VERTICAL HYDROLOGIC EXCHANGE AND ECOSYSTEM METABOLISM IN A SONORAN DESERT STREAM

Hyporheic metabolism in a Sonoran Desert stream was examined, focusing on the sources of detritus supporting hyporheic respiration. Two alte rnative hypotheses were specifically addressed: (1) organic matter der ived from the surface stream supports hyporheic respiration, and (2) d etritus buried during flash floods supports hyporheic respiration. As predicted for the surface-derived organic matter hypothesis, respirati on was lowest immediately following flash floods and increased signifi cantly with time after flood (P < 0.001). Hyporheic respiration ranged from 0.05 mgO(2) . L sediments(-1). h(-1) immediately following a fla sh flood to as high as 4.41 mgO(2) . L sediments(-1). h(-1) late in al gal succession. Respiration was significantly correlated with surface algal biomass during two spring/summer successional sequences (P < 0.0 5; partial correlation coefficients 0.58 and 0.88). Respiration was al so consistently higher in downwelling than upwelling zones with overal l mean rates of 1.12 and 0.46 mgO(2) . L sediments(-1). h(-1), respect ively. Respiration exhibited a distinct diel pattern with highest rate coinciding with time of maximum photosynthesis and was also significa ntly correlated with dissolved organic carbon concentration (P < 0.05) , further supporting the hypothesis of hyporheic dependence on algal p roduction. Flash floods bury organic matter that is also respired in t he hyporheic zone; however, based upon storage of organic carbon immed iately following floods, an average of only 15% of the observed respir ation could be supported. We conclude that hyporheic respiration in Sy camore Creek is tightly linked to surface production. It is spatially distributed in biotic ''hot spots'' where surface waters enter hyporhe ic sediments and is most likely supported by organic matter that is su pplied as dissolved organic carbon, perhaps from algal production.