The influence of particle size and mineralogy on both phosphorus retention and release by streambed sediments

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.