Phosphorus and dissolved silicon dynamics in the River Swale catchment, UK: a mass-balance approach

The internal riverine processes acting upon phosphorus and dissolved silicon were investigated along a 55 km stretch of the River Swale during four monitoring campaigns. Samples of river water were taken at 3 h intervals at sites on the main river and the three major tributaries. Samples were analysed for soluble reactive phosphorus, total dissolved phosphorus, total phosphorus, dissolved silicon and suspended solid concentration. Mass-balances for each determinand were calculated by comparing the total load entering the river with the total load measured at the downstream site. The difference, i.e. the residual load, showed that there was a large retention of phosphorus and silicon within the system during the March 1998 flood event, but the other three campaigns produced net-exports. Cumulative residual loads were calculated for each determinand at 6 h intervals throughout each campaign. This incremental approach showed that the mass-balance residuals followed relatively consistent patterns under various river discharges. During stable low-flow, there was a retention of particulate phosphorus within the system and also a retention of total dissolved phosphorus and soluble reactive phosphorus, most likely caused by the sorption of soluble phosphorus by bed-sediments. In times of high river-discharge, there was a mobilization and export of stored bed-sediment phosphorus. During overbank flooding, there was a large retention (58% of total input) of particulate phosphorus within the system, due to the mass deposition of phosphorus-rich sediment onto the floodplain. Soluble phosphorus was also retained within the system by sequestration from the water column by the high concentration of suspended solids. The dissolved silicon mass-balance residuals had a less consistent pattern in relation to river discharge. There was a large retention of dissolved silicon during overbank flooding, possibly due to sorption onto floodplain soil, and net-exports during periods of both stable low how and rising limbs of hydrographs, due to release of dissolved silicon from pore-waters. Copyright (C) 2001 John Wiley & Sons, Ltd.