Distribution of solutes, microbes and invertebrates in river sediments along a riffle-pool-riffle sequence

1. Unlike riffles, research has focused rarely on the hyporheic zone of pools. To highlight the functioning of a pool, field investigations were performed in a riffle-pool-riffle sequence by integrating simultaneously physico-chemistry, microbes and invertebrates. The study was conducted in a channel characterised by strong downwelling of surface water. 2. To include the downstream flux of water within the sediment, a longitudinal profile was studied along six stations situated: at the centre (Station 1) and at the lower end (Station 2) of the first riffle, at the upstream part (Station 3), at the centre (Station 4, at the inflection point) and at the lower end (Station 5) of the pool, and at the centre of the second riffle (Station 6). At each station, three replicate samples were taken and three sample depths were investigated (0.2, 0.5, and 1.0 m below the stream bed) on two dates. 3. Physico-chemical parameters (vertical hydraulic gradient, oxygen concentration and specific conductance) differed between stations depending on infiltration rates. In contrast, organic matter and microbial parameters presented patchy distributions linked with factors other than the geomorphological pattern. Despite not very pronounced geomorphologic features, the slope variation at the centre of the pool (at the inflection point) affects the distribution of epigean and hypogean invertebrates. 4. Based upon faunal parameters, the pool could be divided into an upstream and a downstream part, the latter being more strongly influenced by surface water. 5. The pool should be considered as a heterogeneous area. In that respect, the inflection point of a pool may be as important as the top of a riffle in the functioning of river sediments.