The effects of weathering variability and anthropogenic pressures upon silicon cycling in an intertropical watershed (Tana River, Kenya)

We present the first study of the Si isotopic composition of dry season river waters from the Tana River, Kenya. Data encompasses the entire river basin, with samples collected from headwaters to the estuary, thereby capturing a salinity gradient. In the headwaters, the isotopic signature is affected by climate, as a result of its control on soil drainage and weathering. The δ30Si signatures in the basin range from + 0.69‰ up to + 2.23‰. Signatures are clearly affected by dams: an increase in δ30Si ratios of 0.54‰ and a decrease in the dissolved Si (DSi) concentration by 41% were observed downstream of the Masinga dam, the largest of a succession of 5 hydroelectric dams. This reduction in Si load is most likely due to increased diatom productivity as the corresponding change in δ30Si signature is consistent with the known fractionation by these organisms. The δ30Si composition of waters entering the estuary is ca. + 2‰ and DSi concentrations are 349 μM. In the estuary, the DSi concentrations decrease linearly following the salinity, while the δ30Si ratio remains stable, indicating the absence of processes affecting the DSi pool for the studied range of salinity.