Spatial variation in the coupling of nitrogen and phosphorus uptake in stream ecosystems

In streams, nutrient uptake rates are likely to be influenced by both organic matter inputs and food web structure, which vary predictably with drainage area (DA). We investigated nitrogen and phosphorus uptake in three streams in the South Fork Eel River watershed with DA 0.58 – 17 km².  We measured stream uptake response to a range of nitrogen and phosphorus concentrations and assessed interaction of these nutrients with a series of addition experiments.  In each stream, NH₄, PO₄, NO₃ and a cocktail of all three were added, with conservative tracers, on consecutive days.  During each addition, uptake length was estimated at three different nutrient addition rates.  This method allowed us to estimate ambient uptake lengths and to assess stream response to increased nutrient levels.   In all three streams, N uptake lengths were shorter than P uptake lengths, suggesting N limitation.  Addition of P did not strongly affect N uptake lengths in any of the three streams, but N addition shortened P uptake lengths in the two smaller streams.  The magnitude of increase in P uptake length with increasing P input decreased with DA, while the magnitude of increase in N uptake with increasing N input increased with DA.  These results suggest the availability of N relative to P increases with DA, consistent with an observed increase in N:P in the water column.    Overall, these results suggest a decrease in the severity of N limitation with an increase in DA, consistent with observed increases in nitrogen-fixing biomass as streams increase in size.