Evaluation of the Degradation Level of a Fluvial Basin in the Western Area of Côte d'Ivoire: Case of the Cavally River

Abstract

The present study is a diagnosis of the level of degradation of Cavally River in Cote d'Ivoire. Several abiotic indicators such as physico-chemical parameters were appreciated. These abiotic indicators were supported by biotic indicators sensitive to some variations in quality of water. That is the use of benthic macro-invertebrates as some species of freshwater shrimps of the genus Macrobrachium. Sampling was made between September 2015 and August 2016 on the upper course of Cavally River and the area on either side of the mining zone "Ity". Measures of physico-chemical parameters such as dissolved oxygen, temperature of water, hydrogen potential, electrical conductivity, mercury rate and water transparency were made by season using multi parameters. Shrimps were caught monthly using dip nets and keep nets. The extreme value of physicochemical parameters is for dissolved oxygen value were 4.90 mg/l, temperature of water (25.55°C), hydrogen potential (8.48), electrical conductivity (53.14 μS/cm), mercury rate (5.75.10-3 mg/kg) and water transparency (27.06 cm). In general, the results showed that the extreme values of physico-chemical parameters were recorded in areas with high concentrations of anthropogenic activities. In the mining areas, high levels of mercury have been recorded above those of the WHO guideline for surface water. In total, M. vollenhovenii was the most abundant species with 383 individuals out of 879 specimens caught (43.57%). It is followed by M. dux with 321 individuals (36.51%) and M. macrobrachion with an abundance of 175 individuals (19.90%). Spatially, these species have been more abundant in the upstream and downstream areas which are more or less conserved environments. However, they were less abundant in mining areas. These results show that the upper course of the Cavally River is deteriorating. That constitutes a real threat to aquatic organisms and biodiversity.