PREFLOOD AND POSTFLOOD 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 subsyste m in Sycamore Creek, Arizona; and 3) to compare the short-term nutrien t addition technique with computations based upon natural nutrient gra dients. From June to September 1995, we did 8 short-term nitrate and c hloride 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 r etention efficiency of nitrate. We also calculated nitrate uptake leng ths based on a natural downstream decline in nitrate concentration, us ing data from the addition dates and from previous studies. Nitrate up take lengths measured in Sycamore Creek were short (<120 m) compared t o published values from other streams, indicating a high retention eff iciency of nitrate in this nitrogen-limited stream. A midsummer flood caused a 2-fold decrease in retention efficiency of nitrate in the rea ch (i.e., nitrate uptake length increased from 61 to 124 m); however, this change was within the range of variation measured before the floo d. Rapid algal recovery (23 d), the dramatic decrease in discharge, an d a large transient storage zone may account for the apparent high res ilience of nutrient retention efficiency to disturbance. Most of the t emporal variation in nitrate uptake length during the study period was attributed to changes in the algal assemblage. In particular, retenti on efficiency of nitrate decreased when nitrogen fixers were abundant. Uptake lengths calculated from additions were always shorter than tho se from natural nitrate declines, supporting our hypothesis that nutri ent uptake lengths from short-term nutrient additions reflect gross, r ather than net, nutrient uptake. Uptake rates from short-term addition s and from natural declines of nitrate over post-flood succession show ed a similar temporal pattern, but the ratio between them increased la te 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.