Understanding Phosphorus Mobility and Bioavailability in the Hyporheic Zone of a Chalk Stream

This paper investigates the changes in bioavailable phosphorus (P) within the hyporheic zone of a groundwater-dominated chalk stream. In this study, tangential flow fractionation is used to investigate P associations with different size fractions in the hyporheic zone, groundwater and surface water. P speciation is similar for the river and the chalk aquifer beneath the hyporheic zone, with 'dissolved' P (< 10 kDa) accounting for similar to 90% of the P in the river and > 90% in the deep groundwaters. Within the hyporheic zone, the proportion of 'colloidal' (< 0.45 mu m and > 10 kDa) and 'particulate' (> 0.45 mu m) P is higher than in either the groundwater or the surface water, accounting for similar to 30% of total P. Our results suggest that zones of interaction within the sand and gravel deposits directly beneath and adjacent to river systems generate colloidal and particulate forms of fulvic-like organic material and regulate bioavailable forms of P, perhaps through co-precipitation with CaCO(3). While chalk aquifers provide some degree of protection to surface water ecosystems through physiochemical processes of P removal, where flow is maintained by groundwater, ecologically significant P concentrations (20-30 mu g/L) are still present in the groundwater and are an important source of bioavailable P during baseflow conditions. The nutrient storage capacity of the hyporheic zone and the water residence times of this dynamic system are largely unknown and warrant further investigation.