Carbon cycling in the Amazon River: Implications from the 13C compositions of particles and solutes

Abstract

The 13C:12C of suspended particu-late organic C (POC), dissolved organic C (DOC), and dissolved inorganic C (DIC) were measured during 1982–1984 at 11 main-channel and 7 tributary stations over an 1,800 km reach of the Amazon River between Vargem Grande and Obidos, Brazil. The measured δ13C range vs. marine carbonate (PDB) was –32 to −26‰ for suspended POC, −30 to −28‰ for DOC, and −26 to −12% for DIC. The δ13C of the fine particulate organic C (FPOC) decreased downriver from Vargem Grande, with values lowest during the falling-water portion of the runoff cycle; these trends were the result primarily of input of 13C-depleted FPOC from tributaries draining the lowland regions of the Amazon basin and floodplain soils. The δ13C of the FPOC at Obidos implies that at least 35% of the POC exported by the Amazon River is derived from the lowland portion of the Amazon basin. The δ13C of DIC decreased downriver with the lowest values measured during falling water; these trends were due primarily to within-river respiration and tributary input. The δ13C of the DIC suggests that ∼40% of the organic matter being respired in the river is C4 plant material derived from floodplain grasses.