Responses of the macroinvertebrate community to urban wastewater pollution in the upper Ouémé Basin in Benin

Abstract

In Benin, most of urban wastewaters are discharged into rivers without any prior treatment. The objective of this study was to assess the effects of urban wastewater on the macroinvertebrate communities of the upper Ouémé River in Benin. To address this question, 30 stations located on five rivers were monitored in the dry and the wet seasons. For each station and each season, 12 samples of macroinvertebrates following standardized French multi-habitat sampling protocol were collected and physico-chemical parameters were recorded. Three types of stations were chosen on each river: two control stations located upstream of the wastewater discharge points, two stations impacted in the urban area and receiving urban wastewaters, and two stations downstream of the wastewater discharge points to measure the resilience of a set of river characteristics. Urban wastewater impacted the water quality by mainly increasing electrical conductivity and the nutrient concentrations. Wastewaters also deeply impacted the diversity and the composition of the invertebrate community. The Indval index highlighted three indicator taxa for the control stations (Caenidae, Baetidae and Ephemerellidae), one for the impacted stations (Chironomidae), and two for the downstream stations (Libellulidae and Lestidae). We also observed ecosystem resilience a few hundred meters downstream of the discharge points. These results challenge managers on the degradation of river water quality in the upper Ouémé River, but also reveal good self-purification capacities of the watercourses likely to promote the resilience of these ecosystems.