PHOSPHATE DYNAMICS IN AN ACIDIC MOUNTAIN STREAM - INTERACTIONS INVOLVING ALGAL UPTAKE, SORPTION BY IRON-OXIDE, AND PHOTOREDUCTION

Acid mine drainage streams in the Rocky Mountains typically have few a lgal species and abundant iron oxide deposits which can sorb phosphate . An instream injection of radiolabeled phosphate ((PO4)-P-32) into St . Kevin Gulch, an acid mine drainage stream, was used to test the abil ity of a dominant algal species, Ulothrix sp., to rapidly assimilate p hosphate. Approximately 90% of the injected phosphate was removed from the water column in the 175-m stream reach. When shaded stream reache s were exposed to full sunlight after the injection ended, photoreduct ive dissolution of iron oxide released sorbed P-32, which was then als o removed downstream. The removal from the stream was modeled as a fir st-order process by using a reactive solute transport transient storag e model. Concentrations of P-32 mass(-1) of algae were typically 10-fo ld greater than concentrations in hydrous iron oxides. During the inje ction, concentrations of P-32 increased in the cellular P pool contain ing soluble, low-molecular-weight compounds and confirmed direct algal uptake of (PO4)-P-32 from water. Mass balance calculations indicated that algal uptake and sorption on iron oxides were significant in remo ving phosphate. We conclude that in stream ecosystems, PO4 sorbed by i ron oxides can act as a dynamic nutrient reservoir regulated by photor eduction.