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

Fluvial systems play a distinct role in monitoring the global carbon budget in the biosphere, because they not only act as the carbon carriers from the terrestrial source to the marine environment, but also stimulate various in situ processes that could significantly affect the carbon content. The carbon concentration, including its form, could be altered by natural processes such as gas exchange with the atmosphere (particularly the degassing of CO2), storage as river-bottom sediments (by microbial degradation of organic matter) and higher plankton production mainly by anthropogenic activities leading to higher nutrient input from sources such as agricultural land, domestic sewage and industry. In freshwater river systems, dissolved organic carbon (DOC), mainly originating from plant decomposition, is a very dynamic and reactive fraction of the whole carbon cycle on Earth. In contrast, the dissolved inorganic carbon (DIC) is derived from atmospheric CO2 through weathering of silicate and carbonate rock, and soil organic matter consumption. To determine the sources of inorganic carbon in river water, it is essential to estimate the amount of CO2 consumed from the atmosphere during rock weathering and to understand the biogeochemical cycling of carbon. One of the more reliable methods adopted to understand the different sources of DIC in rivers is the measurement of carbon isotopes in DIC,because the fractionation factors between the different carbonate species in dissolved river water and gaseous CO2 are well established.[1–3] The stable isotope ratios of oxygen (d18O values) and hydrogen (d2Η values) in river water are well established for determining the sources of these waters in the river basins. This would also determine the signatures of carbon in the river water accurately