Fluvial Carbon Fluxes from Tropical Mountainous Rivers: Spatial and Seasonal Variations

Rivers are one of the major active geomorphological agents that exports the fluvial carbon from land to ocean. The previous global scale investigations have estimated fluvial carbon flux transport by rivers to coastal seas to be 0.80-1.33×10¹⁵ gm (or Pg) C y^-1, and up to 60% of it comes from the tropical regions. However, most of previous estimations are based on carbon discharge by a few major rivers. Despite being large in numbers and having high yields, the coastal/mountainous rivers have received less attention. To fill the gap, we investigated 70 west-flowing coastal Indian rivers, draining the Western Ghats (WG) for their carbon transport characteristics. These rivers deliver approximately 0.005 Pg carbon to the Arabian Sea annually. It means that despite occupying only 1% of Asia's area, this region exports 3% of fluvial carbon load. Of this, 42.9% is dissolved inorganic carbon (DIC), 12.9% is dissolved organic carbon (DOC), 10.1% is particulate inorganic carbon (PIC), and 34.2% is particulate organic carbon (POC). Inorganic carbon concentrations were comparably higher in non-monsoon seasons throughout the region. Strong region variability observed in terms of carbon yields and type of major carbon species across the region. The WG region with a load of 44.38 gC m^-2 y^-1 is one of the highest carbon yielding region in the world. Intense precipitation, high topographic relief, catchment lithological, and presence of thick vegetation cover with varying soils from south to north of the Western Ghats are the major factors make this region unique for fluvial carbon transport.