Effect of seasonal biogenic silica variations on dissolved silicon fluxes and isotopic signatures in the Congo River

We report 2 yr of monthly data from the Congo River (the second largest river in the world) on dissolved silicon concentrations, biogenic silica particle concentrations, isotopic signatures of dissolved silicon, and biogenic siliceous particle counts. Diatoms predominated in the biogenic silica fraction, especially during low flow; phytoliths and sponge spicules were more abundant than diatoms only during the rainy season, when biogenic silica fluxes are low. Biological processes dominated the seasonal variations of the dissolved δ30Si ratio that are superimposed on a constant abiotic δ 30Si value of +0.70‰ ± 0.05‰ throughout the year. The measured biogenic silica concentration is less than the amount required to explain the monthly variations of dissolved δ 30Si signatures. We use these signatures and a Rayleigh isotopic fractionation model to calculate that 82% ± 7% of the diatoms produced each month would be exported out of the water column, probably through settling in the Malebo Pool or further upstream. The uptake of dissolved silicon by diatoms during low water flow periods could explain the absence of the dilution effect observed for the other major elements. Annual Si export to the estuary is 1.17 × 1010 mol yr−1 in the form of biogenic silica and 2.23 × 1011 mol yr−1 in dissolved Si form, with a mean dissolved δ 30Si of +0.96‰ ± 0.27‰. Phytoliths make only a minor contribution to the annual biogenic silica flux, and dissolved Si fluxes predominate over biogenic Si fluxes.