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

The 13 C: 1 2 C 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 δ 13 C range vs. marine carbonate (PDB) was –32 to −26‰ for suspended POC, −30 to −28‰ for DOC, and −26 to −12% for DIC. The δ 13 C 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 13 C‐depleted FPOC from tributaries draining the lowland regions of the Amazon basin and floodplain soils. The δ 13 C 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 δ 13 C 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 δ 13 C of the DIC suggests that ∼40% of the organic matter being respired in the river is C 4 plant material derived from floodplain grasses.