An Assessment of the Spatial Variability, Channel Storage Mechanisms and Bioavailability of Reactive Phosphorus in the Upper River Taw

The ambitious and pressing objectives of the European Water Framework Directive have triggered the need for research to address phosphorus (P) as it is currently the failing feature of the majority of surficial water bodies in England. This work focuses on the sediments of the upland low alkalinity River Taw. Point and diffuse sources of P have been identified and are currently being addressed; however it is unknown what affect these sources have had on the river’s sediments and how they will react to diminishing sources of P in the water column. The Diffusive Gradient in Thin Films (DGT) method is utilised in situ to understand, quantify and identify bioavailable P fluxes and potential storage mechanisms at sites of known diffuse and point P inputs. It was found that at the heavily contaminated point source, sediments were likely acting as a sink even after potential input P reduction. The proposed mechanism for this is the formation of a ‘calcium cap’ which co-precipitates with available phosphorus rendering it unavailable. The strong correlation of calcium with total P at all sites, with one exception, potentially validates this mechanism. The exception is subject to agricultural and point P sources and it is suggested that natural calcium at this site isn’t proportional to P loading resulting in sediments acting as a source of P to the water column. It was also found that the molybdenum blue method of quantifying soluble reactive phosphorus is not equivalent to the DGT bioavailable P pool, especially at pristine or moderately point/diffuse influenced sites. This is likely due to the desorption of weakly bound P from colloids, which is unavailable to DGTs. These results have the potential to be scaled up to the catchment or other catchments which exhibit similar sediment physical and chemical composition