Aim To test the hypothesis that macroscale environmental variables explain local taxonomic richness of stream macroinvertebrates, and then to identify the relationships between these variables and benthic fauna richness in North Africa.Location North Africa, from West Morocco to East Tunisia.Methods Using a large-scale data base made of 211 sampling sites gathered from an area of 500,000 km 2 , an artificial neural network model has been built to identify and predict the influence of macroscale environmental variables on local macroinvertebrate richness. ResultsThe correlation coefficient (r) between observed and estimated taxon richness values was 0.75 (P < 0.001), and the model explained more than 55% (r 2 ¼ 0.563) of the macroinvertebrate richness variation. Macroinvertebrate richness was, therefore, accurately predicted using only three environmental variables accounting for hydrology (number of rainy days), geographical factors, i.e. connections between European and North African land masses (longitude) and climatic gradient (altitude).Main conclusions Stream macroinvertebrate richness in North Africa results from a combination of climatic, geographical and hydrological parameters. Although consistent with current biogeographic and ecological concepts mainly derived from European and North American streams, this study underlines the specificity of dry Mediterranean ecosystems. The shape of diversity patterns results from climatic conditions and their associated environmental seasonal dynamics, which screens geographical processes.

The mid Tafna wadi (N-W Algeria) is exposed to several man induced disturbances including urban, industrial, and agricultural effluents, and alteration of the natural flow regime. The recent installation of a reservoir upstream of the study area has strongly modified the flow regime by extending the period of low water level, and this stream section is only occasionally fed by water releases. Investigations on the hyporheos and benthos of this stream have allowed the comparison of densities, taxa richness and diversity, during three hydrological periods (natural high water, low flow and reservoir water releases), with the aim to highlight effects of the reservoir management on downstream hydrosystems. Species richness and abundance of stygobite crustaceans were low in the hyporheos. Surface water Crustaceans represented 53% of the overall fauna, with Cyclopoidae composing 50% of the Crustaceans. The dominance of the Cyclopoidae was also observed in the hyporheos. Daphniidae were more abundant (37%) in the benthic habitat than in the hyporheic zone. The alternation of high and low discharges appeared to affect the hyporheos more than the benthic communities. The stability concept of the hyporheic habitat does not fit with our results showing a great range of spatial (mosaic habitat) and temporal changes according to the flow regime.

Nitrogen retention was measured along the Tafna wadi downstream of a heavily polluted reservoir in North-West Algeria to understand the role of the hyporheic zone (HZ) in nitrogen dynamics. Nutrient concentrations were measured monthly for 2 years within the bed sediments of a 300 m reach located 20 km downstream from the dam. Due to strong hydrological fluctuations hyporheic water was analysed during natural low and high water (HW) periods, and during water reservoir releases. Nutrient concentrations in surface water (SW) increased during water releases and in the HZ during the low water (LW) periods. Surface/hyporheic water interactions were characterized by determining the vertical hydraulic gradient (VHG) and the chemical signature of the ground water (GW). The latter was obtained from regional GW monitoring. Hyporheic chemistry was strongly influenced by patterns of surface flow. Hyporheic and SWs had similar chloride concentrations during high flow when they were significantly lower than those of the regional GW. GW was generally richer in nitrates and nitrites, but was lower in ammonium concentrations than interstitial and river waters. Nitrates decreased significantly from upstream to downstream within the HZ throughout the hydrological period even though temporal fluctuations were high. Ammonium concentrations in interstitial water (IW) were significantly higher than in SW and generally increased from upstream to downstream. This study demonstrates the importance of the HZ in altering the dissolved inorganic nitrogen composition and concentrations of heavily polluted arid streams. The study is of interest because it documents a large 'natural experiment' that being the effect of periodic water release from a reservoir with serious water quality problems on the water quality dynamics (particularly nitrogen) of subsurface and SWs downstream.