Regulation of bioavailable nutrients in the hyporheic zone of a Chalk stream in South East England

In this study we have used tangential flow fractionation (TFF) to investigate P-colloid associations in the hyporheic zone of a groundwater dominated chalk stream, as well as the association of phosphate (PO4) with laboratorygenerated chalk and clay colloids. Phosphorus (P) speciation is similar for the River Lambourn and the deeper Chalk aquifer beneath the hyporheic zone, with ‘dissolved’ P (<10 kDa) accounting for c.90% of the P in the River and >90% in the deep groundwaters. Within the hyporheic zone the proportion of ‘colloidal’ (<0.45 um>10 kDa) and ‘particulate’ (>0.45 um) P is high, accounting for 30% of total P. Our results suggest that zones of interaction within the sand and gravel deposits directly beneath and adjacent to the river system generate colloidal and particulate forms of fulvic-like organic material and regulate bioavailable forms of P, perhaps through co-precipitation with CaCO3. Organic matter decomposition and nitrification in the hyporheic zone could be the source of both fulvic-like dissolved organic matter and NO3-N. While the aquifer provides some degree of protection to the sensitive surface water ecosystems through physiochemical processes of P removal, where flow is maintained by groundwater, ecologically significant P concentrations (20-30 ug/L) are still present in the Chalk groundwater and may be an important source of bioavailable P during baseflow conditions. Synthetic colloidal suspensions of chalk and clay were found to be very effective at removing PO4 (100 ug-P/L) from solution (>90% for chalk and >80% for clay after 1hour) and may be an important sink for bioavailable P within streambed sediments. The nutrient storage capacity of the hyporheic zone and the water residence times of this dynamic system are largely unknown and warrant further investigation.