VERTICAL HYDROLOGIC EXCHANGE AND ECOLOGICAL STABILITY OF A DESERT STREAM ECOSYSTEM

The influence of hydrologic linkage between hyporheic and surface subs ystems was investigated in sand-bottomed reaches of a desert stream. D irection of hydrologic exchange was measured as vertical hydraulic gra dient (VHG) using mini-piezometers. Maps of VHG indicated upwelling (d ischarge from the interstitial regions into surface water) at the base s of riffles and heads of runs; downwelling (infiltration of surface w ater into the hyporheic zone) occurred at the bases of runs. Dissolved NO3-N in surface water was higher over or immediately downstream from upwelling zones. Loss of continued supply from the hyporheic zone and intense assimilatory demand by surface autotrophs generated longitudi nal declines in NO3-N and lower nutrient concentrations in downwelling zones. Algal standing crop (as chlorophyll a) was significantly highe r in upwelling zones than in areas without positive VHG. Postflood tra jectories of chlorophyll a indicated that algae at upwelling zones rec overed from disturbance significantly faster than those at downwelling zones. Recovery rate was related to supply of NO3-N from enriched int erstitial water in the hyporheic zone. Hydrologic linkage integrates s urface and hyporheic subsystems and increases ecosystem stability by e nhancing resilience of primary producers following flash flood disturb ance.