Abstract
Historically, coastal ecosystems (tidal marshes, mangrove forests, seagrass meadows) have been impacted and degraded by human intervention, mainly in the form of land acquisition. With increasing recognition of the role of blue carbon ecosystems in climate mitigation, protecting and rehabilitating these ecosystems becomes increasingly more important. This study evaluated the potential carbon gains from rehabilitating a degraded coastal tidal marsh site in south-eastern Australia. Tidal exchange at the study site had been restricted by the construction of earthen barriers for the purpose of reclaiming land for commercial salt production. Analysis of sediment cores (elemental carbon and Pb-210 dating) revealed that the site had stopped accumulating carbon since it had been converted to salt ponds 65 years earlier. In contrast, nearby recovered (control) tidal marsh areas are still accumulating carbon at relatively high rates (0.54 tons C ha(-1) year(-1)). Using elevation and sea level rise (SLR) data, we estimated the potential future distribution of tidal marsh vegetation if the earthen barrier were removed and tidal exchange was restored to the degraded site. We estimated that the sediment-based carbon gains over the next 50 years after restoring this small site (360 ha) would be 9,000 tons C, which could offset the annual emissions of similar to 7,000 passenger cars at present time (at 4.6 metric tons pa.) or similar to 1,400 Australians. Overall, we recommend that this site is a promising prospect for rehabilitation based on the opportunity for blue carbon additionality, and that the business case for rehabilitation could be bolstered through valuation of other co-benefits, such as nitrogen removal, support to fisheries, sediment stabilization, and enhanced biodiversity.
Highlights
Coastal wetlands, such as seagrass meadows, tidal marshes, and mangrove forests, have the ability to sequester organic carbon in their sediments over millennial time scales at rates 30–50-fold greater than the soils of terrestrial forests (Duarte et al, 2013)
At the newly rehabilitated site, the locations were split into three zones: (1) Sarcocornia, (2) Sarcocornia/Suaeda, and (3) Sarcocornia/Tecticornia/Suaeda an equal mix of the three species (Figure 2b). These nine cores from each of the two rehabilitation sites were used for carbon analysis to calculate the mean carbon density at each sample depth and inundation category. While both above- and below-ground plant biomass and soil are used to determine carbon stocks in coastal wetlands, this study focused on the below ground biomass + sediment in the calculations, since this component represents 65–95% of the total stored carbon for tidal marshes (Howard et al, 2014)
The low mean carbon density (13.7 ± 1.4 mg C cm−3) and lack of sediment accretion measured in the salt pond is an indication that either the ponds are not accumulating carbon or accumulation is undetectable
Summary
Coastal wetlands, such as seagrass meadows, tidal marshes, and mangrove forests, have the ability to sequester organic carbon in their sediments over millennial time scales at rates 30–50-fold greater than the soils of terrestrial forests (Duarte et al, 2013). Coastal wetlands represent less than 3% of terrestrial forests coverage, they are able to sequester similar amounts of organic carbon annually (Duarte et al, 2013) These ecosystems remove CO2 from the atmosphere by storing carbon in their living biomass through photosynthesis, they have the ability to trap externally produced organic carbon suspended in tidal flows and terrestrial runoff, which they continually accrete within their sediments over time (Mcleod et al, 2011). Mangrove rehabilitation projects implemented in Mexico to increase carbon storage benefited from coastal protection and water purification ecosystem services (Curado et al, 2013, 2014; Adame et al, 2015a) Such projects assist in mitigating climate change, and provide additional benefits to the surrounding environment and community
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