NITRIFICATION IN THE HYPORHEIC ZONE OF A DESERT STREAM ECOSYSTEM

Nitrification in the hyporheic zone of Sycamore Creek, a Sonoran Deser t stream, was examined, focusing on the association between respiratio n and nitrate production. Subsurface respiration in Sycamore Creek is highest in regions of hydrologic downwelling where organic matter deri ved from the stream surface is transported into the hyporheic zone. Si milarly, nitrification was closely related to hydrologic exchange betw een the surface and hyporheic zone. Nitrification in downwelling regio ns averaged 13.1 mu gNO(3)-N . L sediments(-1). h(-1) compared with 1. 7 mu gNO(3)-N . L sediments(-1). h(-1) in upwelling regions. Hyporheic respiration also varies temporally as a result of flash floods which scour and remove algae from the stream and thus reduce the pool of org anic matter to support subsurface metabolism. Nitrification was also s ignificantly affected by flooding; nitrification increased from an ave rage of only 3.0 mu gNO(3)-N . L sediments(-1). h(-1) immediately foll owing floods to 38.5 mu gNO(3)-N . L sediments(-1). h(-1) late in succ ession. Nitrification was significantly correlated with hyporheic resp iration, supporting the hypothesis that nitrification is fueled by min eralization of organic nitrogen to ammonium. The coupling between subs urface respiration and nitrification is one step in a cyclic interacti on between surface and hyporheic zones and serves to transform nitroge n from an organic to inorganic form.