Foraminiferal assemblage as environmental indicators: A case study from Ebrie (Cöte d'Ivoire) and Densu (Ghana) Estuaries, West Africa.

In order to assess and monitor the health of West African ecosystems, a vast biomarker-based BEACON program has been initiated. This study presents preliminary and partial results comparing two sectors of the Ebrié (Jacqueville bridge) and Densu (Gahna) lagoons. It is based on the analysis of benthic foraminifera from thirty-three (33) samples of surface sediments from these lagoon beds, including fifteen (15) from Ghana and eighteen (18) from Côte d'Ivoire. On the whole, the foraminifera identified fall into ten genera left in open nomenclature. These are the genera Ammotium, Ammobaculites, Ammonia, Amphistegina, Quinqueloculina, Cribloelphidium, Nonion, Miliammina, with a few rare planktonic individuals in the genera Globigerina, Globorotalia (cultrata). A qualitative analysis of these benthic individuals was carried out. The genera Amphistegina, Ammonia, Cribloelphidium and Nonion were found to have calcareous haline tests coiled in a planispiral or trochospiral mode. We also find the Quinqueloculina genus, with a porcelain test in an elongated and milioline mode. The living conditions of these foraminifera are closely linked to the existence of an aerated environment favoring the permanent renewal of oxygen in the bottom sediments. They are abundant in the Densu lagoon and in very low proportions in the Ebrié lagoon. In contrast, benthic forms with elongated, agglutinated testes, such as Ammotium, Ammobaculites and Miliammina, characterize poorly oxygenated waters, i.e. oxic to anoxic or eutrophic waters. They are very well represented in the Ebrié lagoon and rare in the Densu lagoon. From the above, this distribution of benthic foraminifera shows that the Ebrié lagoon (Jacqueville bridge) is very confined and disoxic compared with the Densu lagoon.  The presence of planktonic foraminifera such as the genera Globigerina, Globorotalia suggests a marine influence in both lagoons.

Keywords : Ebrié lagoon, Densu lagoon, foraminifera, biomarkers, anoxia