Pre- and post-flood retention efficiency of nitrogen in a Sonoran Desert stream

The objectives of this study were 1) to compare Sonoran Desert streams with other streams in terms of retention efficiency of nitrate; 2) to examine the effects of a flood on nitrate retention and to determine which factors control nitrate retention in the surface stream subsystem in Sycamore Creek, Arizona; and 3) to compare the short-term nutrient addition technique with computations based upon natural nutrient gradients. From June to September 1995, we did 8 short-term nitrate and chloride additions (4 additions before and 4 after a flood) in a 240-m reach to measure nitrate uptake length as an index of surface stream retention efficiency of nitrate. We also calculated nitrate uptake lengths based on a natural downstream decline in nitrate concentration, using data from the addition dates and from previous studies. Nitrate uptake lengths measured in Sycamore Creek were short (<120 m) compared to published values from other streams, indicating a high retention efficiency of nitrate in this nitrogen-limited stream. A midsummer flood caused a 2-fold decrease in retention efficiency of nitrate in the reach (i.e., nitrate uptake length increased from 61 to 124 m); however, this change was within the range of variation measured before the flood. Rapid algal recovery (23 d), the dramatic decrease in discharge, and a large transient storage zone may account for the apparent high resilience of nutrient retention efficiency to disturbance. Most of the temporal variation in nitrate uptake length during the study period was attributed to changes in the algal assemblage. In particular, retention efficiency of nitrate decreased when nitrogen fixers were abundant. Uptake lengths calculated from additions were always shorter than those from natural nitrate declines, supporting our hypothesis that nutrient uptake lengths from short-term nutrient additions reflect gross, rather than net, nutrient uptake. Uptake rates from short-term additions and from natural declines of nitrate over post-flood succession showed a similar temporal pattern, but the ratio between them increased late in succession. This result suggests that, during late successional stages, nutrient release processes became more important than nutrient uptake processes, a prediction that is consistent with the ecosystem succession and nutrient retention hypothesis.