Extent change of protected mangrove forest and its relation to wave power exposure on Aldabra Atoll

Assessing blue carbon in mangrove ecosystems of Seychelles

Mangrove forests play an important role in climate change mitigation and adaptation, globally recognized as natural climate solution. The protection and restoration of mangrove ecosystems are especially important to Small Island Developing States, like Seychelles, due to their vulnerability to the impacts of climate change, such as sea level rise and tropical cyclones. Therefore, it is crucial for countries like Seychelles to develop baseline information on the status of their mangrove forests to guide conservation and management actions. In this study, we conducted a field campaign to collect local data on plant (i.e., aboveground and belowground) and soil carbon from representative mangrove forests in the inner and outer islands of Seychelles. We used this data to develop, for the first time, a blue carbon assessment for Seychelles' mangrove ecosystems. Seychelles holds 2195 ha of mangrove forests, with ∼80% of them found on the outer island of Aldabra Atoll. Seychelles mangrove ecosystems store 688,091 ± 18,353 tonnes of organic carbon (or 2.5 million tonnes CO2e) and an average of 477.0 ± 16.2 tonnes of carbon per hectare, with 70% of their total carbon stocks stored in their soils. Aldabra Atoll holds the highest total carbon stocks, accounting for 67% of Seychelles' mangrove stocks, despite having a relatively shallow soil organic layer (∼40 cm) due to the dominance of limestone and 'coral champignon' below 40 cm depth. Seychelles currently protects ∼85% of its mangrove extent including the Aldabra Atoll, a UNESCO World Heritage and Ramsar site, and Port Launay, a Ramsar site. Overall, field data from this study demonstrates the important climate mitigation potential of Seychelles' mangrove forests and the important role they play in supporting Seychelles in achieving its Nationally Determined Contributions (NDCs) commitments.

Seabird nutrient subsidies enrich mangrove ecosystems and are exported to nearby coastal habitats

Eutrophication by human-derived nutrient enrichment is a major threat to mangroves, impacting productivity, ecological functions, resilience, and ecosystem services. Natural mangrove nutrient enrichment processes, however, remain largely uninvestigated. Mobile consumers such as seabirds are important vectors of cross-ecosystem nutrient subsidies to islands but how they influence mangrove ecosystems is poorly known. We assessed the contribution, uptake, cycling, and transfer of nutrients from seabird colonies in remote mangrove systems free of human stressors. We found that nutrients from seabird guano enrich mangrove plants, reduce nutrient limitations, enhance mangrove invertebrate food webs, and are exported to nearby coastal habitats through tidal flow. We show that seabird nutrient subsidies in mangroves can be substantial, improving the nutrient status and health of mangroves and adjacent coastal habitats. Conserving mobile consumers, such as seabirds, is therefore vital to preserve and enhance their role in mangrove productivity, resilience, and provision of diverse functions and services.

Hydrodynamic Limitations to Mangrove Seedling Retention in Subtropical Estuaries

Mangrove-forest sustainability hinges upon propagule recruitment and seedling retention. This study evaluates biophysical limitations to mangrove-seedling persistence by measuring anchoring force of two mangrove species (Rhizophora mangle L. and Avicennia germinans (L.) L.). Anchoring force was measured in 362 seedlings via lateral pull tests administered in mangrove forests of two subtropical estuaries and in laboratory-based experiments. Removal mechanism varied with seedling age: newly established seedlings failed due to root pull-out while seedlings older than 3 months failed by root breakage. The anchoring force of R. mangle seedlings was consistently and significantly greater than A. germinans (p = 0.002); however, force to remove A. germinans seedlings increased with growth at a faster rate (p < 0.001; A. germinans: 0.20–0.23 N/g biomass; R. mangle: 0.04–0.07 N/g biomass). Increasing density of surrounding vegetation had a positive effect (p = 0.04) on anchoring force of both species. Critical velocities at which seedlings become susceptible to instantaneous uprooting estimated from anchoring forces measured in the field were 1.20 m/s and 1.50 m/s, respectively, for R. mangle and A. germinans. As estimated critical velocities exceed typical flow magnitudes observed in field sites, removal of established seedlings likely occurs following erosion of sediments from the seedling base.

Nature-based solutions for atoll habitability

Atoll societies have adapted their environments and social systems for thousands of years, but the rapid pace of climate change may bring conditions that exceed their adaptive capacities. There is growing interest in the use of ‘nature-based solutions' to facilitate the continuation of dignified and meaningful lives on atolls through a changing climate. However, there remains insufficient evidence to conclude that these can make a significant contribution to adaptation on atolls, let alone to develop standards and guidelines for their implementation. A sustained programme of research to clarify the potential of nature-based solutions to support the habitability of atolls is therefore vital. In this paper, we provide a prospectus to guide this research programme: we explain the challenge climate change poses to atoll societies, discuss past and potential future applications of nature-based solutions and outline an agenda for transdisciplinary research to advance knowledge of the efficacy and feasibility of nature-based solutions to sustain the habitability of atolls.

This article is part of the theme issue ‘Nurturing resilient marine ecosystems’.

Atoll inland and coastal mangrove climate change vulnerability assessment

Climate change threatens global mangroves, which are already among the world's most impacted ecosystems. Vulnerability components of exposure, sensitivity and adaptive capacity were evaluated on mangroves of atoll settings on Jaluit Atoll, in the Marshall Islands, assessing spatial changes of mangrove cover 1945-2018/19, sea-level trends 1968-2019, and reviewing available information. Inland mangrove depressions occur on Jaluit, as well as coastal lagoon margin mangroves, and both were assessed using the same methods. Spatial analysis results showed both inland and coastal mangroves have increased in area. Inland mangroves on eight of Jaluit's islands mostly expanded after 1976 from 40 to 50 hectares, with progradation and tidal creek infill closing lagoon connections. Shoreline mangroves showed 88-100% of transects prograding 0.1-0.51 m year^-1 and 0-11.5% of transects eroding 0-0.18 m year^-1. Assessment of a combination of aerial/satellite images, literature and on-the-ground photos indicated that the mangroves are in healthy condition. Vulnerability assessment results showed both inland and coastal mangroves to have similar strengths and weaknesses in resilience, with intrinsic areas of vulnerability persisting during increased future sea level rise, limited sediment supply and extremely low elevations.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11273-022-09878-0.

Assessing Potential Climatic and Human Pressures in Indonesian Coastal Ecosystems Using a Spatial Data-Driven Approach

Blue carbon ecosystems are key for successful global climate change mitigation; however, they are one of the most threatened ecosystems on Earth. Thus, this study mapped the climatic and human pressures on the blue carbon ecosystems in Indonesia using multi-source spatial datasets. Data on moderate resolution imaging spectroradiometer (MODIS) ocean color standard mapped images, VIIRS (visible, infrared imaging radiometer suite) boat detection (VBD), global artificial impervious area (GAIA), MODIS surface reflectance (MOD09GA), MODIS land surface temperature (MOD11A2), and MODIS vegetation indices (MOD13A2) were combined using remote sensing and spatial analysis techniques to identify potential stresses. La Niña and El Niño phenomena caused sea surface temperature deviations to reach −0.5 to +1.2 °C. In contrast, chlorophyll-a deviations reached 22,121 to +0.5 mg m−3. Regarding fishing activities, most areas were under exploitation and relatively sustained. Concerning land activities, mangrove deforestation occurred in 560.69 km2 of the area during 2007–2016, as confirmed by a decrease of 84.9% in risk-screening environmental indicators. Overall, the potential pressures on Indonesia’s blue carbon ecosystems are varied geographically. The framework of this study can be efficiently adopted to support coastal and small islands zonation planning, conservation prioritization, and marine fisheries enhancement.

Assessment of water contamination by potentially toxic elements in mangrove lagoons of the Red Sea, Saudi Arabia

Mangrove (Avicennia marina) forests in the Red Sea cost have great concern from environmental, biological, economic, and social points of view. Therefore, assessing water contamination in this ecosystem is worth to be investigated. Consequently, here we aimed to examine the levels of salinity, acidity, and the total content of Fe, Mn, Cu, Zn, Cd, Cr, Ni, and Pb in water samples collected from the upper, middle, and lower part of three mangrove lagoons (i.e., Al-Shuaiba, Yanbu, and Jeddah), Red Sea, Saudi Arabia. The total metal content (µg L^-1) in water samples differed significantly among the studied areas and ranged from 286.2 to 4815.0 for Fe, 86.4-483.0 for Mn, 22.9-468.8 for Cu, 199.2-366.6 for Zn, 44.1-99.8 for Cd, 25.6-80.3 for Cr, 11.6-41.5 for Ni, and from 17.7 to 102.0 for Pb. The mean values of Cu, Zn, Cd, and Pb were higher than the WHO water quality standards for fisheries. Water samples in Yanbu were more contaminated and contained higher concentrations of all metals than Jeddah and Al-Shuaiba, due to the petrochemical industries in this industrial area. Our findings suggest that the high metal content in the water of these mangrove sites, particularly in Yanbu, should be considered due to the high potential environmental and human health risks in these ecosystems. These results may help for demonstrating effective approaches for the management of these lagoons. More studies will be carried out on the sediment and mangrove plants in this ecosystem.