A seasonal cycle in the carbon‐isotope composition of organic carbon in the Sanaga River, Cameroon

We report measurements of the carbon‐isotope composition (δ 13 C value) of particulate organic carbon (POC) in the suspended sediment load of the Sanaga and Mbam Rivers, Cameroon. The results demonstrate that there is a seasonal cycle in δ 13 C value of 3.3%0 (−26.6 to −23.3%0) in the Sanaga River and 5.9%0 (−27.1 to −21.2%0) in the Mbam River that is directly related to seasonal changes in the discharge of the rivers. During periods of high discharge, δ 13 C values of POC are high, reflecting an increase in the proportion of POC derived from C, plants, including C 4 ‐derived carbon transported overland by wet season rains from savanna regions in the catchment hinterland. During periods of low discharge, the δ 13 C values of POC are low, reflecting derivation of the carbon primarily from C 4 plants growing close to the riverbanks. Results suggest that the prime determinant of POC δ 13 C values in tropical rivers that carry a significant suspended sediment load is the δ 13 C value of the catchment vegetation.