Blue Carbon Storage in Tropical Seagrass Meadows Relates to Carbonate Stock Dynamics, Plant–Sediment Processes, and Landscape Context: Insights from the Western Indian Ocean

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 (C org ) and inorganic (C carb ) 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 C org (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 (C carb ) clearly corresponded to C org 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 C org and C carb stocks in seagrass habitats were pri- Electronic supplementary material: The online version of this article (doi:10.1007/s10021-017-0170-8) contains supplementary material, which is available to authorized users. Author contributions MG, MB, and HWL conceived and designed the study. MG, LDL, GSS, ME, EA, LMR, SB, and LMN performed field research and laboratory analyses. MG, LDL, MD, GSS, AK, and MB analysed data. MG led the writing of the paper with substantial input from MB, MD, and LDL; other authors commented on and edited the manuscript.*Corresponding author; e-mail: martin.gullstrom@su.se Ecosystems (2018) 21: 551-566 DOI: 10.1007/s10021-017-0170-8 Ó 2017 The Author(s). This article is an open access publication 551marily 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.

High midday temperature stress has stronger effects on biomass than on photosynthesis: A mesocosm experiment on four tropical seagrass species

“…It is, however, likely that the biomass effect observed in this study is a consequence of a combined effect of temperature on both the internal physiology of the seagrass plants, and sediment processes within the rhizosphere. The organic carbon content of the selected study site is relatively low, averaging around 0.5% (Gullström et al., ), which would by itself not cause harmfully low oxygen levels. The seagrass die‐back seen in this study would, however, increase the oxygen consumption in the sediment while the belowground material is degraded.…”

“…In the Western Indian Ocean (WIO), shallow‐water environments are largely inhabited by seagrasses, forming extensive lush meadows (Aleem, ; Gullström et al., ) providing important ecosystem services such as the functioning as habitat and nursery ground for fish and invertebrates (de la Torre‐Castro & Rönnbäck, ) and the sequestration and storage of coastal “blue” carbon (Gullström et al., ). This region encompasses a high diversity of seagrass species of which many inhabit the upper subtidal and lower intertidal.…”

Blue Carbon Storage Capacity of Temperate Eelgrass (Zostera marina) Meadows

“…Although the contribution of seagrasses to global oceanic carbon storage has been quantitatively acknowledged, most estimates come from just a few sites and seagrass species (Dahl et al, 2016;Greiner et al, 2013;Gullström et al, 2018;Macreadie et al, 2013;Miyajima et al, 2015;Serrano et al, 2014Serrano et al, , 2015Röhr et al, 2016). Importantly, the anomalously high belowground accumulation of carbon in P. oceanica meadows might lead to overestimation of the global seagrass C org stock if values for this species are applied as broad proxies for other seagrass species.…”

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

“…Although these characteristics—high primary productivity, ability to capture particles and low oxygen sediments—yield high potential for significant carbon storage, the specific biological, chemical, and physical environments are inevitably variable among seagrass meadows, regions, and species. Understanding regional and local‐scale factors controlling the magnitude of blue carbon stocks and accumulation rates could improve estimates of blue carbon potential and assist with effective local management of seagrass ecosystems (Lavery et al ; Mtwana Nordlund et al ; Gullström et al ).…”

“…Such heterogeneity has been attributed to a wide array of habitat characteristics, including species composition, hydrodynamic regimes, and seagrass and sediment characteristics Serrano et al 2014Serrano et al , 2016Samper-Villarreal et al 2016;Kindeberg et al 2018;Mazarrasa et al 2018;Santos et al 2019). At the regional or seascape scale, enhanced carbon stocks are typically associated with higher seagrass structural complexity (Jankowska et al 2016;Samper-Villarreal et al 2016;Mazarrasa et al 2018), higher proportions of fine sediments (Dahl et al 2016;Röhr et al 2016;Gullström et al 2017;Miyajima et al 2017), and sheltered sites with reduced wave height and exposure (Samper-Villarreal et al 2016;Mazarrasa et al 2017a). However, many of these same factors, such as seagrass density and sediment grain size, also vary at the meadow scale.…”