Multi-proxy evidence for compositional change of organic matter in the largest tropical (peninsular) river basin of India

Abstract

• We report OM source variability in largest monsoonal river of India. • We examine the impact of natural and man made impact on OM distribution. • Effect of land use changes on the OM source have been highlighted. • Effect of reduced monsoon and dams on the OM source change is described. The distribution and compositional changes of organic matter (OM) within the Godavari river system is increasingly influenced by reduced monsoon rainfall and an increased number of damming. To track these changes stable isotopes of organic carbon and concentrations of lignin phenols were analyzed in total suspended matter (TSM), sediments, agriculture soils and plants from Godavari basin. The results indicated that the upper tributaries drained heavier carbon (δ 13 C org = −20.4 ± 2.2‰) than the lower tributaries (δ 13 C org = −25.4 ± 1.5‰) owing to the regional vegetation in the upper to lower basins. OM originating from algae near dam impoundments was incorporated into TSM and sediment due to extreme drought condition. The organic carbon (OC) content was higher in TSM and in the sediment of the region after the middle reach dam (Sriram Sagar) than before (2.2 ± 1.6 vs. 1.0 ± 0.1% OC and 2.1 ± 2.3 vs. 0.6 ± 0.2% OC, respectively). The lignin yield (Λ8) was lower in TSM and in the sediment after the dam impoundment than before (0.37 vs. 1.94 mg/100 mg OC and 2.9 ± 1.1 vs. 5.4 ± 2.3 mg/100 mg OC, respectively) due to an increased contribution of lignin free OC from algae and degraded soil. Less rainfall and dam impoundments enhanced the fraction of labile OM from freshwater algae and an estuarine phytoplankton bloom in the study year. Our study is the first to document lignin fluxes from an Indian monsoonal river (Godavari). The flux of lignin phenols of 7.26 × 10 9 g yr −1 is much lower than those of most world rivers except the rivers from polar arctic.