Dynamic of particulate and dissolved organic carbon in small volcanic mountainous tropical watersheds

Abstract

In the tropical zone, small watersheds are affected by intense meteorological events. These events play an important role in the erosion of soils and therefore on the associated organic carbon fluxes to the ocean. We studied the geochemistry of three small watersheds around the Basse-Terre volcanic Island (French West Indies, FWI) during a four years period, by measuring dissolved organic carbon (DOC), particulate organic carbon (POC) and dissolved inorganic carbon (DIC) concentrations. The mean annual yields ranged between 8.1–15.8tCkm−2yr−1, 1.9–8.6tCkm−2yr−1 and 8.1–25.5tCkm−2yr−1 for DIC, DOC and POC, respectively. Floods and extreme floods (i.e., extremely high discharge associated to extreme meteorological events such as cyclones or tropical storms) account for 42.6% of the yearly water flux and represent 54.5% of the annual DOC flux, and more than 85% of the annual POC flux. The DIC flux occurs essentially during low water levels with 75% of the annual flux. During low water levels and floods, the dissolved carbon is exported in majority under the inorganic form (DIC/DOC=2.6±2.1), while during extreme floods, the dissolved carbon transported is mostly organic (DIC/DOC=0.7±0.2). The partial “residence time” taking into account only the physical processes (erosion and transport) in Guadeloupean soils has been estimated between 381 and 1000years. These relatively short times could be linked to the intensity of meteorological events rather than the frequency of meteorological events. The total export of organic carbon coming from small tropical and volcanic mountainous rivers is estimated at 2.4±0.6MtCyr−1 for DOC and at 5.9±2.4MtCyr−1 for POC, emphasizing that these carbon fluxes are significant and should be included in global carbon budgets. In addition, the quality of terrestrial organic matter (POC/DOC, and C/N ratios) arriving to the ocean is different from the one of large river origin. These inputs are responsible for a fast transport of terrestrial organic matter to the ocean but their effect on regional and global carbon budget is still a matter of debate.