We contrasted leaf processing rates among five streams in the Upper Hudson watershed to describe how ecosystem function varied with water chemistry. Although most temperate streams are Phosphorous-limited (median N:P = ~50), this shortage was particularly pronounced in our study streams (mean N:P ± SD = 122 ± 28). Intriguingly, total P was not correlated with N:P (because among-stream variation in N was almost twice as great for P and streams with greater total P also tended to include greater amounts of N). We used the in-stream processing rates of 3g senesced knotweed (Polygonatum cuspidatum) leaves in June 2007 as an indicator of ecosystem function, and found greater than four-fold variation in processing rates among streams. A model that included both P availability (μg L^-1 of Total P) and the relative availability of Nitrogen to P (N:P) explained 98% of the among-stream variation in leaf processing rates. In our system, the influence of particular nutrients cannot be predicted from their concentrations alone. Here, leaf processing (a measure of ecosystem functioning) was fastest where Phosphorous is abundant both in absolute terms (μg L^-1) and abundant relative to Nitrogen.