The metabolism of organic matter in the hyporheic zone of a mountain stream, and its spatial distribution

Community respiration in hyporheic sediments (HCR) was studied in a characteristic riffle-pool-sequence of a mountain stream. HCR activity at the riffle site strongly exceeded that at the corresponding pool site with a mean ratio of 5.3. The vertical distribution of HCR activity was homogeneous in the pool, while there was a distinct maximum in the uppermost layer in the riffle. Similarly, the spatial distribution of certain fractions of particulate organic matter (POM), and their turnover, was largely determined by stream morphology. Mean annual HCR per unit area of stream bed was estimated as 1.71 g O₂ m⁻² d⁻¹. Hence, HCR contributes significantly to total heterotrophic activity in streams, thus enhancing the relative importance of heterotrophic processes in running waters containing hyporheic zones.