Does spatial heterogeneity of hyporheic fauna vary similarly with natural and artificial changes in braided river width?

Heterogeneity of hyporheic fauna is associated with geomorphological features and related vertical water exchanges. Constrictions on river floodplain are known to induce groundwater inputs and increase stygobite fauna. Two floodplain constrictions were studied in a large braided river (the Drôme River): one linked to a natural process (valley narrowing), another to an artificial river regulation (early 20th embankment). Spatial distribution of hyporheic organisms were sampled upstream and downstream of the two constrained sections, at 9 stations, 3 positions (left and right sides, centre of the braided strip), 3 replication points and at a depth of 50 cm in the river sediment. The spatial heterogeneity in community composition was higher near the banks than at the centre of the braided strip, no matter the width of the strip. The artificial constriction induced a decrease in spatial heterogeneity of the benthic fraction of the hyporheic fauna, but no changes were detected for the stygofauna. The natural valley narrowing reduced width and thickness of the alluvium and induced an inflow of groundwater resulting in an increase in stygofauna abundance. Natural floodplain narrowing linked to geology thus control the distribution of stygobite species, while artificial constrictions only modify the spatial distribution of the benthic fraction of the hyporheic fauna.