Diatoms modify the relationship between dissolved silicon and bicarbonate in the impounded rivers

In order to investigate the relationship between dissolved silicon (DSi) and bicarbonate (HCO₃^–), we analysed water samples from the main Changjiang channel and its main tributaries in August 2006 and, during 2007-2009, the contrasting tributaries Wujiang (WJ) – mainly dominated by carbonate weathering, and Ganjiang – mainly dominated by silicate weathering. The DSi: HCO₃^– ratio ranged from 0 to 0.67, which is in agreement with mixing between the weathering of carbonate or silicate. A negative correlation between DSi and HCO₃^– was observed and interpreted as the imprint of regional geology on water chemistry. This relationship and the DSi: HCO₃^– ratios illustrate the predominant role of carbonate weathering on the riverine HCO₃^–, even forcatchments where silicate rocks are dominant. In contrast, a cascade of dams in WJ tributary influenced the concentrations of Dsi and HCO₃^– and resulted in a positive correlation between DSi and HCO₃^–. This is because reservoirs allowed populations of diatomsto develop and diatoms stoichiometrically sequestered DSi and HCO₃^– causing a negative feedback regulation on the DSi: HCO₃^– ratio. Our study demonstrates that the relationship between DSi and HCO₃^–can reflect not only their geological background but also the influence of algal activity (diatom uptake) within rivers.