Experimental releases of radiotracer and stable PO4 were conducted on the same or consecutive days in a temperate first-order woodland stream to determine if both techniques resulted in similar measures of PO4 uptake length (Sw). For each set of experiments, Sw measured by radiotracer releases was substantially shorter than Sw measured by stable PO4 releases. For stable PO4 releases, Sw was shorter for releases resulting in small increases in PO4 concentration in water (7–12 μgP/L) than for releases resulting in larger increases in PO4 concentration (84–163 μgP/L). Although the uptake rate of PO4 increased with increasing [PO4] the increase in uptake rate was not sufficient to offset increases in downstream flux of PO4 with stable PO4 additions, thus resulting in longer Sw. A plot of PO4 uptake 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 PO4 releases will overestimate ambient Sw unless PO4 increases are sufficiently small that they do not approach saturation of the biological community.
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