Blue carbon effects of mangrove restoration in subtropics where Spartina alterniflora invaded

Functional traits of macrobenthos substantially indicated habitat change from the invasive saltmarsh to introduced mangrove

Mangrove afforestation is usually thought to be beneficial to mitigate the degradation and loss of mangroves. In Southern China, planting mangroves with the introduced Sonneratia apetala is also supportive to remove the invasive Spartina alterniflora. However, the influence of mangrove afforestation dominated by introduced species on macrobenthos, a vital joint of energy flow and nutrient cycling in mangroves, remains unclear. We explored the linkage between the functional traits of macrobenthos and the physicochemical properties of sediments in a coastal continuum including the mudflat (MF), exotic Spartina alterniflora saltmarsh (SL), natural Avicennia marina forest (AM), and introduced S. apetala afforestation (SA) via a seasonal field survey. After removing the S. alterniflora invaded into mudflat via S. apetala afforestation, the sediment C/N ratio decreased compared to that of natural forest, while the concentrations of microphytobenthic chlorophyll-a increased. The macrobenthic inhabiting mode shifted from epifaunal to infaunal as well. The biomass and density of microbenthic community decreased along MF, SL, AM, and SA. SL had greater C/N ratio and smaller functional richness (FR) than MF. AM was characterized by similar functional diversities, and pH value and salinity of sediment to those of MF, and greater microphytobenthic chlorophyll-a was found in AM. Compared to AM, the introduced S. apetala substantially engineered the habitat due to its flourishing above-ground pneumatophore system which caused faster deposition process, subsequently changed the resource utilization strategies of macrobenthos considerably. Overall, the use of Sonneratia afforestation on Spartina removal could not replace the contribution of natural Avicennia forest with respect to the functional traits of macrobenthos. Careful consideration on ecosystem functionalities would be indispensable for conducting saltmarsh eradication and mangrove afforestation in the future.

Photovoltaic Power Station Impacts on the Benthic Ecosystem and Sediment Carbon Storage in Tidal Flats in China

Photovoltaic power is a rapidly growing component of the renewable energy sector. Photovoltaic power stations (PVPSs) on coastal tidal flats offer benefits, but the lack of information on the effects of PVPSs on benthic ecosystems and sediment carbon storage can hamper the development of eco-friendly renewable energy. We sampled the macrobenthos and sediment cores at a PVPS on a coastal tidal flat in eastern China. The biodiversity indicators and benthic ecological quality based on macrobenthos were mostly higher under the photovoltaic panels than elsewhere. These variations were primarily driven by pH, sediment grain size, and chlorophyll-a content. However, the PVPS had exerted a considerable influence on the macrobenthic community structure. Furthermore, the carbon stocks in the sediment cores from under the photovoltaic panels were similar to those in the reference sites. These results suggest that this PVPS has not had discernible short-term adverse effects on the benthic ecosystems or sediment carbon storage of the tidal flat. Nevertheless, the potentially long-term and cascading risks throughout the ecosystem warrant caution. Therefore, we recommend that policymakers adopt a cautious development strategy and implement long-term, high-frequency monitoring to ensure the sustainability of renewable energy production on coastal tidal flats.

Replacing Spartina alterniflora with northward-afforested mangroves has the potential to acquire extra blue carbon

Climate change provides an opportunity for the northward expansion of mangroves, and thus, the afforestation of mangroves at higher latitude areas presents an achievable way for coastal restoration, especially where invasive species S. alterniflora needs to be clipped. However, it is unclear whether replacing S. alterniflora with northward-afforested mangroves would benefit carbon sequestration. In the study, we examined the key CO2 and CH4 exchange processes in a young (3 yr) northward-afforested wetland dominated by K. obovata. We also collected soil cores from various ages (3, 15, 30, and 60 years) to analyze the carbon storage characteristics of mangrove stands using a space-for-time substitution approach. Our findings revealed that the young northward mangroves exhibited obvious seasonal variations in net ecosystem CO2 exchange (NEE) and functioned as a moderate carbon sink, with an average annual NEE of -107.9 g C m-2 yr-1. Additionally, the CH4 emissions from the northward mangroves were lower in comparison to natural mangroves, with the primary source being the soil. Furthermore, when comparing the vertical distribution of soil carbon, it became evident that both S. alterniflora and mangroves contributed to organic carbon accumulation in the upper soil layers. Our study also identified a clear correlation that the biomass and carbon stocks of mangroves increased logarithmically with age (R2 = 0.69, p < 0.001). Notably, both vegetation and soil carbon stocks (especially in the deeper layers) of the 15 yr northward mangroves, were markedly higher than those of S. alterniflora. This suggests that replacing S. alterniflora with northward-afforested mangroves is an effective long-term strategy for future coasts to enhance blue carbon sequestration.

Dynamics of benthic microeukaryotic communities in a mangrove wetland invaded by Spartina alterniflora: Effects of vegetation, seasonality, and sediment depth

Benthic microeukaryotes are crucial mediators of biogeochemical cycles in coastal wetland ecosystems, yet their spatial and temporal variability remains poorly understood. This study delineates the diversity patterns of benthic microeukaryotes in a Spartina alterniflora-invaded mangrove ecosystem in Fujian, China. Using high-throughput sequencing of 18S rRNA gene transcripts, we identified the influences of vegetation, seasonality, and sediment depth on microeukaryotic communities. We discovered that vegetation cover significantly affects community composition, primarily driven by nutrient concentrations and pH. The community structure of microeukaryotes varied seasonally and vertically, correlating with changes in sediment temperature, pH, salinity, and fucoxanthin concentration. Notably, invasive Spartina alterniflora habitats showed enhanced heterotrophic interactions, suggesting that invasive species can reshape benthic microeukaryotic co-occurrence patterns. Seasonal co-occurrence patterns revealed dominant Bacillariophyta assemblages exhibited distinct network modules enriched in the cold (spring) and warm (summer and fall) seasons, respectively, which indicated potential ecological niche differentiation. Our findings reveal the complex relationships between environmental factors and benthic microeukaryotic diversity, offering insights into microbial responses to natural and invasive vegetation influences.

Mapping trade-offs among key ecosystem functions in tidal marsh to inform spatial management policy for exotic Spartina alterniflora

Invasive Spartina alterniflora has become a global management challenge in coastal wetlands. China has decided to eradicate it completely, but the high costs and its provision of beneficial ecosystem functions (EF, in the form of blue carbon and coastal protection) have raised concerns about its removal. Here, using the Yangtze Estuary as a case study, we explore a reasonable pathway of S. alterniflora management that balanced control of invasive species and EF. We simulated the spatial patterns of two key EF - blue carbon storage and wave attenuation - and identified appropriate zones for eradicating S. alterniflora based on their trade-offs. We observed contrasting patterns along the land-sea gradient for S. alterniflora community, with a decrease in blue carbon storage and an increase in wave attenuation. Notably, pioneer S. alterniflora near the foreshore displayed a high cluster of blue carbon storage (63.61 ± 7.33 Mg C ha-1) and dissipated nearly 70% of wave energy by a width of 163 m. The trade-offs between the two EF indicated that the eradication project should be implemented along the seawall rather than the foreshore. Even in the scenario of prioritized shore defense with the largest eradication zone, S. alterniflora still stored 43.1% more carbon (10.67 Gg C) compared to complete eradication and dissipated over 70% of wave energy in extreme events. Our study innovatively integrates eradication and reservation in S. alterniflora management, providing a sustainable and flexible spatial strategy that meets the needs of stakeholders.

Variations in Soil Blue Carbon Sequestration between Natural Mangrove Metapopulations and a Mixed Mangrove Plantation: A Case Study from the World's Largest Contiguous Mangrove Forest

Sundarban is the world's largest mangrove wetland. This study, conducted in 2016, to compare blue carbon sequestration with different natural metapopulations and a four-year-old Avicennia marina (30% area)-Rhizophora mucronata (70% area)-mixed mangrove plantation under anthropoganic stress. The aims of the study is to find out the variations in soil ecological function indicators (pH, electrical conductivity, bulk density, soil texture, available nitrogn, phosphorus and soil organic carbon) and key ecological service indicator (soil blue carbon pool) between sites. Simpson's Index of dominance, diversity and Shannon-Weiner Index revealed that all the sites are under ecological stress, with the Suaeda maritima-dominated mudflat having the least biodiversity. It is also revealed that pH and electrical conductivity were highest in Suaeda maritima and Phoenix padulosa-dominated metapopulations, whereas organic carbon was the highest under the mangrove plantation and Avicennia marina-dominated site. Available nitrogen was recorded highest in the community with the Sonneretia sp.-Avicennia marina association. The mixed mangrove plantation had the highest blue carbon pool. The species diversity was not found to be related with the distance from the nearby conserved mangrove forest, contrary to the island biogeography theory. This study concludes with a recommendation of mixed mangrove plantations to restore the degraded saline mudflats along the human settlements across the globe.