The effects of an urban effluent from the city of Taourirt on macroinvertebrate community structure in the Za wadi, (Eastern Morocco)
The present work fits in the framework of a survey of the impact of domestic discharges from the city of Taourirt on the physicochemical and biological quality of the running waters of the Za wadi, which is a permanent tributary of the Moulouya River. The domestic releases do not receive any treatment before their discharge. The study was carried out at three stations: a reference station upstream from the discharge point and the two downstream stations. The quantitative collection of the macroinvertebrates was performed with a Surber net (0.4 mm aperture mesh) that allowed sampling of a reduced surface area of 0.025 m2. The inventory of the benthic fauna revealed a taxonomic diversity of 47 families and 64 taxa harvested in the quantitative samples. The spatiotemporal monitoring of several physicochemical and biological variables provided evidence of a relatively intense pollution. This pollution resulted in an important inorganic and organic load to the wadi downstream from the discharge point. This load decreased downstream as a result of the degradation of part of the organic load, due to the action of heterotrophic micro-organisms, and as a result of sedimentation, dilution and natural self-purification phenomena. Use of various metrics and biotic indices (e.g., Biological Monitoring Working Party (BMWP) score, Average Score Per Taxa (ASPT), Shannon-Wiever Diversity Index (H), Biotic Index (IB), taxonomic richness and equitability) differentiated the reference site (Bridge of Taourirt) from the slightly affected (Ouled Lefkir) and deteriorated (Principal Discharge) sites. These biotic indices decrease with increased degradation of water quality. On the other hand, the Simpson index (D) and the Modified Hilsenhoff Biotic Index (mHBI) increase in poor water quality. The organization of the communities at the different sampling sites appeared to be closely linked to the environmental conditions: hydrological mode, chemical characteristics and riparian vegetation controlled the taxonomic richness and abundance of the macroinvertebrates.