The McMurdo Dry Valleys of South Victoria Land, Antarctica, contain numerous glacial meltwater streams that drain into lakes on the valley floors. Many of the streams have abundant perennial mats of filamentous cyanobacteria. The algal mats grow during streamflow in the austral summer and are in a dormant freeze-dried state during the rest of the year. NO₃ and soluble reactive P (SRP) concentrations were lower in streams with abundant algal mats than in streams with sparse algal mats. NO₃ and SRP concentrations were higher in the hyporheic zone of a stream with abundant algal mats than in the stream itself. An experimental injection of LiCl, NaNO₃, and K₃PO₄ was conducted in Green Creek, which has abundant algal mats. Substantial hyporheic exchange occurred. The NO₃ and PO₄ concentrations at 50 m below the injection were 55 μM and 18 μM, respectively, during the experiment. NO₃and PO₄ concentrations were below the detection limit of 1 to 2 μM at a site 497 m below the injection during the Cl tracer arrival, indicating a high capacity for nutrient uptake by algal communities. NO₂ and NH₄ were present at sites 226 and 327 m below the injection, indicating that, in addition to denitrification and algal uptake, dissimilatory NO₃ reduction to NO₂ and NH₄ may be a NO₃ sink during transport. Transport modelling with nutrient uptake represented as a 1^st-order process yielded reach-scale parameters of 4.3 × 10⁻⁵ to 3.9 × 10⁻⁴/s and 1.4 × 10⁻⁴ to 3.8 × 10⁻⁴/s for uptake of NO₃ and PO₄, respectively. The best match with the observed data was a model in which PO₄ uptake occurred only in the main channel and NO₃ uptake occurred in the main channel and in the hyporheic zone. Hyporheic NO₃ uptake was 7 to 16% of the total uptake for the different stream reaches. These results demonstrate that nutrient flux to the lakes is controlled by hyporheic exchange and nutrient uptake by algal mats in dry valley streams. Streams without algal mats contribute more nutrients to the lakes than streams with algal mats.