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Effect of forest and savanna vegetation on the carbon-isotope composition of sediments from the Sanaga River, Cameroon

1994; Wiley; Volume: 39; Issue: 8 Linguagem: Inglês

10.4319/lo.1994.39.8.1845

1939-5604

Michael I. Bird, Pierre Giressè, Allan R. Chivas,

Aquatic Ecosystems and Phytoplankton Dynamics

The carbon‐isotope composition ( δ 13 C) of organic matter from modern sediments in the Sanaga River basin, Cameroon, shows a progressive downriver decrease in δ 13 C values. Sediments in tributaries draining montane grassland regions have consistent δ 13 C values of ∼−19‰, while rivers draining savanna regions have δ 13 C values of around −22‰. After the river passes through extensively forested regions, the 13 C values of the sediments closest to the mouth of the river decrease to −25‰. Samples collected from rivers with entirely forested catchment had low δ 13 C values (from −27 to −29‰) consistent with a pure C 3 vegetation source. The downriver carbon‐isotope trends are consistent with the hypothesis that observed variations are due to changes in the relative proportions of C 3 and C 4 carbon in the sediments, which are derived from terrestrial vegetation in the river catchment. Some sediments from the Mbam River, the principal tributary of the Sanaga River, have extremely high δ 13 C values (up to −16.7‰). These high values may be the result of severe erosion in the densely populated and intensively farmed Bamileke region in the headwaters of the Mbam. An underrepresentation of C 4 ‐derived carbon in the upper reaches of the Sanaga is possibly due to the damming of the Djerem River at Mbakaou. Since the dam was constructed <25 yr ago, this suggests that the δ 13 C value of river sediments responds rapidly to changes in the basin.