Geographic variability in organic carbon stock and accumulation rate in sediments of East and Southeast Asian seagrass meadows

Abstract

Organic carbon (OC) stored in the sediments of seagrass meadows has been considered a globally significant OC reservoir. However, the sparsity and regional bias of studies on long-term OC accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global OC sink. We evaluated the amount and accumulation rate of OC in sediment of seagrass meadows and adjacent areas in East and Southeast Asia. In temperate sites, the average OC concentration in the top 30 cm of sediment was higher in seagrass meadows (780-1080 μmol g . Carbon isotope mass balancing suggested that the contribution of seagrass-derived carbon to OC stored in sediments was often relatively minor (temperate: 10-40%; subtropical: 35-82%; tropical: 4-34%) and correlated to the habitat type, being particularly low in estuarine habitats. Stock of OC in the top meter of sediment of all the studied meadows ranged from 38 to 120 Mg ha À1 . The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32-1.34 mm yr À1). The long-term OC accumulation rates calculated from the sediment accumulation rate and the top 30 cm average OC concentration for the seagrass meadows (24-101 kg ha À1 yr À1 ) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha À1 yr À1 on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.

Blue carbon stocks in Baltic Sea eelgrass (<i>Zostera marina</i>) meadows

“…The average sizes of C org stocks in Finnish and Danish eelgrass meadows were also considerably lower than the mean values reported by Alongi (2014)for tropical seagrass meadows (14 270 t C km −2 ). In contrast, our estimates for the carbon stock in the top 25 cm for Danish and Finnish meadows (627-6005 g C m −2 ) are comparable to Australian (262-4833 g C m −2 : Lavery et al, 2013) and Asian estimates (3800-12 000 g C m −2 : Miyajima et al, 2015).…”

“…As proposed in previous work, accumulation of fine-grained size fractions in seagrass sediments, relative to those accumulated in bare sediments, appears to be one of the major factors influencing the carbon sink capacity of seagrass meadows (Kennedy et al, 2010;Miyajima et al, 2015) and may thus be a useful proxy for the sink capacity. In addition, it is well known that seagrasses modify sediments by reducing water flow and consequently increasing particle trapping and sedimentation and reducing resuspension (Fonseca and Fisher, 1986;Fonseca and Cahalan, 1992;Gacia et al, 2002;Hendriks et al, 2008;Boström et al, 2010).…”

“…Results for carbon accumulation (applied by multiplying the depthintegrated C org stock by regional seagrass area and sediment accumulation rate estimate from the literature) in each area are given as C org accumulation (t yr −1 ). Due to the lack of long-term monitoring of sediment accumulation in eelgrass meadows, we used available minimum, average and maximum sediment accumulation rates in seagrass meadows obtained from the literature (Duarte et al, 2013b;Serrano et al, 2014;Miyajima et al, 2015).…”

“…Consequently, there is a pressing need to better un-derstand which factors are causing variability in carbon storage (C org stocks, carbon stored in living and dead seagrass biomass and sediments) and the capacity of seagrass meadows to sequester carbon (C org accumulation) in seagrass sediments. Indeed, recent studies show considerable influence of seagrass habitat setting, sediment characteristics and speciesspecific traits on the variability in carbon storage capacity in seagrass meadows (Duarte et al, 2013a;Lavery et al, 2013;Miyajima et al, 2015). Such differences contribute to uncertainty in local and global estimates of the carbon storage capacity and carbon dynamics in coastal seagrass areas.…”

Key biogeochemical factors affecting soil carbon storage in <i>Posidonia</i> meadows

“…The seagrass itself may exert a primary control on C org storage through its biomass, productivity and nutrient content (Lavery et al, 2013;Serrano et al, 2014;Miyajima et al, 2015), all of which are highly variable depending upon seagrass species and habitat conditions (Alcoverro et al, 1995;Collier et al, 2007). Seagrass density, biomass and productivity are strongly related to the underwater light penetration (Dennison, 1987;Duarte, 1991).…”