results show that Hg concentrations fluctuated from ∼1300 BCE to 100 CE with a shift in the paleorecord around 350 BCE, which could potentially be associated with an intensified El Niño Southern Oscillation period. From around 100 until ∼1880 CE, the Hg concentration remained relatively constant (∼2 μg kg−1) with an increase up to 8 μg kg−1 from 1880 CE until present. Although Hg accumulation doubled since European settlement in WA (from 2.4 to 4.8 μg m−2 yr−1), such an increase can be explained by enhanced organic carbon accumulation, rather than changes in external Hg fluxes. Therefore, our study showcases the importance of considering biogeochemical processes when reconstructing long-term Hg accumulation based on sedimentary archives. Furthermore, the results demonstrate the key role of seagrass meadows as natural paleoarchives of environmental contamination, providing valuable insights into human impacts on coastal areas and to establish pre-anthropogenic baseline metal concentrations.

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. 

Aim We decipher global patterns in seagrass growth and structure to advance seagrass science, facilitate multidisciplinary research, and promote effective management strategies and policy development for seagrass habitats.Methods A global data set on seagrass total biomass, total net primary productivity (NPP), shoot density, and percent cover was compiled across 11 genera. We tested how these variables change across seagrass genera, life-history strategy, and geographic location; and then modelled the environmental drivers (temperature, nutrients, light availability) of the patterns observed.Results The mean (+/- SE) total biomass of seagrasses globally was estimated at 340 +/- 10 g DW m-2 and total NPP was estimated at 5.4 +/- 0.4 g DW m-2 d-1. Mean shoot density across all genera was 1,780 +/- 67 shoots m-2 and percent cover was 42% +/- 1%. Persistent genera showed the highest total biomass followed by opportunistic then colonising genera, whereas shoot density was higher for colonising than persistent, which had the highest percent cover. Latitudinal and bioregional patterns were identified for some genera and were mostly driven by genus-specific traits, followed by localised environmental factors.Main Conclusions Trends in seagrass meadow ecology were largely influenced by seagrass genera and their life-history strategy, with localised environmental factors, such as temperature and nutrients, also playing a role. Global estimates of seagrass biomass and NPP were 2-times higher than previously reported. Seagrass life-history strategies modulate the trade-offs between NPP and biomass to thrive in specific habitats. This study provides the most updated global synthesis of patterns in seagrass growth and structure.

Seagrass ecosystems are recognized for their capacity to sequester and store organic carbon, but there is large variability in soil organic carbon stocks associated with plant traits and environmental conditions, making the quantification and scaling of carbon storage and fluxes needed to contribute to climate change mitigation highly challenging. Here, we provide estimates of carbon stocks associated with seagrass systems (biomass and soil) through analyses of a comprehensive global database including 2700+ seagrass soil cores. The median global soil Corg stock estimate is 24.2 (12.4 - 44.9) Mg Corg ha-1 in the top 30 cm of soil, 27% lower than estimates from previous global syntheses, refining the IPCC Tier 1 soil Corg stock currently used for carbon accounting in places without local data. We estimate that seagrass carbon stocks at risk of degradation could emit 1,154 Tg (665 - 1699) CO2 with a social cost of $213 billion (2020 US dollars), if no action is taken to conserve these habitats.

In this study, we investigate the interplay between relative sea-level changes, the development of human settlements and land-cover changes in the Vastervik-Gamlebyviken region on the southeast coast of Sweden, an important archaeological area from the Mesolithic until recent times. The reconstruction of shore displacement was based on diatom analysis of radiocarbon-dated sediment cores from three lake basins combined with previously published lake isolation data. The resulting curve was used to construct palaeogeographical maps for selected time windows. Land-cover changes were inferred from pollen data from three lakes using the Landscape Reconstruction Algorithm with its two models REVEALS and LOVE. Our data suggest that people took advantage of the land gained due to an overall fall in relative sea level from similar to 35 to similar to 3 metres above sea level (m a.s.l.) over the last 10 000 years, interrupted by periods of transgression and highstands. A sea-level regression of similar to 16 m occurred between 10 000 and 8500 cal a BP followed by an similar to 3-4-m sea-level rise, reaching similar to 22 m a.s.l. at similar to 7500 cal a BP, which corresponds to the maximum Littorina Sea shoreline in the area. The available archaeological findings for the Mesolithic and Early Neolithic (8950-5450 cal a BP) agree well with the shore displacement curve showing that settlements and human activities were concentrated along or above the shorelines as defined from our study. During the transgression after 8500 cal a BP, however, seasonal settlements were submerged (as shown by findings of polished stone tools and hearths buried in sand) and used again during the subsequent regression after 4600 cal a BP. The Iron Age (2450-900 cal a BP) corresponds partly to a highstand at similar to 11 m a.s.l. between 3600 and 2000 cal a BP and partly to a rapid regression of similar to 8 m between 2000 and 1500 cal a BP, and both periods coincide with known human activities along the contemporaneous shoreline. The rapid regression after 2000 cal a BP corresponds to an increase of both regional and local landscape openness and the beginning of a continuous record of crop cultivation.