Abstract
Globally, seagrass ecosystems are considered major blue carbon sinks and thus indirect contributors to climate change mitigation. Quantitative estimates and multi-scale appraisals of sources that underlie long-term storage of sedimentary carbon are vital for understanding coastal carbon dynamics. Across a tropical–subtropical coastal continuum in the Western Indian Ocean, we estimated organic (Corg) and inorganic (Ccarb) carbon stocks in seagrass sediment. Quantified levels and variability of the two carbon stocks were evaluated with regard to the relative importance of environmental attributes in terms of plant–sediment properties and landscape configuration. The explored seagrass habitats encompassed low to moderate levels of sedimentary Corg (ranging from 0.20 to 1.44% on average depending on species- and site-specific variability) but higher than unvegetated areas (ranging from 0.09 to 0.33% depending on site-specific variability), suggesting that some of the seagrass areas (at tropical Zanzibar in particular) are potentially important as carbon sinks. The amount of sedimentary inorganic carbon as carbonate (Ccarb) clearly corresponded to Corg levels, and as carbonates may represent a carbon source, this could diminish the strength of seagrass sediments as carbon sinks in the region. Partial least squares modelling indicated that variations in sedimentary Corg and Ccarb stocks in seagrass habitats were primarily predicted by sediment density (indicating a negative relationship with the content of carbon stocks) and landscape configuration (indicating a positive effect of seagrass meadow area, relative to the area of other major coastal habitats, on carbon stocks), while seagrass structural complexity also contributed, though to a lesser extent, to model performance. The findings suggest that accurate carbon sink assessments require an understanding of plant–sediment processes as well as better knowledge of how sedimentary carbon dynamics are driven by cross-habitat links and sink–source relationships in a scale-dependent landscape context, which should be a priority for carbon sink conservation.
Highlights
Understanding the strength and variability of sedimentary carbon storage is crucial for accurately assessing coastal carbon dynamics (Lavery and others 2013; Watanabe and Kuwae 2015) and for developing efficient management strategies to absorb atmospheric CO2, thereby contributing to climate change mitigation (Duarte and others 2011)
The sedimentary Corg content in seagrass habitats was higher in most tropical sites than in subtropical sites, with the highest average levels found in Chwaka Bay, followed by Pongwe, ZanMbweni, and Fumba (Figure 3A)
Our spatially widespread survey indicated that the patterns of variability in sedimentary Corg and carbon as carbonate (Ccarb) in seagrass habitats were largely related to sediment properties and partly related to seagrass structural complexity
Summary
Understanding the strength and variability of sedimentary carbon storage is crucial for accurately assessing coastal carbon dynamics (Lavery and others 2013; Watanabe and Kuwae 2015) and for developing efficient management strategies to absorb atmospheric CO2, thereby contributing to climate change mitigation (Duarte and others 2011). Evidence of the rapid degradation and loss of vegetated blue carbon habitats (Davy and others 2009; Waycott and others 2009; Spalding and others 2010; Mcleod and others 2011; Macreadie and others 2015; Serrano and others 2016a) due to anthropogenic activity stresses the urgent need for widespread assessments of natural blue carbon sinks to spur directed conservation and restoration efforts (Laffoley and Grimsditch 2009; Duarte and others 2013) as well as for predictive modelling of habitat–carbon dynamics under global change (Macreadie and others 2013) linked to accurate estimates of future economic impacts (Pendleton and others 2012). Seagrass meadows are among the Earth’s most productive aquatic ecosystems (Duarte and Cebrian 1996) and provide efficient habitats for the longterm burial of sedimentary Corg (Smith 1981; Duarte and others 2005, 2010; Serrano and others 2016b), well exceeding the accumulation rates of terrestrial ecosystems (Mateo and others 2006; Mcleod and others 2011). Along with the metabolic source of sedimentary Corg from seagrass tissue (Duarte and others 2011), seagrass canopies trap suspended organic matter, retaining it in the sediment as accumulated organic matter
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