干旱区生态环境知识资源中心

The hyporheic zone is of great interest for stream ecologists because of its role in stream biogeochemical processing. Our study addresses the effects of leaf-litter inputs and varying discharge on surface-hyporheic water exchange and their possible consequences for the hyporheic zone biogeochemistry. Our study was conducted during autumn in Riera de Santa Fe (northeastern Iberian Peninsula), a stream with a well developed deciduous riparian canopy. We placed 15 wells spaced at 5-m intervals longitudinally down the study reach and measured surface and hyporheic nutrient and dissolved O-2 (DO) concentrations on 23 sampling dates (15 during the leaffall period and 8 after a flood that washed out 65% of the accumulated leaf biomass). We assessed changes in surface-water exchange and in hyporheic NH4-N and soluble reactive P (SRP) uptake via coinjection of a conservative tracer and nutrients. Compared to surface water, hyporheic water had lower DO, higher SRP and NO3-N concentrations, and similar NH4-N concentration. Hyporheic water had higher DO saturation (p = 0.00) and higher NH4-N concentration (p = 0.00) in downwelling than in upwelling wells, whereas SRP and NO3-N concentrations did not differ significantly between well types (p > 0.05). Hydrologic connectivity was higher in downwelling than in upwelling wells and decreased with leaf-litter accumulation in the stream channel and increased with stream discharge. Increased connectivity after a flood reduced the difference in DO between surface and hyporheic compartments in upwelling and downwelling wells and in NO3-N in upwelling wells. NH4-N and SRP uptake responded differently to these changes. Hyporheic SRP uptake rate was controlled by hyporheic SRP concentration, which did not vary with changes in connectivity, whereas NH4-N uptake rate was indirectly affected by changes in connectivity through the influence of connectivity on DO availability. Last, although no NO3-N was added during the solute injections, we observed an increase in hyporheic NO3-N that probably was caused by nitrification. Together these results illustrate how the combination of stream hydrology and organic matter accumulation can dictate seasonal changes in hyporheic biogeochemistry.