Comparative Ecology of Phytoplankton and Trophic Status in the Estuaries of Bandama (Côte d'Ivoire) and Volta (Ghana)

Abstract

Background: Tropical estuaries are vital ecosystems that, despite being affected by human activities, remain under-researched. Aims: This study evaluated the phytoplankton diversity and trophic pollution status of the Bandama Estuary in Côte d'Ivoire and the Volta Estuary in Ghana, both connected to significant rivers. Methodology: A total of fifteen sampling sites were chosen, with six in the Bandama Estuary and nine in the Volta Estuary. Physico-chemical parameters were measured using a Hanna HI9829 portable multiparameter device, while microalgae were collected using a 20 μm mesh plankton net and a 1.5 L Niskin bottle. Phytoplankton identification and counting were performed with a Nikon Eclipse 0.90 Dry photonic microscope. The collection was carried out from February to July 2023 in the estuaries of the Volta and Bandama. Results: In the Volta Estuary, 30 phytoplankton taxa were identified, primarily from three phyla: 20 Chlorophyta (67%), 7 Bacillariophyta (23%), and 3 Euglenophyta (10%). The Chlorophyceae class was the most abundant, with 2,400,000 cells/L (35%). Conversely, the Bandama Estuary revealed 52 phytoplankton taxa across five phyla, with Cyanophyta dominating at 92%. The Bandama Estuary showed hypoxic dissolved oxygen levels (0.72 mg/L), while the Volta Estuary was well-oxygenated (6.23 mg/L).  The trophic index calculated for both estuaries indicates a eutrophic environment. In contrast, the Volta Estuary shows a score of 15, indicating moderate organic pollution, providing a foundation for future biomonitoring of anthropogenic pollution in the region. Conclusion: This research highlights the ecological significance of these estuaries and the need for ongoing studies to understand their health and resilience.