Recent studies of nitrogen (N) and phosphorus (P) cycling in stream networks reveal a complex relationship between N and P availability and uptake.  In the South Fork Eel River (SFER), recent additions of nitrogen to streams across a drainage area gradient have resulted in strong increases in P uptake from the water column. The magnitude of increase in P uptake in response to added N declines as drainage area increases, suggesting that N availability increases downstream while P becomes relatively scarce, and larger streams approach co-limitation by N and P.  These results suggest that increasing N availability leads to an increase in P uptake with increasing drainage area.  One possible explanation is an increase in N fixation rates in larger streams. We hypothesize that shifts in biofilm community composition from low-light heterotrophic streams to larger high-light algal dominated streams coincide with an increase in the predominance of nitrogen-fixing cyanobacteria, leading to increased N fixation with increased drainage area.  In this project, we measured nitrogen fixation by isolated epilithic biofilm assemblages (n=7) in five streams in SFER watershed that ranged in drainage area from 0.58 – 148 km². Fixation was measured in-situ during 2-hour incubations using acetylene-reduction assays in re-circulating closed chambers.

Results/Conclusions

Across the drainage area gradient (0.58-148 km²), nitrogen fixation rates increased significantly, exhibiting a strong linear relationship with catchment area (R²=0.98, p<0.001).  Maximum rates observed were quite high, ranging from 50-200 ug N h^-1 m^-2.  Observed spatial patterns in nitrogen fixation were also positively correlated with previously established patterns in epilithon N:P and P uptake.  These results are consistent with our hypothesis that increases in N fixation may be responsible for increasing N availability in the drainage network, leading to higher rates of P uptake.  Observed biogeochemical patterns suggest that shifts in microbial and algal community composition associated with stream network position may differentially affect availability of N and P, and potentially alter the limiting nutrient further downstream.  Thus, N fixation may be an important driver of biogeochemical cycling in watershed ecosystems and deserves more consideration in watershed studies.