Seasonal Variation of the Physicochemical Quality of Water and the Structure of Benthic Macroinvertebrate Communities in the Middle Reach of the Chari River in Chad (Sudano-Saharan Zone-Central Africa)

Abstract

Physicochemical analyses coupled with the study of benthic macroinvertebrate community structures constitute an excellent tool for assessing the ecological health status of lotic environments. However, very few studies have addressed the ecological characteristics and functioning of the Chari River, and the influence of anthropogenic pressures and seasonal variability on this group of bioindicators remains poorly understood. This study aimed to assess the seasonal trend of the physicochemical water quality and the benthic macroinvertebrate community structure of the middle course of the Chari River in Chad. Twelve sampling stations were selected along the middle reach of the Chari River. Data were collected monthly during the two climatic seasons, namely in November, February, March, and April during the dry season, and in May, August, September, and October during the rainy season. Water physicochemical parameters were measured both in situ and in the laboratory following standard methods, whereas benthic macroinvertebrates were collected following the multi-habitat approach, using a long-handled kick net. Water quality indices showed that waters of the middle reach of the Chari River fall within the good-to-moderate quality range, with moderate and mostly biodegradable organic pollution and high nitrogen contamination linked to anthropogenic inputs. Concerning macrofauna, a total number of 14,871 individuals, distributed in 3 phyla, 7 classes, 17 orders, 82 families, and 136 genera, were identified. The orders Hemiptera, Coleoptera, and Gastropoda were the most abundant during the rainy season, while Ephemeroptera, Unionoida, and Odonata were dominant during the dry season. The analysis of interactions between abiotic and biotic factors revealed a seasonal structuring of the benthic macrofauna of the middle Chari along an ecological quality gradient. This gradient opposed well-oxygenated environments characterized by relative hydrological stability—associated with high taxonomic richness and high community diversity in the dry season—to environments enriched with nitrogenous compounds and organic matter of anthropogenic origin during the rainy season, where pollution-tolerant taxa such as Chironomidae dominate. Overall, these results show that the middle course of the Chari River remains moderately impacted by anthropogenic pressures, with fairly good water quality. These results highlight the importance of establishing a regular monitoring program, as well as controlling diffuse pollution sources in the Chari catchment, to prevent increasing degradation of this aquatic ecosystem.