7 - Community respiration in the hyporheic zone of a riffle-pool sequence

Summary

ABSTRACT A portable incubation device was used to study community respiration activity in hyporheic sediments of a mountain stream. Substantial heterotrophic activity was measured in these sediments, which contribute significantly to total organic matter processing in stream ecosystems. Hyporheic community respiration (HCR), which was mainly attributable to microbial metabolism, decreased with sediment depth in a riffle, with a minimum value at 20 cm. At all depths, HCR activity, loosely-associated particulate organic matter (LAPOM), and the LAPOM protein content were all much lower in the pool sediments than in the riffle sediments. This implies that the hyporheic zone investigated here was probably not supplied with significant amounts of nutrients from downwelling water in the pool, as implied by current concepts of the hydraulics in riffle-pool sequences.

INTRODUCTION

In running waters, organic matter (OM) is subjected to biological transformation as well as physical transportation due to the flowing water. In streams with a moderate or high gradient, the residence time of water is short and the biological activity in the open water is low (e. g. Meyer et al., 1990). However, the sediment interstices act as effective retention sites for particulate organic matter (POM) (e. g. Schwoerbel, 1961; Metzler & Smock, 1990; Leichtfried, 1991). Thus most biological activity in streams is associated with the sediments (Naimanef et al., 1987).

In many cases, the stream channel cuts into an alluvial floodplain, and surface waters are in contact with phreatic groundwater. In the transition zone between superficial sediments and deeper layers of alluvium, which is known as the hyporheic zone, characteristics of both habitats occur (Schwoerbel, 1961).