Dryland streams of the Pilbara region of Western Australia have iron- and calcium-rich sediments which effectively ‘lock up’ the little phosphorus (P) available for production. It is hence widely assumed that productivity within these streams is strongly P limited. Yet water column measurements of bioavailable nutrients (DIN and SRP) across a range of Pilbara streams indicated that nitrogen (N) concentrations are also much depleted and potentially limiting (DIN = 0 – 36 µg l^-1, SRP = 1 – 4 µg l^-1_, N:P ratio = 0 – 12). We measured the response of seston and periphyton to nutrient substrate addition in a series of bottle assays to determine which nutrient is most strongly limiting. Bottles were filled with either stream water (seston measurements) or stream water + periphyton inoculum (periphyton measurements). Nitrogen (NO₃ + NH₄) and phosphorus (PO₄) substrates were added to bottles to enable 10x ambient concentrations. A full factorial design was employed with treatments of control, dark, +N, +P, and +N+P. Dissolved oxygen concentrations were measured at the start and completion of 6 hr in-stream incubations to calculate productivity and respiration. Results from bottle assays were compared to ecosystem respiration measured by the single station method for each site. This poster presents initial findings and experimental design of further investigations underway.