First assessment of water and carbon cycles in two tropical coastal rivers of south-west India: an isotopic approach

RATIONALE The contribution of tropical coastal rivers to the global carbon budget remains unmeasured, despite their high water dynamics, i.e. higher run-off with their basin characteristic of warm temperature. Two rivers draining the western part of the Western Ghats, the Swarna (length 80 km) and Nethravati (147 km) Rivers, were studied for water and carbon cycles. METHODS The stable isotope ratios of oxygen (O-18 values), hydrogen (H-2 values) and carbon (C-13 values) were used to understand the water circulation, the weathering processes and the carbon biogeochemical cycle. The river water samples were collected during the dry post-monsoonal season (November 2011). RESULTS The O-18 and H-2 values of river water suggested that the monsoonal vapour source and its high recycling have a dominant role because of the orographical and tropical conditions. The absence of calcareous rocks has led to dissolved inorganic carbon (DIC) mainly originating from atmospheric/soil CO2, via rock-weathering processes, and the low soil organic matter combined with high run-off intensity has led to low riverine dissolved organic carbon (DOC) contents. The C-13 values increase from upstream to downstream and decrease with increasing pCO(2). There is a positive relationship between the C-13(DIC) values and the DOC concentrations in these two rivers that is contrary to that in most of the studied rivers of the world. CONCLUSIONS The higher evapotranspiration supported by tropical conditions suggests that there are higher vapour recycling process in the Swarna and Nethravati basins as studied from the water O-18 and H-2 values. The basin characteristics of higher rainfall/run-off accompanied by warm temperature suggest that the C-13 value of riverine DIC is mainly controlled by the weathering of source rocks (silicates) with variation along the river course by CO2 degassing from the river water to the atmosphere and is less dominated by the oxidation of DOC. Copyright (c) 2013 John Wiley & Sons, Ltd.