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 ~90% of
the P in the river and >90% in the deep groundwaters.
Within the hyporheic zone, the proportion of
‘colloidal’ (<0.45 μm and >10 kDa) and ‘particulate’
(>0.45 μm) 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 coprecipitation
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 (20–30 μg/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