Measurement of Phosphorus Uptake Length in Streams: Comparison of Radiotracer and Stable PO4Releases

Abstract

Experimental releases of radiotracer and stable PO₄were conducted on the same or consecutive days in a temperate first-order woodland stream to determine if both techniques resulted in similar measures of PO₄uptake length (S_w). For each set of experiments, S_wmeasured by radiotracer releases was substantially shorter than S_wmeasured by stable PO₄releases. For stable PO₄releases, S_wwas shorter for releases resulting in small increases in PO₄concentration in water (7–12 μgP/L) than for releases resulting in larger increases in PO₄concentration (84–163 μgP/L). Although the uptake rate of PO₄increased with increasing [PO₄] the increase in uptake rate was not sufficient to offset increases in downstream flux of PO₄with stable PO₄additions, thus resulting in longer S_w. A plot of PO₄uptake rate against [PO₄] suggested two uptake mechanisms: biologically controlled uptake at low [PO₄] that appeared to become saturated at concentrations above approximately 5 μgP/L, and physical/chemical adsorption at high [PO₄] that continued to increase with increasing concentration. Results indicate that stable PO₄releases will overestimate ambient S_wunless PO₄increases are sufficiently small that they do not approach saturation of the biological community.