Silicon depletions in some Norfolk rivers

Abstract

SummaryWater samples collected weekly from the rivers Yare, Tud, Wensum and Tas in Norfolk, England, displayed marked depletions in the spring and summer of the concentration of dissolved silicon. These were unconnected with any hydrological event and were assumed to be due to the assimilation of silicon by diatoms. Equilibrium concentrations were maintained in the Yare and Tud during the weeks prior to and following the spring bloom. It was estimated that 35 Mg (51%) of the predicted load were removed from the Yare during the 11 weeks of this depletion and 6‐0 Mg (45%) from the Tud. The lowest observed concentration (0‐4 mg/l) occurred during the first week in May when over 90% of the silicon had been removed. However, the maximum amount of removal and hence maximum diatom productivity occurred earlier at a time of higher water discharge. A similar pattern was observed in the Yare during the spring of 1971. Two small blooms occurred later in the summer of 1970 in the Yare and Tud. It was estimated that 15% ofthe Yare's dissolved silicon load of 263 Mg was in the assimilated form and 12‐5%of the Tud's output of 56‐6 Mg. The weathering of silicate minerals was probably the source of almost all the silicon and the outputs represented a silicon erosion rate of 1‐15 Mg/km2/year for the Yare and 0‐77 Mg/kmVyear for the Tud.