Organic carbon in coastal sediments and blue carbon estimation at two sites in the Canary Islands: a preliminary approach

Abstract

Seagrass meadows are increasingly recognized as effective blue carbon ecosystems due to their ability to trap and store organic carbon (OC), both within their biomass and within the underlying sediments. This study evaluated the spatial variability of organic carbon and organic matter in sediments from Cymodocea nodosa meadows and adjacent unvegetated sandy areas at two sites in the Canary Islands: El Pajar (Gran Canaria) and Abades (Tenerife). The content of OC, in %, was estimated using two analytical methods: Walkley-Black - wet oxidation (WB) and Loss on Ignition (LOI). These methods were applied to surface sediments (0-2 cm) and sediment cores (0–16 cm). The %OC in surface sediments ranged from 0.63% to 0.306%, while core sediments reached 0.381% with WB method. On average, the LOI method yielded OC estimates that were approximately 66% higher than those obtained using the WB method. Using a measured glucose recovery factor of 0.915 for the WB method resulted in OC estimates that were 16% lower compared to using the conventional factor of 0.77. There was no correlation between carbon content in C. nodosa biomass (mean 0.681 g C in El Pajar and 0.709 g C in Abades) and sedimentary OC, suggesting that hydrodynamic processes, physico-chemical characteristics, and sediment characteristics strongly influence OC accumulation in sediments and the abiotic factors of coastal areas have to be attended in terms of predicting the carbon storage capacity. The carbon stocks were estimated, using a dry bulk density (DBD) of 1.03 g cm-3 for the LOI carbon estimation, and reached 272.72 MgC ha⁻¹ in sediments and 12.478 gC ha⁻¹ in biomass. Based on carbon market prices from 2023 to 2025, the estimated economic value ranged from €4,211.03 to €6,154.01 ha⁻¹. These findings emphasize the importance of seagrass ecosystems as carbon sinks, as well as highlighting the importance of method selection, site-specific calibration and consideration of adjacent unvegetated areas in blue carbon assessments.