Physico-chemical characterization of the Niger river in Niamey and its impacts on Mormyridae biodiversity
Résumé
Abstract Freshwater ecosystems in tropical regions face escalating threats from climate variability and anthropogenic activities, yet their ecological responses remain understudied. This study examines the seasonal and the spatial dynamics of key physico-chemical parameters in the Niger River (Niamey, Niger) during the rainy season (August–September) and post-rainy transition (October–November), and their cascading effects on Mormyridae family a bioindicator family group reliant on stable physico-chemical conditions such as conductivity, temperature, and turbidity for effective electrosensory functioning. Field data were collected during the rainy season (August-September) and the post-rainy transition (October–November) of 2022 across three stations: Tondibia, Barrage Yantala, and Gamkalley. Key parameters such as dissolved oxygen (DO), transparency, conductivity, pH, and temperature were analyzed alongside Mormyridae biodiversity indices (species richness, abundance, Shannon diversity). Results revealed pronounced seasonal shifts: high turbidity (transparency = 4.0 ± 1.0 cm) and low DO (5.06 mg l −1 ) during the rainy season likely impaired electrosensory function and reduced species presence (Brienomyrus Niger absent in August). Post-rainy improvements in transparency and DO (transparency = 8.33 ± 4.51 cm; DO = 7.29 mg l −1 ) facilitated biodiversity rebounds, with Pollimyrus isidori abundance peaking at 127 in November. Urban-influenced sites exhibited persistent degradation throughout the study period, with elevated conductivity (38.33 ± 2.89 µ S cm −1 ) correlating with suppressed biodiversity ( Mormyrops anguiloides : 51 individuals total). Barrage Yantala demonstrated intermediate conditions, serving as refuge for species like Marcusenius cyprinoides (65 individuals in November). Overall, transparency showed weak positive correlations with Shannon H′ ( r = 0.34, p > 0.05) and Margalef ( r = 0.39), while DO exhibited moderate negative correlations with Simpson and Shannon indices ( r = —0.49 and—0.44, respectively). Conductivity showed a significant positive correlation with Margalef richness ( r = 0.61, p < 0.05). The study underscores the vulnerability of electrosensory fish to anthropogenic pressures and highlights the dual role of seasonality and human activities in shaping tropical river ecosystems. Immediate conservation measures such as riparian buffer zones, pollution mitigation, and habitat restoration are urged to safeguard Niger River biodiversity.
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