Abstract
Estimates of organic carbon (Corg) storage by seagrass meadows which consider inter-habitat variability are essential to understand their potential to sequester carbon dioxide (CO2) and derive robust global and regional estimates of blue carbon storage. In this study, we provide baseline estimates of seagrass extent, and soil Corg stocks and accumulation rates from different seagrass habitats at Rottnest Island (in Amphibolis spp., Posidonia spp., Halophila ovalis and mixed Posidonia/Amphibolis spp. meadows). The Corg stocks in 0.5 m thick seagrass soil deposits, derived from 24 cores, were 5.1+-0.7 kg Corg m-2 (mean+-S.E, ranging from 0.05 to 12.9 kg Corg m-2), accumulating at 23.2+-3.2 g Corg m-2 yr-1 (ranging from 0.22 to 58.9 g Corg m-2 yr-1) over the last decades. There were significant differences in Corg content (%) and stocks (mg Corg cm-3), stable carbon isotope composition of the soil organic matter (d13C) and soil grain size among the seagrass meadows studied, highlighting that biotic and abiotic factors influence seagrass soil Corg storage. Mixed meadows of Posidonia/Amphibolis spp. and monospecific meadows of Posidonia spp. and Amphibolis spp. had the highest Corg stocks (ranging from 6.2 to 6.4 kg Corg m-2), while Halophila spp. meadows had the lowest Corg stocks (1.2+-0.6 kg Corg m-2). We estimated a total soil Corg stock of 48.1±8.5 Gg Corg beneath the 755 ha of Rottnest Island’s seagrasses, and a Corg sequestration capacity of 0.81+-0.06 Gg Corg yr-1, which is equivalent to the sequestration of ~22% of the island’s current annual CO2 emissions. Our results contribute to the existing global dataset on seagrass soil Corg storage and show a significant potential of seagrass to sequester CO2, which are particularly relevant in the context of achieving carbon neutrality through conservation actions in environmentally-marketed, tourist destinations such as Rottnest Island.
Highlights
While this study focuses on one region, the results obtained contribute to understand the differences in Corg storage between seagrass species, highlighting the importance of accounting for habitat variability when scaling up estimates
The dry bulk density (DBD), proportion of sediment particles < 0.125 mm, and δ13C signatures increased with soil depth within the top 10 cm of Amphibolis spp. and H. ovalis cores, and were relatively constant or declined in the Posidonia spp. and mixed Posidonia/Amphibolis cores (Supplementary Figure S2)
The soil Corg content (%) in Posidonia spp. and Amphibolis spp. showed no clear trends with soil depth, but Corg content declined in Posidonia/Amphibolis spp. and H. ovalis cores below 20 cm depth
Summary
The carbon storage capacity of seagrass has been recognized since the early 1980s (e.g., Smith, 1981) but interest has recently intensified with the recognition of blue carbon ecosystems and their potential to contribute to climate change mitigation (Duarte et al, 2005, 2013; Nellemann and Corcoran, 2009; Mcleod et al, 2011; Fourqurean et al, 2012a). Once Corg is buried in the soil, biotic and abiotic factors are likely to control the degree of Corg accumulation and preservation (Mateo et al, 2006) These factors include the rates of sediment accumulation, sediment grain-size, and biochemical composition of the organic matter (Keil and Hedges, 1993; Torbatinejad et al, 2007; Serrano et al, 2016a), and vary among seagrass meadows (De Falco et al, 2000; Kennedy et al, 2010). In order to improve existing estimates of Corg storage in seagrass ecosystems, further studies that expand the current knowledge on geomorphological and biological factors driving Corg storage are needed (Serrano et al, 2016a; Gullström et al, 2018; Mazarrasa et al, 2018)
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