Changes in the vertical distribution of hyporheic and benthic fauna associated with low flow conditions in the headwaters of the Tafna river (northwest Algeria)

Abstract

This study investigated temporal changes in the vertical distribution of invertebrates during periods of low flows in the Tafna wadi, a semi-arid Mediterranean river characterised by a general decline in average flows. The hyporheos was sampled along a 130-m stretch at depths of 0.3, 0.6 and 1 m in a pool-riffle sequence and a lateral gravel bar. Benthic fauna was collected on each sampling occasion from different locations in the studied sections. Sampling was conducted in two consecutive years (2014 and 2015) at the end of the spring (high waters) and over a 2-month period during the low summer flow. The results indicated that high surface water temperatures may initiate the movement of some taxa into the interstitial habitat, thus supporting the hyporheic refuge hypothesis. In the hyporheic zone, the low oxygen rates recorded deeper in the sediment and in the last low-flow months caused significant declines in taxonomic richness, and lower pH values resulted in a reduction in invertebrate abundances. In the benthic zone, high flow velocities increased richness but decreased invertebrate abundances. A large number of taxa were found in the gravel bar section reflecting the more suitable living conditions in this habitat compared with the river reach (high DO rates).