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 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. 

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. © 2024. The Authors.