In polar regions, where many glaciers are cold‐based (frozen to their beds), biological communities on the glacier surface can modulate and transform nutrients, controlling downstream delivery. However, it remains unclear whether supraglacial streams are nutrient sinks or sources and the rates of nutrient processing. In order to test this, we conducted tracer‐injections in three supraglacial streams (62 to 123 m long) on Canada Glacier in the Taylor Valley, of the McMurdo Dry Valleys, Antarctica. We conducted a series of additions including: nitrate (N), N + phosphate (P), N+ P + glucose (C), and N+C. In two reaches, N‐only additions resulted in N uptake. The third reach showed net N release during the N‐only addition, but high N uptake in the N+P addition, indicating P‐limitation or N+P co‐limitation. Co‐injecting C did not increase N‐uptake. Additionally, in these systems at low N concentrations the streams can be a net source of nitrogen. We confirmed these findings using laboratory‐based nutrient incubation experiments on sediment collected from stream channels on Canada Glacier and two other glaciers in the Taylor Valley. Together, these results suggest there is active biological processing of nutrients occurring in these supraglacial streams despite low sediment cover, high flow velocities and cold temperatures, modifying the input signals to proglacial streams. As glaciers world‐wide undergo rapid change, these findings further our understanding of how melt generated on glacier surfaces set the initial nutrient signature for subglacial and downstream environments.