Effect of forest and savanna vegetation on the carbon isotope composition of sediments from the Sanaga River, Cameroon. Limnology and Oceanography 39

The carbon-isotope composition (&13C) of organic matter from modern sediments in the Sanaga River basin, Cameroon, shows a progressive downriver decrease in 6°C values. Sediments in tributaries draining montane grassland regions have consistent 613C values of-- 19?4m, while rivers draining savanna regions have 613C values of around-22?&0. After the river passes through extensively forested regions, the 6°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 613C values (from-27 to-29%~) consistent with a pure C3 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, and C, 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 613C values (up to- 16.7o/oo). 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,-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 tha