Carbon budgets of multispecies seagrass beds at Dongsha Island in the South China Sea
Introduction
Seagrass beds are considered to be significant sinks of Blue Carbon (Nellemann et al., 2009). On an areal basis, seagrass beds are not only highly productive (Duarte and Chiscano, 1999) but also trap allochthonous suspended organic carbon (C) in the sediments (Fourqurean et al., 2012). Despite the small amount of living biomass in a seagrass bed, the bed may hold as much C as the same surface area of a tropical forest (Pendleton et al., 2012). On a global scale, seagrass beds can store 19.9 Pg of organic C in living seagrass biomass and the underlying sediments (Fourqurean et al., 2012). While the accumulation rate of organic C in the sediments of seagrass beds has been estimated (e.g. Lavery et al., 2013), little is actually known about the fate of production by seagrasses.
Although the average short-term (years) and long-term (decades) storage rates of organic C from seagrass leaves in sediments have been estimated to be 133 (Cebrián, 2002) and 83 g m−2 yr−1 (Duarte et al., 2005), respectively, few studies have tried to quantify and integrate the processes constructing seagrass C budgets. Based on a compilation of globally available data, Duarte and Cebrián (1996) estimated that the majority of seagrass net production is decomposed (50%), with export and herbivory accounting for 24% and 19%, respectively, and the remaining 16% being stored. However, these estimates of processes of C budgets are primarily limited to two seagrass species (Thalassia testudinum and Posidonia oceanica), which have been studied only in the Caribbean and Mediterranean. Although there is a wealth of available data on seagrass leaf production, there are still few field measurements of herbivory, export, decomposition, and storage or of belowground processes (Kennedy and Björk, 2009). To obtain a more accurate global storage rate estimate for seagrass Blue Carbon, reliable estimates of these processes in dominant seagrass species across a broad geographic range and of the C storage potential of each species are needed.
The tropics may include the most biogeochemically active coastal regions and represent potentially important C sinks in the biosphere (Twilley et al., 1992). However, seagrass biomass and production were lower at low latitudes (Duarte and Chiscano, 1999). Tropical seagrass beds are also generally dominated by a variety of smaller-sized seagrasses (Short et al., 2007), which typically have a lower organic C stock in living biomass than the temperate species. The observed lower seagrass biomass and production at low latitudes may be partly attributed to the higher level of grazing on seagrasses (Heck and Valentine, 2006). It is also thought that the higher temperatures in the tropics promote more efficient decomposition of seagrass detritus in the sediment. It appears that the remaining amount of organic C in tropical seagrass beds would be lower than in temperate seagrass beds.
The tropical Indo-Pacific has the highest seagrass biodiversity in the world, with as many as 14 species growing together in mixed meadows (Short et al., 2007). A multispecies seagrass bed may exhibit a higher production rate than a monospecific seagrass bed (Erftemeijer and Stapel, 1999). Empirical work on the processes occurring in tropical multispecies seagrass beds is needed to improve our understanding of the contribution of seagrass ecosystems to Blue Carbon. Dongsha Island (20°42′N, 116°43′E), also known as Pratas Island, is a pristine island that is little affected by human activities and is located in the South China Sea (Fig. 1). There is a lagoon on the western side of Dongsha Island with only one inlet (80 m wide) connecting it to the sea. The shoot density and coverage of seagrasses, as well as their species diversity, are high on this remote island (Lin et al., 2005). In this study, the C budgets of the leaf and belowground production of dominant seagrass species at Dongsha Island were further examined. The objectives of this study were 1) to quantify the biomass, production, consumption, and detrital export and decomposition of dominant seagrass species in the multispecies seagrass beds, 2) to determine whether there is variation in these processes among the dominant species, 3) to determine whether there is seasonal variation in these processes, 4) to examine whether there is spatial variation (in the semi-enclosed lagoon vs. on the open coast) in these processes, and 5) to estimate the organic C storage rates in the multispecies seagrass beds at Dongsha Island by integrating the processes described above.
Section snippets
Study sites
Dongsha Island is 2.80 km long and 0.87 km wide, covering an area of 1.74 km2. The lagoon exhibits a surface area of 0.64 km2, a mean depth of approximately 1 m at low tide, and a small tidal amplitude of approximately 80 cm. Despite the small area of the island, the total coverage area of seagrass beds is 11.85 km2. These beds grow on carbonate sediment in the lagoon and around Dongsha Island. A total of seven seagrass species from six genera and two families have been identified around the
Seagrass biomass and production
The coverage and shoot density of seagrasses were high at Dongsha Island (Table 1). The mean coverage and shoot density in the lagoon were higher than those on the N coast.
In the lagoon, the total biomass of seagrasses averaged 1047 g DW m−2, 57.6% of which consisted of the belowground compartment. There was a significant interaction effect between the seagrass species and seasons on the aboveground biomass (Table 2). The aboveground biomass of TH was greater than those of the other two species
Integrated C budgets of the multispecies seagrass beds
This study is the first to quantify and integrate the processes constructing the C budgets of Thalassia-Halodule-Cymodocea seagrass beds in the South China Sea. Different seagrass species can show substantial variation in biomass (Fourqurean et al., 2012). Our results further showed that production, consumption, and detrital export and decomposition varied among seagrass species in the multispecies seagrass beds at Dongsha Island. The variations in leaf production and consumption in tropical
Acknowledgements
This study was financially supported by Marine National Park Headquarters, Taiwan under Grant 98415. We thank Prof. Laurie Battle for English proofreading.
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