Vertical Dynamics of Culturable Heterotrophic Bacteria Concentrations in the Water Column of Hydroelectric Reservoirs: A Case Study of Kossou, Taabo and Faé in Côte d’Ivoire

Background and Aims: Bacteria play a critical role in trophic chains, particularly by decomposing organic matter into minerals that are required for phytoplankton. They are also actively devoured by zooplankton, allowing carbon to flow through the trophic chain. Several studies have examined the distribution of bacteria in the water column of marine environments, whereas far fewer investigations have focused on freshwater systems. This study aimed to characterize the distribution of heterotrophic bacteria in the water column of the Kossou, Taabo, and Faé hydroelectric reservoirs in Côte d’Ivoire. Methodology: Water samples were collected at each sampling site over a one-year period during eight field campaigns, in 1 L bottles at the surface, at 1 m depth, and at 2 m depth using hydraulic bottles. During January (dry season) and July (rainy season) campaigns, samples were additionally collected every four (4) hours to study the nycthemeral dynamics of bacterial abundance. Heterotrophic bacteria were analyzed using culture methods. Results: Heterotrophic bacteria concentrations were high at the water surface, at depths of 1 meter and 2 meters in April, June, July and September, which correspond to the rainy seasons. The bacterial nycthemeral distribution in the water column remained relatively similar for both the dry and rainy seasons. The only difference was that during the dry season at 18 hours, heterotrophic bacteria concentrations remained higher at the water surface and at a depth of 1 meter, whereas during the rainy season at 18 hours, bacterial concentrations generally remained low throughout the water column. Weak correlations were observed between bacteria and environmental variables, suggesting the involvement of other factors. Conclusion: Thus, heterotrophic bacteria concentrations in the water column of the hydroelectric reservoirs studied are higher during the rainy season, and their nycthemeral distribution varies. However, this nycthemeral variation remains relatively similar for the dry and rainy seasons.