Purpose In many streams worldwide including those on the south coast of Western Australia (WA), sediments of the >2-mm fraction often contribute up to 50% of the streambed. However, most analysis and interpretation of sediment chemistry, including phosphorus (P), is conducted on the 2-mm fraction to P retention and release in sandy-gravely streams.
Material and methods Sediment samples were collected from streams in agricultural catchments, and P retention and release by the 2-mm (typically lateritic; iron rich) sediment fractions were examined using fluvarium and batch experiments. Phosphorus sorbed by sediment was estimated on a mass (mg P kg(-1)) and area basis (mg P m(-2)).
Results and discussion Phosphorus sorption measurements suggested that mineralogy as well as particle size were important factors influencing P retention by stream sediments. Stream sediments retained approximately 30% of added P. In a desorption phase, approximately 8% of the retained P was released into stream water.
Conclusions Stream sediments in south western WA appear to be net immobilisers of P, retaining more P than they release, dependent on the stream P concentration. Exclusion of the >2-mm fraction when determining stream sediment P dynamics may therefore underestimate whole stream sediment P retention and release.