Organic Carbon Fluxes from Fringing Mangroves in a Major Caribbean Delta (Atrato, Urabá Gulf, Colombia): Upscaling from Litterfall Traps to the Macrosystem

Abstract:

Mangrove-dominated coastlines are a salient feature of the Tropics, however little is known about organic carbon fluxes from canopies in Caribbean deltaic settings, while significantly more studies are available for carbonate islands, small embayments, and coastal lagoons. The Atrato River, draining one of the rainiest areas in the World, forms a major delta in the SW Caribbean (Urabá Gulf, Colombia) where highly productive mangroves thrive despite the recurrent ENSO cycle. Previous studies in this delta reported on the spatial variability of daily rates in litterfall (both biomass and carbon) among geomorphic units (GUs) among a single embayment, as well as among and within transects at each geomorphic unit. This contribution was aimed at comparing the results of upscaling from traps (0.25 m²) to the entire embayment (776.3 ha) and the delta (3846 ha) using stratified (5 GUs measured three times during 6 mo-period) and unstratified (1 GU measured monthly during a 15 mo-period) sampling designs. Stratified sampling showed significant variation among GUs (1.60±0.74 gC m^-2 d^-1; Kruskal-Wallis Chi-squared = 19.994, df = 4, p < 0.001), while the unstratified sampling showed a random but unsignificant temporal variability (2.16±0.35 gC m^-2 d^-1; Kruskal-Wallis Chi-squared = 6.99, df = 9, p = 0.63). The estimates using two temporal efforts for a single GU (365 ha) were 2.5 and 2.9 GgC y^-1. The estimates for the entire embayment (776.3 ha) using the two sampling designs were 4.5 and 6.1 GgC y^-1. These results demonstrate that upscaling from traps to entire embayments produce greater error than upscaling from traps to a single GU, because in this particular area spatial variability overrides temporal variability. Finally, upscaling from a single embayment to the entire delta using the two methods produced estimates ranging from 22.3 and 30.8 GgC y^-1. Despite of the significant differences in organic C fluxes from the mangrove canopy to the ground obtained by using to sampling designs, this study points to the potential significantly high contribution of this delta to atmospheric C sinking at a regional scale. Moreover, sustaining an extensive mangrove interior with a high contribution of recalcitrant C in the form of woody material and propagules, and under a microtidal regime, make this delta a suitable site for blue carbon projects.