Trophic status assessment of the Napié reservoir (Côte d'Ivoire): physico-chemical analysis and chlorophyll-a biomass

The overall objective of this study is to identify the physico-chemical properties influencing seasonal variations in chlorophyll-a levels and to assess the trophic status of the Napié reservoir in northern Côte d'Ivoire. The physico-chemical properties were determined in situ using a multiparameter instrument (HANNA 928) between 6 a.m. and 7:30 a.m. At four stations, water samples were collected monthly during the dry season (January, March, and April 2024) and the rainy season (June, July, and October 2024). Nutrient and chlorophyll-a levels were also analyzed. The results show that water is moderately warm, with moderately basic pH values and high nutrient levels during the dry season. Our hierarchical dendrogram shows three distinct groups of stations based on physico-chemical properties. Chlorophyll-a levels varied seasonally, with the highest levels recorded during dry seasons (81.9–275.33 μg/L) and the lowest during rainy seasons (40.9–86.20 μg/L). Regression analysis revealed both positive and negative associations between chlorophyll-a biomass and various physico-chemical properties. The trophic assessment, based on chlorophyll-a levels and the Organization for Economic Cooperation and Development classification system, indicates that the Napié reservoir is hypereutrophic, raising concerns about water quality and highlighting the need for sustainable management of this aquatic ecosystem.