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 decomposition and nutrient turnover in many river ecosystems. We determined ecosystem respiration, hyporheic community respiration, and respiration of the epilithic community at the sediment surface in the Necker, a gravel-bed river, during August 1994. Ecosystem respiration, assessed using single-station diel oxygen curves, varied between 4.6 and 7.0 g O₂ m^-2 d^-1. To estimate the respiration of the epilithic community, stones were collected from the surface layer and enclosed in flow-through respiration chambers. The respiration of the epilithic community (mainly algae) ranged from 1.0 to 1.5 g O₂ m^-2 d^-1, contributing 4-19% of the river ecosystem respiration. Two different methods were used for measuring hyporheic community respiration. First, chambers were incubated at 30 cm sediment depth for direct assessment of respiration rates. 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 respiration and respiration of the epilithic community was used for calculating the hyporheic contribution. Hyporheic respiration rates ranged from 3.9 to 5.9 g O_-2 m² d^-1, contributing 76-96% of ecosystem respiration.