I examined nutrient limitation, nitrogen demand and nitrogen procesing in streams of the Hubbard Brook Experimental Forest (HBEF). Despite draining remarkably similar forested watersheds, HBEF streams vary in their nitrate concentrations. I examined spatial and seasonal variation in nutrient limiation of stream autotrophhs and whole stream ecosystem nutrient demand. I found that nutrients rarely limit algal biomass in these streams, and nitrogen may instead inhibit algal growth through modification of competitive interactions between algae and microbes. Whole-stream demand for both NH4 and NO3 appears to be primarily controlled by stream microbial communities while PO4 demand is primarily controlled by abiotic sorption capacity. Instream PO4 demand appears to be related to forest age, probably because of long-term changes in the physical structure of streambeds as the surrounding forest ages. Uptake of NH4 and NO3 were not closely tied to these differences in physical structure. Much of the NH4 removed from the water column is subsequently nitrified, I found that variation in nitrification potential was significantly correlated with streamwater NO3 concentrations. It appears that higher DIN concentrations facilitate greater NH4 availability to stream nitrifiers, suggesting that there may be a positive feedback linking rates of NO3 loss from forests to nitrification rates in the receiving streams. Competition for N within the stream microbial community appears to be intense, when streamwater NH4 concentrations were experimentally doubled in summer, uptake rates also doubled, indicating a large capacity for NH4 uptake in these streams. By continuously adding a labile form of DOC (CH3COOK) to a headwater stream for two months I increased demand for nitrogen, and was able to test several predictions about the outcome of N competition. Adding DOC to the stream stimulated heterotrophic bacteria, increasing their biomass and activity and leading to high demand for both NH4 and NO3. This increased demand reduced NO3 concentrations to less than 5 micrograms N per L for the duration of the experiment. Both nitrification and denitrification were reduced as a result of DOC addition, indicating that nitrifiers and denitrifiers lost out in competition for available DIN. Streams of the HBEF actively process DOC, DON and PO4.