Vegetated coastal ecosystems, often termed 'Blue Carbon' ecosystems, play a crucial role in climate change mitigation by sequestering significant amounts of atmospheric carbon dioxide. Mangroves, seagrasses, and salt marshes store large carbon stocks in their biomass and soils. However, regional carbon inventories and sequestration rates for these ecosystems remain poorly understood, particularly in the Southwestern Atlantic (SWA). This lack of data hinders the development of effective conservation and climate change mitigation strategies. The SWA, extending from Guyana to Argentina, presents a unique opportunity for studying these ecosystems due to its extensive coastal wetlands. These regions are under increasing pressure from human activities and climate change, highlighting the urgent need for comprehensive research to assess their carbon storage capacity and vulnerability. This study aims to systematically review and synthesize available data on carbon stocks and accumulation rates in SWA vegetated coastal ecosystems, to provide crucial information for policymakers and conservation efforts. The study's significance lies in its potential to inform the development of nature-based solutions for climate change mitigation and adaptation in the region, and contributes to the global understanding of Blue Carbon ecosystems.

Existing literature highlights the global importance of vegetated coastal ecosystems (VCEs) as carbon sinks, but data is geographically biased, with significant gaps in the Southwestern Atlantic. Previous studies have shown the high carbon storage capacity of mangroves, seagrasses, and salt marshes, but regional-scale assessments for the SWA are lacking. The existing data reflects varying research efforts and capabilities, creating inconsistencies and limitations in understanding the region's contribution to global carbon cycles. This study addresses this knowledge gap by compiling and analyzing available data, aiming to provide a more comprehensive understanding of the SWA's blue carbon potential. The importance of VCEs as Natural Climate Solutions (NCS) to meet the goals of the Paris Agreement, along with their various ecosystem services, is well established. However, the SWA region lacks this robust data, hindering the implementation of effective national and regional Blue Carbon strategies.

The study employed a systematic review approach, compiling data from peer-reviewed publications, theses, and dissertations. The geographic scope encompassed the Southwestern Atlantic, from Guyana to Argentina. Data on aboveground and belowground organic carbon (OC) stocks, along with carbon sequestration rates, were collected for mangroves, seagrasses, and salt marshes. The study used a variety of sources, including satellite imagery for area estimation and data from sediment cores and field measurements for carbon stocks. Different methods were employed across the studies included in the analysis, creating challenges in data homogenization. The authors used various approaches to standardize data, including conversion factors and extrapolation techniques, while acknowledging the uncertainties associated with these methods. Data gaps were identified and addressed where possible by using global averages or expert knowledge, and the resulting estimations of stocks and rates are reported with acknowledgements of limitations. Upscaling to ecoregions was achieved using area estimates from different sources, including the MapBiomas project for Brazil. Statistical analyses were conducted to assess differences in carbon stocks and sequestration rates among different ecosystems and ecoregions. The study carefully documented its methodology and limitations to ensure transparency and to guide future research.

The study estimated that approximately 0.4 Pg of organic carbon is stored in the Southwestern Atlantic's vegetated coastal ecosystems, representing 2–5% of global coastal carbon stores. Mangroves account for the largest area (1.2 Mha) and contribute 70–80% of annual organic carbon accumulation. Brazil holds the majority (approximately 95%) of the region's mangrove carbon stocks. The annual carbon sequestration rate is estimated to be between 0.5 and 3.9 Tg C yr⁻¹, equivalent to 0.7–13% of global rates. There are significant differences in carbon stocks and accumulation rates between the three VCE types: mangroves have the highest aboveground biomass, while saltmarshes have the highest soil carbon stocks. Significant spatial variability exists across different ecoregions, potentially influenced by factors such as latitude, freshwater discharge, soil type, and anthropogenic impact. The study revealed important data gaps, particularly for seagrasses and saltmarshes. Data on carbon accumulation rates (CAR) were more abundant for mangroves compared to seagrasses and saltmarshes, highlighting a crucial need for further research in these areas. The findings underscore the need for improved mapping and data collection to enhance the accuracy of carbon stock and sequestration estimates. Aboveground and soil organic carbon stocks vary greatly across different ecosystems and ecoregions. For example, mangrove soils in the Amazonian ecoregion exhibit lower organic carbon content compared to other regions, likely due to hydrodynamic conditions and soil pedogenesis. Differences in seagrass stocks are attributed to a complex interplay of factors, including latitude, pedogenesis, and geomorphology. Mangrove carbon accumulation rates also show a latitudinal variation, with higher rates in the Southeastern Brazil ecoregion compared to Amazonia and Guiana. While the total annual sequestration rates for the region are high, the study acknowledges the significant uncertainties due to data gaps and methodological differences, meaning that these estimates are likely conservative and underestimated.

The findings confirm the Southwestern Atlantic as a significant Blue Carbon hotspot, underscoring its potential for climate change mitigation. The substantial carbon stocks and high sequestration rates of the region's vegetated coastal ecosystems highlight the critical need for conservation and restoration efforts. The dominance of mangroves in carbon storage and accumulation emphasizes the importance of protecting these ecosystems from deforestation and degradation. The spatial variability in carbon stocks and accumulation rates, driven by a complex interplay of environmental and anthropogenic factors, emphasizes the need for targeted conservation strategies tailored to specific ecoregions. The data gaps identified, particularly for seagrasses and salt marshes, highlight the need for future research focusing on improving data collection and mapping. These efforts are crucial for enhancing the accuracy of regional carbon budget estimates and for informing effective climate change mitigation policies. The results also highlight the potential for integrating Blue Carbon strategies into national and regional climate commitments, potentially unlocking new sources of finance for conservation and restoration projects. The study's findings have broad implications for conservation planning and policy development. The data collected can inform decisions about protected area designation, restoration prioritization, and carbon offsetting initiatives in the region. The integration of Blue Carbon into national climate commitments presents a significant economic opportunity for the region while simultaneously contributing to climate change mitigation and biodiversity conservation.

The Southwestern Atlantic holds substantial blue carbon stocks and high sequestration rates, primarily driven by mangroves. The study emphasizes the critical role of these ecosystems in climate change mitigation and highlights the urgent need for conservation and restoration efforts. Significant knowledge gaps exist, particularly regarding seagrass and saltmarsh ecosystems, requiring further research. The findings support the inclusion of blue carbon in national and regional climate strategies, opening avenues for carbon finance and ecosystem protection.

The study acknowledges limitations due to data scarcity and variability in methodologies across studies. Extrapolation techniques and data standardization introduce uncertainties in the estimates of carbon stocks and sequestration rates, particularly for seagrasses and saltmarshes. The uneven spatial distribution of data, with more information available for mangroves than other VCE types, potentially biases the regional assessment. The conservative nature of the estimates suggests the actual carbon storage might be considerably larger. Future research should focus on improving data availability and standardization, leading to more accurate estimates for each ecosystem.