Exportation of organic carbon from the Amazon River and its main tributaries



As part of a joint Brazilian–French project, entitled ‘Hydrology and Geochemistry of the Amazon Basin’, we carried out a seven-year study (1994–2000) on the distribution, behaviour and flux of particulate and dissolved organic carbon in the Amazon River and its main tributaries (the Negro, Solimões, Branco, Madeira, Tapajós, Xingú and Trombetas rivers).

The concentrations of particulate and dissolved organic carbon varied from one river to another and according to the season, but dissolved organic carbon (DOC) always accounted for about 70% of the total organic carbon (TOC). The mean concentration of dissolved organic carbon was 6·1 mg l−1 in the Madeira River, 5·83 mg l−1 in the Solimões River and 12·7 mg l−1 in the Negro River. The percentage in weight of the particulate organic carbon decreased as the concentration of suspended matter increased. The Solimões River contributed the most carbon to the Amazon River: about 500 kg C s−1 during the high water period and about 300 kg C s−1 during the low water period. However, the temporal variations in organic carbon in the Amazon River (i.e. downstream of Manaus) are basically controlled by inputs from the Negro River and its variations. The Negro River does not produce a simple dilution effect. During the high water period (between March and August) the TOC flux, calculated as the sum of the Solimões, Negro and Madeira tributaries, was about 5·7 × 1013 g C yr−1, whereas during the low water period (between September and February) the TOC flux was about 2·6 × 1013 g C yr−1.

The mean annual flux of TOC at Óbidos (the final gauging station upstream of the estuary) was about 3·27 × 1013 g C yr−1 (i.e. 32·7 ± 3·3 Tg yr−1). Of this, the flux of DOC represents about 2·7 × 1013 g C yr−1 and the flux of particulate organic carbon (POC) represents about 0·5 × 1013 g C yr−1. The mean annual input of TOC by all tributaries (Negro, Solimões, Madeira, Trombetas, Tapajós and Xingú) was about 2·8 × 1013 g C yr−1. When we compared this input with the output recorded at Óbidos (3·27 × 1013 g C yr−1), we found that the amount of organic carbon increased (about 0·4 × 1013 g C yr−1). This shows that other important sources of organic carbon exist in the lower reaches of the Amazon River. These inputs can be attributed to the adjacent floodplain lake system, called ‘várzea’. Copyright © 2003 John Wiley & Sons, Ltd.