CONTRIBUTION OF THE HYPORHEIC ZONE TO ECOSYSTEM METABOLISM IN A PREALPINE GRAVEL-BED RIVER

The hyporheic zone is assumed to be an important site for decompositio n and nutrient turnover in many river ecosystems. We determined ecosys tem respiration hyporheic community respiration, and respiration of th e epilithic community at the sediment surface in the Necker, a gravel- bed river, during August 1994. Ecosystem respiration, assessed using s ingle-station diel oxygen curves, varied between 4.6 and 7.0 g O-2 m(- 2) d(-1). To estimate the respiration of the epilithic community, ston es were collected from the surface layer and enclosed in flow-through respiration chambers. The respiration of the epilithic community (main ly algae) ranged from 1.0 to 1.5 g O-2 m(-2) d(-1), contributing 4-19% of the river ecosystem respiration. Two different methods were used f or measuring hyporheic community respiration First, chambers were incu bated at 30 cm sediment depth for direct assessment of respiration rat es. Respiration rates of the hyporheic sediment were measured using a closed chamber system and extrapolated for comparison over 0.5 m and 1 m sediment depth. Second, the difference between whole-ecosystem resp iration and respiration of the epilithic community was used for calcul ating the hyporheic contribution. Hyporheic respiration rates ranged f rom 3.9 to 5.9 g O-2 m(-2) d(-1), contributing 76-96% of ecosystem res piration.