Both local and watershed-scale processes influence nutrient dynamics of streams but are most often studied in separately. We examined patterns and controls of dissolved nutrient concentration, speciation, and stoichiometry throughout a stream network ranging from headwater springs to small rivers in northern California. Dissolved inorganic nutrients were extremely low and showed modest decreases with increasing stream size. Particulate and dissolved organic species showed stronger patterns, with particulate phosphorus (P), dissolved organic carbon (DOC), and nitrogen (DON) increasing between 2 to 6 fold from small headwater streams to mid sized rivers. An in-stream source of dissolved and particulate organic matter was evident in the larger and more productive stream reaches. Increases in aquatic N fixation by cyanobacteria appear to elevate DON inputs at these sites. As a result, opposite trends for dissolved total P and N concentrations with stream size were observed, driving strong increases in N:P. This increase may indicate a spatial shift from N to P limitation of algal production as stream size increased in the watershed. Under summer baseflow conditions, downstream increases in productivity lead to strong in-stream influences on nutrient dynamics in the stream network.