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 poo ls. To highlight the functioning of a pool, field investigations were perfo rmed in a riffle-pool-riffle sequence by integrating simultaneously physico -chemistry, microbes and invertebrates. The study was conducted in a channe l characterised by strong downwelling of surface water. 2. To include the downstream flux of water within the sediment, a longitudi nal 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 p art (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 riff le (Station 6). At each station, three replicate samples were taken and thr ee sample depths were investigated (0.2, 0.5, and 1.0 m below the stream be d) on two dates. 3. Physico-chemical parameters (vertical hydraulic gradient, oxygen concent ration and specific conductance) differed between stations depending on inf iltration rates. In contrast, organic matter and microbial parameters prese nted patchy distributions linked with factors other than the geomorphologic al pattern. Despite not very pronounced geomorphologic features, the slope variation at the centre of the pool (at the inflection point) affects the d istribution 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 surfac e 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 i n the functioning of river sediments.