Comparison of dissolved inorganic and organic carbon export in the rivers of tropical volcanic island; example from Guadeloupe, French West Indies
Résumé
Organic matter is an important factor that cannot be neglected when considering global carbon cycle. New data including organic matter geochemistry at the small watershed scale are needed to elaborate more constrained carbon dynamic and climatic models. The objectives are to estimate the DOC and DIC fluxes in well constrained geochemical systems that are small tropical rivers and to characterize and identify the different sources of the organic carbon exported under different hydrological regime (i.e. low water level vs flood level). To answer these questions, we have studied the geochemistry of eleven small watersheds around Basse-Terre Island in the French West Indies during different hydrological regime from 2006 to 2008. Together with water discharge, DOC concentrations, DIC concentrations were measured as well as their isotopic composition (δ13CDOC and δ13CDIC). Spectroscopic indicators of the nature of the organic matter are used to characterize the sources of DOC in the rivers. The ratio DOC/DIC varied between 0.070 and 1.971, with the highest values corresponding to flood conditions, indicating that these rivers export more organic matter during the flood events. The isotopic composition of DOC, ranged from -33.5 to -29.5 ‰ during low water level and from -30.1 to -27.2 ‰ during flood levels, indicates that rivers are fed by different sources during these two hydrological stages. During low water level the rivers were mainly fed by perennial groundwater flow, while during flood level the rivers were fed by lixiviation of top soil solutions. The mean annual fluxes ranged from 2.5 to 5.7 t km-2 yr-1 for the DOC and from 4.8 to 19.6 t km-2 yr-1 for the DIC. The DOC/DIC ratios exhibit a non-linear relationship with the slopes of watershed. Our results emphasize the importance of DOC fluxes in the carbon mass balance in small tropical rivers.