Vertical Hydrologic Exchange and Ecosystem Metabolism in a Sonoran Desert Stream

Abstract

Hyporheic metabolism in a Sonoran Desert stream was examined, focusing on the sources of detritus supporting hyporheic respiration. Two alternative hypotheses were specifically addressed: (1) organic matter derived from the surface stream supports hyporheic respiration, and (2) detritus buried during flash flood supports hyporheic respiration. As predicted for the surface—derived organic matter hypothesis, respiration was lowest immediately following flash floods and increased significantly with time after flood (P < 0.001). Hyporheic respiration ranged from 0.05 mgO 2 °L sediments — 1 °h — 1 immediately following a flash flood to as high as 4.41 mgO 2 °L sediments 0 1 °h — 1 late in algal succession. Respiration was significantly correlated with surface algal biomass during two spring/summer successional sequences (P < 0.05; partial correlation coefficients 0.58 and 0.88). Respiration was also consistently higher in downwelling than upwelling zones with overall mean rates of 1.12 and 0.46 mgO 2 °L sediments — 1 °h — 1 , respectively. Respiration exhibited a distinct diel pattern with highest rate coinciding with time of maximum photosynthesis and was also significantly correlated with dissolved organic carbon concentration (P < 0.05), further supporting the hypothesis of hyporheic dependence on algal production. Flash floods bury organic matter that is also respired in the hyporheic zone; however, based upon storage of organic carbon immediately following floods, an average of only 15% of the observed respiration could be supported. We conclude that hyporheic respiration in Sycamore Creek is tightly linked to surface production. It is spatially distributed in biotic "hot spots" where surface water enter hyporheic sediments and is most likely supported by organic matter that is supplied as dissolved organic carbon, perhaps from algal production.