Abstract
Nitrogen fixation, the conversion of unreactive N2 gas from the atmosphere into a biologically usable form NH3, is the least studied process of the N cycle in stream ecology (Grimm and Petrone 1997). Tapping into the atmospheric N pool can only be accomplished naturally by N-fixers. These diazotrophic organisms can contribute significantly to the autochthonous inputs of nitrogen into streams (Grimm and Petrone 1997). Without further investigations into N-fixation we would ignore a potentially major N flux and transfonnation. Not only do we need to quantify this process, but also examine what factors control N-fixation in streams.