Diversity and spatio-seasonal dynamics of micro-algae in the comoe river (Côte d’Ivoire)

Water remains the most essential natural resource for life. In Côte d'Ivoire, the catchment areas of rivers are the environments most exploited by human populations. These practices have consequences for the watercourses, which also receive various types of waste through run-off water. This study was initiated between February 2020 and January 2021, with a view to assessing the diversity and dynamics of micro-algae in the Comoé River. The composition, diversity, occurrence and spatio-seasonal abundances of micro-algae were studied. Physico-chemical variables were measured in situ and in the laboratory. Microalgae were collected using a plankton net and observed under optical and electron microscopes for identification. A total of 190 taxa were inventoried, divided into 79 genera, 39 families, 23 orders, 10 classes and 5 phyla. The most diverse phyla were Euglenophyta with 59 taxa, Chlorophyta with 55 taxa and Heterokontophyta with 51 taxa. The study revealed a positive upstream-downstream gradient in specific richness and diversity. According to their frequency of occurrence, there were 32 constant taxa, 72 accessory taxa and 86 accidental taxa. We also noted an increase in micro-algal abundance from the upper to the lower reaches of the river. High algal abundances were obtained in the dry seasons at most stations. The lowest abundances were observed at the Kafolo station and the highest at Kokonou and Grand-Bassam.