J. N. Am. Benthol. Soc., 1995, 14(2):249-258 O 1995 by The North American Benthological Society Nitrification in the hyporheic zone of a desert stream ecosystem JEREMY B. JONES, JR.', STUART G. FISHER, AND NANCY B. GRIMM Department of Zoology, Arizona State University, Tempe, Arizona 85287 USA Abstract. Nitrification in the hyporheic zone of Sycamore Creek, a Sonoran Desert stream, was examined, focusing on the association between respiration and nitrate production. Subsurface res- piration in Sycamore Creek is highest in regions of hydrologic downwelling where organic matter derived from the stream surface is transported into the hyporheic zone. Similarly, nitrification was closely related to hydrologic exchange between the surface and hyporheic zone. Nitrification in downwelling regions averaged 13.1 ygN0,-N.L sediments-l.h-l compared with 1.7 ygN0,-N.L sediments-'.h-' 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 organic matter to support subsurface metabolism. Nitrification was also significantly affected by flooding; nitrification increased from an average of only 3.0 ~gN0,-N.L sediments-'.h-' immediately fol- lowing floods to 38.5 pgN0,-N.L sedimentsl.h' late in succession. Nitrification was significantly correlated with hyporheic respiration, supporting the hypothesis that nitrification is fueled by mineralization of organic nitrogen to ammonium. The coupling between subsurface respiration and nitrification is one step in a cyclic interaction between surface and hyporheic zones and serves to transform nitrogen from an organic to inorganic form. Key words: nutrient dynamics, nitrate, nitrification, desert streams, hyporheic zone, hydrologic exchange. In nutrient-limited ecosystems, factors gov- erning import and transformation of nutrients can have strong effects on production. Streams are composed of a mosaic of interacting sub- systems (e.g., surface-stream, riparian, hypor- heic and parafluvial zones) that can function as sources or sinks for nutrients (e.g., Peterjohn and Correll1984, Ford and Naiman 1989, Triska et al. 1989a, Valett et al. 1994). Nutrient demand in particular subsystems, such as that caused by photoautotrophic production in surface chan- nels (Elwood et al. 1981, Newbold et al. 1983, Triska et al. 1989b), and denitrification in ri- parian zones (Peterjohn and Correll 1984, Low- rance et al. 1985, Duff and Triska 1990), may reduce nutrient concentration. Nutrient trans- formation or release from other subsystems may elevate nutrient concentration, for example ni- trogen fixation in surface streams (Howarth et al. 1988) and groundwater input (Rutherford and Hynes 1987, Ford and Naiman 1989, Hen- dricks and White 1991).Elucidation of processes occurring within subsystems, as well as inter- Present address: Environmental Sciences Divi- sion, Oak Ridge National Laboratory, P.O. BOX2008, Building 1000, Oak Ridge, Tennessee 37831-6335 USA. actions between them, is critical to understand- ing stream ecosystem functioning. Primary production in streams of the Sonoran Desert is nitrogen limited (Grimm and Fisher 1986a, 1986b). An important source of nitrogen for photoautotrophic production is hydrologic linkage with the nutrient-rich hyporheic zone (Valett et al. 1994). As water flows from the surface through hyporheic and parafluvial zones, nitrate concentration is elevated through nitrification (Grimm et al. 1991, Holmes et al. 1994). This nitrate-rich subsurface water even- tually reenters the surface through hydrologic exchange, and alleviates local nutrient limita- tion (Valett et al. 1994). In fact, nitrate produc- tion in hyporheic sediments may be great enough to account for 20-100% of algal nitro- gen demand (Grimm 1992). The objectives of our study were to determine factors regulating nitrification in hyporheic sediments and the contribution of nitrification as a source of inorganic nitrogen for photoau- totrophic production. Grimm (1992) and Holmes et al. (1994) speculated that nitrification was fueled by mineralization of organic nitrogen to ammonium during decomposition. This Paper specifically focuses on the association between hyporheic respiration and nitrification. Nitri- 249