『Abstract
　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 〜90％ of the P in the river and ＞90％ in the deep groundwaters. Within the hyporheic zone, the proportion of ‘colloidal’ (＜0.45μm and＞10 kDa) and ‘particulate’ (＞0.45μm) P is higher than in either the groundwater or the surface water, accounting for 〜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 CaCO3. 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 (20-30μg/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.

Keywords: nutrients; Phosphorus; Hyporheic; Groundwater; River; Chalk; Tangential flow fractionation (TFF)』

1. Introduction
2. Site description
　2.1. Geology and hydrogeology
　2.2. Study site
3. Methodology
　3.1. Groundwater and surface water sampling
　3.2. Tangential flow fractionation
　3.3. Chemical analysis
4. Results and discussion
　4.1. Colloid-phosphorus association in surface waters, hyporheic waters and groundwaters
　4.2. Sources and mobility of colloid-bound and dissolved phosphorus
5. Conclusions
Acknowledgements
References