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

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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 (Sw). For each set of experiments, Swmeasured by radiotracer releases was substantially shorter than Swmeasured by stable PO4releases. For stable PO4releases, Swwas 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 Sw. 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 Swunless PO4increases are sufficiently small that they do not approach saturation of the biological community.