Seagrass losses since mid-20th century fuelled CO2 emissions from soil carbon stocks

Microalgal assemblages response to water quality remediation in coastal waters of Perth, Australia

Nutrient reduction is an essential environmental policy for water quality remediation, but climate change can offset the ecological benefits of nutrient reduction and lead to the difficulty of environmental evaluation. Here, based on the records of three lipid microalgal biomarkers and stable isotopes of carbon and nitrogen in two sediment cores from the embayment of Perth, Australia, we reconstructed the microalgal biomasses (diatoms, dinoflagellates and coccolithophores) over the past century and evaluated the ecological effects of nutrient reduction on them, using Change Point Modeling (CPM) and redundancy analysis (RDA). The CPM result showed that total microalgal biomarkers increased by 25% and 51% in deep and shallow areas, respectively, due to nutrient enrichment caused by industrial wastewater in the 1950s and the causeway construction in the 1970s, and dinoflagellates were beneficiaries of eutrophication. The nutrient reduction policy since the 1980s had not decreased total microalgal biomass, and diatoms were beneficiaries of this period. RDA based on time series of sediment cores and water monitoring data revealed that the increase of sea-surface temperature and the decrease of rainfall since the 1980s may be important factors sustaining the high total microalgal biomass and increasing the degree of diatom dominance. The result also indicated that the variations of microalgal assemblages may better explain the effect of nutrient reduction rather than total microalgal biomass.

Large-scale environmental signals in seagrass blue carbon stocks are hidden by high variability at local scales

Increasing focus on nature-based climate change mitigation and adaptation strategies has led to the recognition of seagrasses as globally significant organic carbon (Corg) stocks. However, estimates of carbon stocks have been generally confined to a few regions, with few African studies represented in global datasets. In addition, the extent to which biogeographical and environmental variation shape carbon stocks in marine vegetated environments remains uncertain. For South Africa, Zostera capensis is the dominant seagrass species with limited mapping and quantification of its Corg stocks. Here, we measured Z. capensis Corg stocks at six South African estuaries spanning ∼1800 km of the cool-temperate to subtropical marine environmental gradient. Targeting the intertidal zone of the upper and lower estuary reaches, we collected Z. capensis sediments to a depth of 50 cm and measured the Corg, with the median Corg stock estimated at 24.11 Mg C ha-1 (40.4 ± 53.02; mean ± SD). While this is lower than the global average, these data demonstrate that Z. capensis ecosystems are important contributors to blue carbon stocks in the region. Measured Corg stocks showed significant differences between sampling sites for estuaries; however, we did not detect significant differences between estuaries due to high intra-estuarine Corg variability. Examination of biogeographical regions, terrestrial and marine environmental variables as drivers of Corg variability revealed that annual mean sea surface temperature may explain variation in Corg stocks. Furthermore, we found evidence of signals of biogeographical regions and precipitation driving some of the variability in Corg stocks; however, this requires further investigation. Overall, our estimates for Z. capensis add to ongoing national and global efforts to quantify seagrass Corg stocks across environmental and biogeographic gradients to better determine their contributions as nature-based solutions to climate change.

Sediment carbon storage differs in native and non-native Caribbean seagrass beds

Non-native species are expanding globally and can alter ecosystem functions, including food web dynamics, community structure and carbon storage. Seagrass are foundation species that contribute a variety of ecosystem services in near-shore coastal ecosystems, including a significant sink of carbon. In the Caribbean, the rapidly expanding non-native Halophila stipulacea has unknown impacts on carbon storage. To investigate the impacts on carbon storage, we quantified organic carbon (Corg) content in sediment and seagrass tissues from monotypic H. stipulacea beds, mixed native seagrass beds dominated by Thalassia testudinum and Syringodium filiforme, and unvegetated substrate in St. John, USVI. We found native seagrass-vegetated sediment contained 1.3 times more Corg than sediment covered by H. stipulacea, and 1.6 times more Corg than unvegetated areas on average. Whereas, H. stipulacea-dominated substrate stored 1.2 times more Corg than unvegetated substrate. Likewise, native species contained 2.2 times more aboveground biomass and 6.0 times more belowground biomass than H. stipulacea. Since seagrasses are critical sources of carbon sequestration, our results suggest that invading H. stipulacea is associated with lower carbon stocks which has potential implications for conservation activities and climate change mitigation.

China's conservation and restoration of coastal wetlands offset much of the reclamation-induced blue carbon losses

China's coastal wetlands have experienced large losses and gains with rapid coastal reclamation and restoration since the end of the 20th century. However, owing to the difficulties in mapping soil organic carbon (SOC) in blue carbon stocks of coastal wetlands on a national scale, little is known about the spatial pattern of SOC stock in China's coastal wetlands and the loss and gain of SOC stock following coastal reclamation, conservation, and restoration over the past decades. Here, we developed a SOC stock map in China's coastal wetlands at 30 m spatial resolution, analyzed the spatial variability and driving factors of SOC stocks, and finally estimated SOC losses and gains due to coastal reclamation and wetland management from 1990 to 2020. We found that the total SOC stocks in China's coastal wetlands were 77.8 Tg C by 2020 with 3.6 Tg C in mangroves, 8.8 Tg C in salt marshes, and 65.4 Tg C in mudflats. Temperature, rainfall, and seawater salinity exerted the highest relative contributions to SOC spatial variability. The spatial trend of SOC density gradually decreased from south to north except for Liaoning province, with the lowest density in Shandong province. About 24.9% (19.4 Tg C) of SOC stocks in China's coastal wetlands were lost due to high-intensity reclamation, but SOC stock gained from conservation and restoration offset the reclamation-induced losses by 58.2% (11.3 Tg C) over the past three decades. These findings demonstrated the great potential of conservation and restoration of coastal wetlands in reversing the loss trend of blue carbon and contributing to the mitigation of climate change toward carbon neutrality. Our study provides significant spatial insights into the stocks, sequestration, and recovery capacity of blue carbon following rapid urbanization and management actions, which benefit the progress of global blue carbon management.

The vulnerability of World Heritage seagrass habitats to climate change

Seagrass is an important natural attribute of 28 World Heritage (WH) properties. These WH seagrass habitats provide a wide range of services to adjacent ecosystems and human communities, and are one of the largest natural carbon sinks on the planet. Climate change is considered the greatest and fastest-growing threat to natural WH properties and evidence of climate-related impacts on seagrass habitats has been growing. The main objective of this study was to assess the vulnerability of WH seagrass habitats to location-specific key climate stressors. Quantitative surveys of seagrass experts and site managers were used to assess exposure, sensitivity and adaptive capacity of WH seagrass habitats to climate stressors, following the Climate Vulnerability Index approach. Over half of WH seagrass habitats have high vulnerability to climate change, mainly from the long-term increase in sea-surface temperature and short-term marine heatwaves. Potential impacts from climate change and certainty scores associated with them were higher than reported by a similar survey-based study from 10 years prior, indicating a shift in stakeholder perspectives during the past decade. Additionally, seagrass experts' opinions on the cumulative impacts of climate and direct-anthropogenic stressors revealed that high temperature in combination with high suspended sediments, eutrophication and hypoxia is likely to provoke a synergistic cumulative (negative) impact (p < .05). A key component contributing to the high vulnerability assessments was the low adaptive capacity; however, discrepancies between adaptive capacity scores and qualitative responses suggest that managers of WH seagrass habitats might not be adequately equipped to respond to climate change impacts. This thematic assessment provides valuable information to help prioritize conservation actions, monitoring activities and research in WH seagrass habitats. It also demonstrates the utility of a systematic framework to evaluate the vulnerability of thematic groups of protected areas that share a specific attribute.

Seagrass ecosystem adjacent to mangroves store higher amount of organic carbon of Andaman and Nicobar Islands, Andaman Sea

This study quantified the organic carbon (Corg) stocks in Thalassia hemprichii meadows that are (i) adjacent to mangroves (MG), and (ii) without mangroves (WMG), in tropical Andaman and Nicobar Islands (ANI) of India. In the top 10 cm of the sediment, Corg content was 1.8-fold higher at the MG sites than the WMG sites. The total Corg stocks (sediment + biomass) in the 144 ha of seagrass meadows at MG sites (988.74 ± 138.77 Mg C) was 1.9-fold higher than in 148 ha of WMG sites. Protection and management of T. hemprichii meadows of ANI can lead to emission avoidance of around 5447.33 (MG; 3595.12 + WMG: 1852.21) tons of CO2. The social cost of the carbon stocks in these T. hemprichii meadows is around US$ 0.30 and 0.16 million at the MG and WMG sites, respectively, showcasing the importance of ANI's seagrass ecosystems as nature-based solutions for climate change mitigation.

Quantifying blue carbon stocks and the role of protected areas to conserve coastal wetlands

Vegetated coastal ecosystems, in particular mangroves, tidal marshes and seagrasses are highly efficient at sequestering and storing carbon, making them valuable assets for climate change mitigation and adaptation. The state of Queensland, in northeastern Australia, contains almost half of the total area of these blue carbon ecosystems in the country, yet there are few detailed regional or state-wide assessments of their total sedimentary organic carbon (SOC) stocks. We compiled existing SOC data and used boosted regression tree models to evaluate the influence of environmental variables in explaining the variability in SOC stocks, and to produce spatially explicit blue carbon estimates. The final models explained 75 % (for mangroves and tidal marshes) and 65 % (for seagrasses) of the variability in SOC stocks. Total SOC stocks in the state of Queensland were estimated at 569 ± 98 Tg C (173 ± 32 Tg C, 232 ± 50 Tg C, and 164 ± 16 Tg C from mangroves, tidal marshes and seagrasses, respectively). Regional predictions for each of Queensland's eleven Natural Resource Management regions revealed that 60 % of the state's SOC stocks occurred within three regions (Cape York, Torres Strait and Southern Gulf Natural Resource Management regions) due to a combination of high values of SOC stocks and large areas of coastal wetlands. Protected areas in Queensland play an important role in conserving SOC assets in Queensland's coastal wetlands. For example, ~19 Tg C within terrestrial protected areas, ~27 Tg C within marine protected areas and ~ 40 Tg C within areas of matters of State Environmental Significance. Using multi-decadal (1987-2020) mapped distributions of mangroves in Queensland; we found that mangrove area increased by approximately 30,000 ha from 1987 to 2020, which led to temporal fluctuations in mangrove plant and SOC stocks. We estimated that plant stocks decreased from ~45 Tg C in 1987 to ~34.2 Tg C in 2020, while SOC stocks remained relatively constant from ~107.9 Tg C in 1987 to 108.0 Tg C in 2020. Considering the level of current protection, emissions from mangrove deforestation are potentially very low; therefore, representing minor opportunities for mangrove blue carbon projects in the region. Our study provides much needed information on current trends in carbon stocks and their conservation in Queensland's coastal wetlands, while also contributing to guide future management actions, including blue carbon restoration projects.

Computational estimation of sediment symbiotic bacterial structures of seagrasses overgrowing downstream of onshore aquaculture

Coastal seagrass meadows are essential in blue carbon and aquatic ecosystem services. However, this ecosystem has suffered severe eutrophication and destruction due to the expansion of aquaculture. Therefore, methods for the flourishing of seagrass are still being explored. Here, data from 49 public coastal surveys on the distribution of seagrass and seaweed around the onshore aquaculture facilities are revalidated, and an exceptional area where the seagrass Zostera marina thrives was found near the shore downstream of the onshore aquaculture facility. To evaluate the characteristics of the sediment for growing seagrass, physicochemical properties and bacterial ecological evaluations of the sediment were conducted. Evaluation of chemical properties in seagrass sediments confirmed a significant increase in total carbon and a decrease in zinc content. Association analysis and linear discriminant analysis refined bacterial candidates specified in seagrass overgrown- and nonovergrown-sediment. Energy landscape analysis indicated that the symbiotic bacterial groups of seagrass sediment were strongly affected by the distance close to the seagrass-growing aquaculture facility despite their bacterial population appearing to fluctuate seasonally. The bacterial population there showed an apparent decrease in the pathogen candidates belonging to the order Flavobacteriales. Moreover, structure equation modeling and a linear non-Gaussian acyclic model based on the machine learning data estimated an optimal sediment symbiotic bacterial group candidate for seagrass growth as follows: the Lachnospiraceae and Ruminococcaceae families as gut-inhabitant bacteria, Rhodobacteraceae as photosynthetic bacteria, and Desulfobulbaceae as cable bacteria modulating oxygen or nitrate reduction and oxidation of sulfide. These observations confer a novel perspective on the sediment symbiotic bacterial structures critical for blue carbon and low-pathogenic marine ecosystems in aquaculture.

The reproducibility of remotely piloted aircraft systems to monitor seasonal variation in submerged seagrass and estuarine habitats

Seasonal variation in seagrass growth and senescence affects the provision of ecosystem services and restoration efforts, requiring seasonal monitoring. Remotely piloted aircraft systems (RPAS) enable frequent high-resolution surveys at full-meadow scales. However, the reproducibility of RPAS surveys is challenged by varying environmental conditions, which are common in temperate estuarine systems. We surveyed three eelgrass ( Zostera marina) meadows in Newfoundland, Canada, using an RPAS equipped with a three-color band (red, green, blue [RGB]) camera, to evaluate the seasonal reproducibility of RPAS surveys and assess the effects of flight altitude (30–115 m) on classification accuracy. Habitat percent cover was estimated using supervised image classification and compared to corresponding estimates from snorkel quadrat surveys. Our results revealed inconsistent misclassification due to environmental variability and low spectral separability between habitats. This rendered differentiating between model misclassification versus actual changes in seagrass cover infeasible. Conflicting estimates in seagrass and macroalgae percent cover compared to snorkel estimates could not be corrected by decreasing the RPAS altitude. Instead, higher altitude surveys may be worth the trade-off of lower image resolution to avoid environmental conditions shifting mid-survey. We conclude that RPAS surveys using RGB imagery alone may be insufficient to discriminate seasonal changes in estuarine subtidal vegetated habitats.

Global trends and prospects of blue carbon sinks: a bibliometric analysis

Blue carbon sinks (mangroves, saltmarshes, and seagrasses) are considered an effective nature-based approach for climate change mitigation. Despite growing interest, a systematic review of this topic is still scarce. This study evaluated 1348 blue carbon sink-related articles from 1990 to 2020 using bibliometric technology. The results from total of 85 countries, 1538 institutions, and 4492 authors indicated that blue carbon sink research shows the characteristics of rapid growth. The most active country, institution, and author were USA, Chinese Academy of Sciences, and Duarte C.M., respectively. Relatively close academic collaboration has formed in blue carbon science. Environmental Sciences was the most popular category with 590 papers. The percentages of articles related to mangroves, saltmarshes, and seagrasses were 63.87%, 40.36%, and 40.65%, respectively. Mangrove carbon sinks are the most popular topic, and stable isotope and remote sensing are the most researched technologies for mapping and quantifying blue carbon sinks. The threats to blue carbon sinks are complex and distinctive. Restoration, conservation, and management of blue carbon ecosystems aimed to improve their carbon sink capacity are becoming hot issues and should be further investigated in the future. These findings provide a scientific roadmap for further research in this field and will enable stakeholders to identify the research trend.

Eutrophication reduced the release of dissolved organic carbon from tropical seagrass roots through exudation and decomposition

Seagrass bed ecosystem is one of the most effective carbon capture and storage systems on earth. Seagrass roots are the key link of carbon flow between leaf-root-sediment, and the release of dissolved organic carbon (DOC) from seagrass roots through exudation and decomposition are vital sources to the sediment organic carbon (SOC) in the seagrass beds. Unfortunately, human-induced eutrophication may change the release process of DOC from seagrass roots, thereby affecting the sediment carbon storage capacity. However, little is known about the effect of nutrient enrichment on the release of DOC from seagrass roots, hindering the development of seagrass underground ecology. Therefore, we selected Thalassia hemprichii, the tropical dominant seagrass species, as the research object, and made a comparison of the release of DOC from roots through exudation and decomposition under different nitrate treatments. We found that under control, 10 μmol L^-1, 20 μmol L^-1 and 40 μmol L^-1 nitrate treatments, soluble sugar of T. hemprichii roots were 71.37 ± 3.43 mg g^-1, 67.03 ± 5.33 mg g^-1, 49.14 ± 3.48 mg g^-1, and 18.51 ± 2.09 mg g^-1, respectively, while the corresponding root DOC exudation rates were 7.00 ± 0.97 mg g DW root^-1 h^-1, 5.11 ± 0.42 mg g DW root^-1 h^-1, 4.08 ± 0.23 mg g DW root^-1 h^-1, and 3.78 ± 0.74 mg g DW root^-1 h^-1, respectively. There was a significant positive correlation between root soluble sugar and DOC exudation rate. DOC concentration of sediment porewater and SOC content also decreased under nitrate enrichment (though not significantly), which were both significantly positively correlated with the rate of root exuded DOC. Meanwhile, nitrate enrichment also reduced the release rate of DOC from seagrass roots during initial decomposition, and the release flux of DOC from decomposition. Therefore, nutrient enrichment could decrease nonstructural carbohydrates of seagrass roots, reducing the rate of root exuded DOC, thereby lowered SOC, as well as the DOC release from seagrass root decomposition. In order to increase the release of DOC from seagrass roots and improve the carbon sequestration capacity of seagrass beds, effective measures should be taken to control the coastal nutrients input into seagrass beds.

Climate-Friendly Seafood: The Potential for Emissions Reduction and Carbon Capture in Marine Aquaculture

Aquaculture is a critical food source for the world's growing population, producing 52% of the aquatic animal products consumed. Marine aquaculture (mariculture) generates 37.5% of this production and 97% of the world's seaweed harvest. Mariculture products may offer a climate-friendly, high-protein food source, because they often have lower greenhouse gas (GHG) emission footprints than do the equivalent products farmed on land. However, sustainable intensification of low-emissions mariculture is key to maintaining a low GHG footprint as production scales up to meet future demand. We examine the major GHG sources and carbon sinks associated with fed finfish, macroalgae and bivalve mariculture, and the factors influencing variability across sectors. We highlight knowledge gaps and provide recommendations for GHG emissions reductions and carbon storage, including accounting for interactions between mariculture operations and surrounding marine ecosystems. By linking the provision of maricultured products to GHG abatement opportunities, we can advance climate-friendly practices that generate sustainable environmental, social, and economic outcomes.

Charting a course for genetic diversity in the UN Decade of Ocean Science

The health of the world's oceans is intrinsically linked to the biodiversity of the ecosystems they sustain. The importance of protecting and maintaining ocean biodiversity has been affirmed through the setting of the UN Sustainable Development Goal 14 to conserve and sustainably use the ocean for society's continuing needs. The decade beginning 2021-2030 has additionally been declared as the UN Decade of Ocean Science for Sustainable Development. This program aims to maximize the benefits of ocean science to the management, conservation, and sustainable development of the marine environment by facilitating communication and cooperation at the science-policy interface. A central principle of the program is the conservation of species and ecosystem components of biodiversity. However, a significant omission from the draft version of the Decade of Ocean Science Implementation Plan is the acknowledgment of the importance of monitoring and maintaining genetic biodiversity within species. In this paper, we emphasize the importance of genetic diversity to adaptive capacity, evolutionary potential, community function, and resilience within populations, as well as highlighting some of the major threats to genetic diversity in the marine environment from direct human impacts and the effects of global climate change. We then highlight the significance of ocean genetic diversity to a diverse range of socioeconomic factors in the marine environment, including marine industries, welfare and leisure pursuits, coastal communities, and wider society. Genetic biodiversity in the ocean, and its monitoring and maintenance, is then discussed with respect to its integral role in the successful realization of the 2030 vision for the Decade of Ocean Science. Finally, we suggest how ocean genetic diversity might be better integrated into biodiversity management practices through the continued interaction between environmental managers and scientists, as well as through key leverage points in industry requirements for Blue Capital financing and social responsibility.

Can the Non-native Salt Marsh Halophyte Spartina alterniflora Threaten Native Seagrass (Zostera japonica) Habitats? A Case Study in the Yellow River Delta, China

Seagrass meadows are critical ecosystems, and they are among the most threatened habitats on the planet. As an anthropogenic biotic invader, Spartina alterniflora Loisel. competes with native plants, threatens native ecosystems and coastal aquaculture, and may cause local biodiversity to decline. The distribution area of the exotic species S. alterniflora in the Yellow River Delta had been expanding to ca.4,000 ha from 1990 to 2018. In this study, we reported, for the first time, the competitive effects of the exotic plant (S. alterniflora) on seagrass (Zostera japonica Asch. & Graebn.) by field investigation and a transplant experiment in the Yellow River Delta. Within the first 3 months of the field experiment, S. alterniflora had pushed forward 14 m into the Z. japonica distribution region. In the study region, the area of S. alterniflora in 2019 increased by 516 times compared with its initial area in 2015. Inhibition of Z. japonica growth increased with the invasion of S. alterniflora. Z. japonica had been degrading significantly under the pressure of S. alterniflora invasion. S. alterniflora propagates sexually via seeds for long distance invasion and asexually by tillers and rhizomes for short distance invasion. Our results describe the invasion pattern of S. alterniflora and can be used to develop strategies for prevention and control of S. alterniflora invasion.

Two decades of seagrass area change: Organic carbon sources and stock

Although seagrass ecosystems provide various ecosystem services, the implications in correspondence with temporal changes of the meadows is lacking. In this study, we analyzed two-decade changes of the seagrass area with the organic carbon storage and the sources at Libong island in Thailand. The seagrass area covered 841 ha in 2019, after two decades of decline (3.2 and 0.6% yr^-1 between 2004 and 2009 and 2009-2019, respectively). Although δ¹³C was not significant between depth layers (p > 0.05), the general trend suggested that the terrestrial source of carbon is dominating bottom depth layer (31.7-37.2%), mixture of terrestrial (19.7-30.3%), seagrass (22.9-29.6%), mangrove (16.8-43.0%) and CPOM (11.2-25.4%) in the middle, and mangroves and seagrasses are dominating surface layer (28.3-66.2 and 29.3-36.5%, respectively). These trends approximately correspond to the areal changes of the meadows, as well as changes of urban area and water quality, providing detailed information on the meadow changes and possible causes.