We analysed Si isotopic signatures (delta Si-30 signature) of dissolved silicon (DSi) in the Scheldt (Belgium) tidal river (four stations) and in its main tributaries (Zenne, Dender, Grote Nete and Kleine Nete). For each station, we analysed four samples representative of the four seasons. These are the first delta Si-30 data at the basin and seasonal scales measured in a watershed under high anthropogenic pressure. In the Scheldt main stem, we show that the role of diatoms on the silicon biogeochemical cycle and delta Si-30 signatures is dominant in summer and sometimes in fall. Out of the diatoms' growing season, the Scheldt main stem exhibits unexpected large delta Si-30 signatures variations, especially in winter. Although these isotopic variations are not reflected in DSi concentrations, they are consistent with delta Si-30 signatures recorded in the main tributaries. This indicates that except for summer diatom blooms, delta Si-30 signatures in the tidal river result from a conservative mixing with tributaries. Overall we suggest that the variations are mainly set by land use and lithology at the sub-basin level. Forest cover (inducing clay dissolution favoured by the presence of humic acids) and wetland cover (yielding to dissolution of biogenic silica) are highly and significantly correlated with delta Si-30 (R-2 > 0.85). Both environments tend to render the tributaries delta Si-30 signatures lighter while agriculture cover has the reverse effect on delta Si-30 values (R-2 = 0.70, p-value < 0.08). In contrast, urban zone cover does not have any noticeable effect on Si isotopic signatures of the Scheldt tributaries. Despite this effect of land use, we estimate a mean annual delta Si-30 signature output from the Scheldt tidal river to the saline estuary at 0.95%., well comparable to other rivers. This results from the mixing of tributaries within the tidal river which levels off at the outlet the Si isotopic differences of the sub-basins. In the Scheldt watershed, it thus seems that Si isotopes could trace anthropogenic activities at the sub-basin scale. In contrast, at the outlet, the variety of land-use and lithology found over the whole watershed are merged into a mean isotopic signature similar to the world river average. As it is now recognised that change in land use alter the silicon biogeochemical continental cycle, our data show that use of Si isotopic signatures might constitute a useful proxy to better trace these modifications on contrasted (sub-)basins. |
