Les lagunes ivoiriennes

Abstract

The first evaluation of the acquired knowledge on the brackish water environment of the Ivory Coast is presented here. Situated along the north coast of the Gulf of Guinea, between 2°50′ W and 5°25′ W, the lagoons have a total surface area of 1200 km 2. The climate of the coast area is similar to the equatorial climate. The annual rainfall is about 2000 mm and distributed in two rainy seasons. Each of the three main lagoons has a different hydrological system which depends on the contribution of the continental fresh water and on exchange with the marine environment. In the Ebrié lagoon, the annual contribution of the fresh water is estimated to be four times the volume of the lagoon. The entry of the sea water is 14 times this volume. The temperature of the water averages 29°C and varies little throughout the year. In the Ebrié lagoon for example, there is considerable variation in salinity over the area depending on the seasonal contribution of the continental fresh water and the exchange with the sea water. The degree of salinity also depends on the morphology and the bathymetry of the different parts. Many intermediate situations are noted, from high gradients (seasonal and vertical) to low gradients within stable oligohaline areas (salinity from 3 to 5‰. On the contrary, the Aby lagoon is less influenced by the marine environment. The salinity gradient is more stable, and the water is more oligohaline. In the marine influenced area, the seasonal variation of pH is the result of mixing between sea and fresh water. However, a low pH is noted particularly in the western part of the Ebrié Lagoon. The concentrations of dissolved oxygen in the surface water are generally 4–7 ppm. Its vertical distribution varies greatly during the different seasons and over the different areas. Deoxygenation conditions are often observed, for example in some polluted bays in the Abidjan area, and in the central basin of the Aby lagoon. Generally, the desalinated water is characterized by its poor nutrient contents and particularly the lack of nitrate. This phenomenon probably is related to the rapid recycling of the nutrients. The biomass of the phytoplankton is generally very high and the concentration of chlorophyll a represents more than 35 mg/m 3 in the western part of the Ebrié lagoon. The corresponding primary production is about 300 mg of carbon/m 3 per hour. The zooplankton is represented mostly by the copepod Acartia clausi. The first estimation of the turnover time of the total biomass is between 1.2 and 3.8 days. In the population of the benthos, molluses represent the bulk of the biomass. The importance of some crustaceans—crabs and shrimps—is noted. Among the ichthyologic fauna, there are many types of biological cycles depending on the various types of salinity tolerances and on the conditions of the reproduction cycle. The lagoon stocks are exploited by means of various types of artisanal fishing gear (individual or collective). The estimation of catches for 1977 was about 7000 tons (of which 60% was represented by Ethmalosa), 70% of the total catch is made by purse seines and beach seines. For the three lagoons, the production should probably be 15–20 000 metric tons, despite obvious signs of over-exploitation. Conclusions are oriented towards a discussion of the heterogeneity, the productivity and the fragility of the lagoon ecosystems.