Ecosystem metabolism controls nitrogen uptake in streams in Grand Teton National Park, Wyoming

Streams and rivers regulate nitrogen transport (N) to downstream ecosystems. Rates of N uptake can be high in streams, but controls on the variation in uptake rates of N among streams are not known. We measured ammonium (NH₄⁺) and nitrate (NO₃^-) uptake velocities (V_f) and compared these with whole-reach estimates of gross primary production (GPP) and community respiration (CR) in 11 low-nitrogen streams in Grand Teton National Park, Wyoming. We predicted that increased metabolism would positively relate to higher N demand because of stoichiometric N requirements associated with carbon fixation. Rates of GPP and CR explained 82% of variation in NH₄⁺ V_f. Nitrate V_f was controlled by GPP, not CR, with GPP explaining 75% of variation in NO₃^- V_f. Nitrate concentrations did not increase downstream during NH₄⁺ addition in all streams, including streams with zero NO₃^- uptake, suggesting low nitrification rates relative to NH₄⁺ uptake. Using a stoichiometric model, we show that areal N uptake estimated from microbial and algal production was similar to measured areal N uptake. High primary production could be a prerequisite for streams exhibiting high NO₃^- uptake rates.