Distribution of C, N and P in the sediments of the Ganges–Brahmaputra–Meghna river system in the Bengal basin

The Bengal basin is one of the geologically youngest and tectonically active drainage regimes of the world which includes the total Lower Ganges–Brahmaputra–Meghna (GBM) river system, one of the highest sediment dispersal systems in the world. The sediment load in this system comprises exclusively fine sand, silt and clay. River bed sediments, and suspended matter from stations situated uniformly all over the basin, have been analyzed in order to understand the spatial distribution and biogeochemical processing of the C, N and P in the system. Among the Ganges, Brahmaputra and Meghna subbasins, variations in total phosphorus (TP) and nitrogen (TN) contents in the bulk and <63 μm fraction of the bed sediments are statistically insignificant, but total carbon (TC) content varies significantly among the three subbasins. There is a gradual increase in the concentration of TC from the Meglna towards the Ganges subbasin, attributable to the nature of the floodplain and bank materials which supply major sediment load to the river channels. The TP concentration is 2 to 5 times more in the suspended matter than that in any particle size fraction of the sediments. Although excellent correlation between Corg and TN suggests their coorigin, the influence of inorganic nitrogen is evident in very low C/N ratios. The C/N ratio varies from 2 to 11, suggesting complete degradation of the organic matter and/or enrichment of inorganic nitrogen in the sediments. The plant available phosphorus (PAP) varies from 1.78% to 6.98% of TP, and indicates a significant anthropogenic contribution. The Corg, TN and C/N ratio in the bed sediments of the Lower GBM system are in the range comparable with other river basins of the subcontinent.