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

We report measurements of the carbon-isotope composition (delta(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 delta(13)C value of 3.3 parts per thousand (-26.6 to -23.3 parts per thousand) in the Sanaga River and 5.9 parts per thousand (-27.1 to -21.2 parts per thousand) in the Mbam River that is directly related to seasonal changes in the discharge of the rivers. During periods of high discharge, delta(13)C values of POC are high, reflecting an increase in the proportion of POC derived from C-4 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 delta(13)C values of POC are low, reflecting derivation of the carbon primarily from C-3 plants growing close to the riverbanks. Results suggest that the prime determinant of POC beta(13)C values in tropical rivers that carry a significant suspended sediment load is the delta(13)C value of the catchment vegetation.