Seagrass blue carbon stocks and sequestration rates in the Colombian Caribbean

Seagrass ecosystems rank amongst the most efficient natural carbon sinks on earth, sequestering CO₂ through photosynthesis and storing organic carbon (C_org) underneath their soils for millennia and thereby, mitigating climate change. However, estimates of C_org stocks and accumulation rates in seagrass meadows (blue carbon) are restricted to few regions, and further information on spatial variability is required to derive robust global estimates. Here we studied soil C_org stocks and accumulation rates in seagrass meadows across the Colombian Caribbean. We estimated that Thalassia testudinum meadows store 241 ± 118 Mg C_org ha^-1 (mean ± SD) in the top 1 m-thick soils, accumulated at rates of 122 ± 62 and 15 ± 7 g C_org m^-2 year^-1 over the last ~ 70 years and up to 2000 years, respectively. The tropical climate of the Caribbean Sea and associated sediment run-off, together with the relatively high primary production of T. testudinum, influencing biotic and abiotic drivers of C_org storage linked to seagrass and soil respiration rates, explains their relatively high C_org stocks and accumulation rates when compared to other meadows globally. Differences in soil C_org storage among Colombian Caribbean regions are largely linked to differences in the relative contribution of C_org sources to the soil C_org pool (seagrass, algae Halimeda tuna, mangrove and seston) and the content of soil particles < 0.016 mm binding C_org and enhancing its preservation. Despite the moderate areal extent of T. testudinum in the Colombian Caribbean (661 km²), it sequesters around 0.3 Tg CO₂ year^-1, which is equivalent to ~ 0.4% of CO₂ emissions from fossil fuels in Colombia. This study adds data from a new region to a growing dataset on seagrass blue carbon and further explores differences in meadow C_org storage based on biotic and abiotic environmental factors, while providing the basis for the implementation of seagrass blue carbon strategies in Colombia.