Spatial and temporal variation of culturable heterotrophic bacterioplankton and their response to nutrient inputs in hydroelectric reservoirs: Case of Kossou, Taabo and Faé in Côte d’Ivoire

Monitoring programs of culturable heterotrophic bacterioplankton abundances in relation to nutrient concentrations was carried out in Kossou, Taabo and Faé hydroelectric reservoirs in Côte d'Ivoire, three important reservoirs for fishing and recreational activities. The study sought to assess the spatial and temporal culturable heterotrophic bacterioplankton distribution and establishthe relationship between these bacterial abundances and nutrient concentrations. Water samples were collected in 8 campaigns throughout a year on linear transect of each reservoir. Culturable heterotrophic bacterioplankton were enumerated using culture methods. Nutrients such as Total nitrogen and total phosphorus were analyzed. Culturable heterotrophic bacterioplankton concentrations ranged from 1.0 × 10⁵ to 1.3 × 10⁸, 1.1 × 10⁵ to 2.4 × 10⁸ and 1.8 × 10⁵ to 9.2 × 10⁸ CFU.100 mL^-1 in the Kossou, Taabo and Faé dam lakes, respectively. Coastal areas close to urban areas and agricultural lands recorded the highest abundances, mainly during the rainy season. Significant differences were respectively observed with those recorded in the deepest areas and during the dry seasons. Generalized additive models (GAM) showed significant variations in culturable heterotrophic bacterioplankton linked to nutrient concentrations, suggesting spontaneous growth of these bacteria as nutrient levels increase in reservoirs. In addition, the culturable heterotrophic bacterial load detected is clear evidence of ongoing bacterial pollution. To limit anthropogenic activities impacts on these reservoirs, potential risk for public health, it is important to establish safety zones around them and to treat wastewater before discharging it.