Stream network position affects epilithic N:P and nutrient uptake in a northern California watershed

Changes in drainage area are associated with physical changes in stream channels that have biological consequences.  Small headwater streams are shaded by a closed canopy and organic matter inputs are primarily terrestrial, while larger, sunny streams are dominated by in-stream production.  In the South Fork Eel River watershed in northern California, these changes influence food webs, but impacts on nutrient uptake are unclear.  We compared uptake of dissolved inorganic nitrogen (DIN) and PO₄ by epilithic biofilms in 5 streams ranging in drainage area from 0.58-140 km².  Uptake was measured by incubating rocks collected from each site in re-circulating chambers.  DIN and PO₄ were added to each chamber and uptake was estimated by change in nutrient concentration over time.  Biofilms were analyzed for organic matter, chlorophyll a, and % carbon, nitrogen and phosphorus.  DIN uptake was higher than PO₄ uptake in all streams.  DIN uptake showed little variation among sites, while PO₄ uptake increased with drainage area, with the largest stream 4 fold higher than the smallest.  The difference between DIN and PO₄ uptake decreased with increasing drainage area, suggesting a shift from N limitation towards P limitation.  DIN uptake was not related to any explanatory variables.  PO₄ uptake was positively related to chlorophyll a and to biofilm N:P, both of which also increased with drainage area.  These results suggest decreasing severity of N limitation with increasing drainage area, leading to an increase in PO₄ uptake as it becomes scarce relative to DIN.  We hypothesize this pattern is the result of higher N fixation in large sunny streams.