Seagrass meadows are globally recognized as critical natural carbon sinks, commonly known as 'blue carbon'. However, seagrass decline attributed to escalating human activities and climate change, significantly influences their carbon sequestration capacity. A key aspect in comprehending the impact of seagrass decline on carbon sequestration is understanding how degradation affects the stored blue carbon, primarily consisting of sediment organic carbon (SOC). While it is widely acknowledged that seagrass decline affects the input of organic carbon, little is known about its impact on SOC pool stability. To address this knowledge, we examined variations in total SOC and recalcitrant SOC (RSOC) at a depth of 15 cm in nine seagrass meadows located on the coast of Southern China. Our findings revealed that the ratio of RSOC to SOC (RSOC/SOC) ranged from 27 % to 91 % in the seagrass meadows, and the RSOC/SOC increased slightly with depth. Comparing different seagrass species, we observed that SOC and RSOC stocks were 1.94 and 3.19-fold higher under Halophila beccarii and Halophila ovalis meadows compared to Thalassia hemprichii and Enhalus acoroides meadows. Redundancy and correlation analyses indicated that SOC and RSOC content and stock, as well as the RSOC/SOC ratio, decreased with declining seagrass shoot density, biomass, and coverage. This implies that the loss of seagrass, caused by human activities and climate change, results in a reduction in carbon sequestration stability. Further, the RSOC decreased by 15 %, 29 %, and 40 % under unvegetated areas compared to adjacent Halophila spp., T. hemprichii and E. acoroides meadows, respectively. Given the anticipated acceleration of seagrass decline due to climate change and increasing coastal development, our study provides timely information for developing coastal carbon protection strategies. These strategies should focus on preserving seagrass and restoring damaged seagrass meadows, to maximize their carbon sequestration capacity.

Wetland area and condition are declining globally despite their importance to climate change mitigation and biodiversity. Introduced ungulate species are contributing to the global decline. Their impacts on wetlands are widespread and varied, however poorly understood. We summarise global impacts of introduced unmanaged and domesticated ungulates on wetlands highlighting potential outcomes of their removal. We place an emphasis on Australia due to the disproportionate impacts of ungulates on wetlands and potential for emerging carbon and biodiversity markets to incentivise private investment in wetland conservation and restoration. Our Systematic Literature Review assessed impacts of cattle, pigs, horses, deer, buffalo, sheep, camels, and other ungulates on wetlands. There were 372 relevant resources from 35 countries, with highest representation from Australia and the United States. The majority related to cattle (29 %) and pigs (19 %). More impacts were reported in freshwater wetlands (51 %) than marine (19 %). A quarter of studies related to riparian habitats. Ungulate impacts varied geographically and among climates. More studies reported soil damage, weed dispersal, decreased vegetation cover, and woody vegetation suppression than neutral or positive changes in these metrics. Decreases in richness and abundance of native flora and fauna were more frequently reported than increases. Of 33 studies reporting wetland carbon impacts, 24 reported increased CO2 emissions due to loss of soil carbon or vegetation biomass. Ungulate exclusion from wetlands could enhance carbon stocks and biodiversity, however further studies comparing wetland typologies and carbon dynamics are needed to quantify levels of enhancement given differences in ungulate species and environments.

Blue carbon has made significant contributions to climate change adaptation and mitigation while assisting in achieving co-benefits such as aquaculture development and coastal restoration, winning international recognition. Climate change mitigation and co-benefits from blue carbon ecosystems are highlighted in the recent Intergovernmental Panel on Climate Change Special Report on Ocean and Cryosphere in a Changing Climate. Its diverse nature has resulted in unprecedented collaboration across disciplines, with conservationists, academics, and politicians working together to achieve common goals such as climate change mitigation and adaptation, which need proper policy regulations, funding, and multi-prong and multi-dimensional strategies to deal with. An overview of blue carbon habitats such as seagrass beds, mangrove forests, and salt marshes, the critical role of blue carbon ecosystems in mitigating plastic/micro-plastic pollution, as well as the utilization of the above-mentioned blue carbon resources for biofuel production, are critically presented in this research. It also highlights the concerns about blue carbon habitats. Identifying and addressing these issues might help preserve and enhance the ocean's ability to store carbon and combat climate change and mitigate plastic/micro-plastic pollution. Checking out their role in carbon sequestration and how they act as the major carbon sinks of the world are integral parts of this study. In light of the global frameworks for blue carbon and the inclusion of microalgae in blue carbon, blue carbon ecosystems must be protected and restored as part of carbon stock conservation efforts and the mitigation of plastic/micro-plastic pollution. When compared to the ecosystem services offered by terrestrial ecosystems, the ecosystem services provided by coastal ecosystems, such as the sequestration of carbon, the production of biofuels, and the remediation of pollution, among other things, are enormous. The primary purpose of this research is to bring awareness to the extensive range of beneficial effects that can be traced back to ecosystems found in coastal environments.

Simple Summary Seafloor biodiversity provides a key ecosystem service, as an efficient route for carbon to be removed from the atmosphere to become buried (long-term) in marine sediment. Protecting near intact ecosystems, particularly those that are hotspots of biodiversity, with high numbers of unique species (endemics), is increasingly being recognised as the best route to protect existing blue carbon. This study measured globally significant stocks of blue carbon held within both rocky (17.5 tonnes carbon km(-2)) and soft (4.1 t C km(-2)) substrata shallow (20 m) seafloor communities along the Antarctic Peninsula. Along the 7998 km of seasonally ice-free shoreline, 59% of known dive sites were classified as rocky and 12% as soft substratum. This gave estimates of 253k t C in animals and plants found at 20 m depth, with a potential sequestration of 4.5k t C year(-1). More carbon was stored in assemblages with greater functional groups. Of the Antarctic Peninsula shore, 54% is still permanently ice covered, and so blue carbon ecosystem services are expected to more than double with continued climate warming. As one of the few increasing negative feedbacks against climate change, protecting seafloor communities around the Antarctic is expected to help tackle both the biodiversity and climate crises. The importance of cold-water blue carbon as biological carbon pumps that sequester carbon into ocean sediments is now being realised. Most polar blue carbon research to date has focussed on deep water, yet the highest productivity is in the shallows. This study measured the functional biodiversity and carbon standing stock accumulated by shallow-water (<25 m) benthic assemblages on both hard and soft substrata on the Antarctic Peninsula (WAP, 67 degrees S). Soft substrata benthic assemblages (391 +/- 499 t C km(-2)) contained 60% less carbon than hard substrata benthic assemblages (648 +/- 909). In situ observations of substrata by SCUBA divers provided estimates of 59% hard (4700 km) and 12% soft (960 km) substrata on seasonally ice-free shores of the Antarctic Peninsula, giving an estimate of 253,000 t C at 20 m depth, with a sequestration potential of ~4500 t C year(-1). Currently, 54% of the shoreline is permanently ice covered and so climate-mediated ice loss along the Peninsula is predicted to more than double this carbon sink. The steep fjordic shorelines make these assemblages a globally important pathway to sequestration, acting as one of the few negative (mitigating) feedbacks to climate change. The proposed WAP marine protected area could safeguard this ecosystem service, helping to tackle the climate and biodiversity crises.

Rising atmospheric CO2 is intensifying climate change but it is also driving global and particularly polar greening. However, most blue carbon sinks (that held by marine organisms) are shrinking, which is important as these are hotspots of genuine carbon sequestration. Polar blue carbon increases with losses of marine ice over high latitude continental shelf areas. Marine ice (sea ice, ice shelf and glacier retreat) losses generate a valuable negative feedback on climate change. Blue carbon change with sea ice and ice shelf losses has been estimated, but not how blue carbon responds to glacier retreat along fjords. We derive a testable estimate of glacier retreat driven blue carbon gains by investigating three fjords in the West Antarctic Peninsula (WAP). We started by multiplying ~40 year mean glacier retreat rates by the number of retreating WAP fjords and their time of exposure. We multiplied this area by regional zoobenthic carbon means from existing datasets to suggest that WAP fjords generate 3,130 tonnes of new zoobenthic carbon per year (t zC/year) and sequester >780 t zC/year. We tested this by capture and analysis of 204 high resolution seabed images along emerging WAP fjords. Biota within these images were identified to density per 13 functional groups. Mean stored carbon per individual was assigned from literature values to give a stored zoobenthic Carbon per area, which was multiplied up by area of fjord exposed over time, which increased the estimate to 4,536 t zC/year. The purpose of this study was to establish a testable estimate of blue carbon change caused by glacier retreat along Antarctic fjords and thus to establish its relative importance compared to polar and other carbon sinks.