Blue carbon stocks in seagrass tissues: A case study from the northern Mexican Caribbean

Blue Carbon ecosystems, such as seagrass meadows, contribute to climate change mitigation through carbon sequestration and storage. Organic carbon (Corg) stocks in seagrass meadows are mainly concentrated in their sediments. The seagrasses themselves are usually ignored in Corg stock assessments, even though in some systems they may provide a considerable contribution. The Corg stock in the living and dead tissues of Thalassia testudinum and Syringodium filiforme was determined at five mixed meadows in biogenic calcareous sediments the northern Mexican Caribbean. Per site, three core samples (11.2 cm diam) were taken, and the samples were sectioned into four strata: above the sediment, 0–10, 10–20 and 20–30 cm deep, separating dead and living tissues. The highest biomass was observed in the belowground strata to 20 cm depth. The seagrass Corg content varied between 18.2 and 27.4 % (average 22.7 %) of dry weight. T. testudinum and S.filiforme contributed on average 83 % and 17 % to the seagrass Corg stock, respectively. On average, the Corg stocks in seagrass biomass was 6.10 ± 0.77 Corg ha⁻¹ (72 % and 28 % in alive and dead tissues, respectively). The total mean Corg stock (in seagrasses and sediments until 30 cm depth combined) in the five studied meadows was 22.64 ± 3.03SE Mg Corg ha⁻¹. The contribution of the seagrass tissue to the total Corg stock varied among the sites (between 15.2 and 38.0 %) with higher contribution in coarser grained sediments. Thus, in some seagrass systems, the seagrass plants, as well as the sediments, can represent a significant share of the Corg stocks.