The potential of agricultural headwater streams to retain soluble reactive phosphorus

The study focuses on the capacity of agricultural headwater streams to retain soluble reactive phosphorus (SRP). In-stream phosphorus uptake was determined via short-term SRP additions in 14 reaches differing in channel morphology and riparian vegetation. In addition, zero equilibrium phosphorus concentrations (EPC 0 ) were estimated for 8 reaches based on adsorption experiments. Average SRP uptake lengths amounted to 3.8 km in channelized sections, 1.9 km in forested sections, and 0.5 km in open meanders. Mass transfer coefficients were highest in open meanders (0.1 cm min −1 ), followed by forested (0.05 cm min −1 ) and channelized sections (0.04 cm min −1 ). EPC 0 ranged from 20 to 1,600 µg SRP l −1 and correlated positively with inorganic P and reductant-soluble P concentrations of the sediments. In 50% of the reaches, phosphorus was released from the sediments at initial water concentrations of up to 500 µg SRP l −1 , indicating a high release potential. Although EPC 0 did not correlate with in-stream SRP uptake, sediments probably play a significant role for the P retention in agricultural headwater streams as they supply the benthic community with phosphorus from the subsurface. Thus, it is crucial that sediment–water interactions are considered in the restoration and management of agricultural headwater streams.