Retention of spike additions of soluble phosphorus in a northern eutrophicstream

We evaluated P transport through a 3(rd)-order eutrophic stream in 2 season s, December 1994 and September 1997 by adding spikes of dissolved P (orthop hosphate) and dye tracer (Rhodamine WT) to a 3-km reach. The December exper iment occurred under ice cover when biological growth was minimal; during t he September experiment plant growth was abundant. Both the dye and P plume s were dispersed in space and time as a result of mixing and transient stor age; only P was retained in the stream reach. Initial reversible short-term P retention in the stream reach was demonstrated in both seasons. In Decem ber, 36% (318 g) of added P was retained for about 2.5 h, then released ove r the next 12 h. In September, 39% (116 g) of added P was retained over 12 h, with negligible subsequent release. All the P added in December was expo rted from the reach within 24 h; 38% of added P (4 mg P/m(2)) was still ret ained in the stream reach at the end of the 48-h September experiment. Phos phorus retention was apparently influenced by now, temperature, concentrati on gradient, and biological activity. Short-term retention probably resulte d from sorption by inorganic sediments and organic biofilms; long-term rete ntion may have been a result of biological uptake. Instream P retention pro cesses may not be capable of reducing P transport significantly during high flows and cold temperatures, but may temporally attenuate inputs delivered during small stormflows in the growing season.