Sedimentary evolution and ecosystem change in Ahémé lake, south-west Benin

Abstract

Tropical moist ecosystems, such as Ahémé lake, south-west Benin, are increasingly marked by water degradation, linked with the activities of increasing riparian populations. The objective of this study is to analyze sedimentary dynamics and its influence on the changing ecosystem of Ahémé lake from 1961–2010. Data used to carry out the study are records of precipitation, flows, turbidity, suspended sediment, mineral elements and bathymetry. Grain size data from the sieving of sediment samples were used to interpret suspended solids distribution in the lake. Linear correlation coefficients were used to assess the degree of dependence between rainfall and runoff inputs to the lake. Lake depth measurements in some areas of the lake serve to determine the rate of infilling. The sorting index was used to highlight the distribution and origin of sediments in the lake. The results show a degradation of the lake Ahémé ecosystem characterized by infilling of its bed, a high correlation (r = 0.90) between rainfall and runoff, seasonal change in physicochemical parameters (total suspended sediment decrease by −91 %) and decrease in fish production by 135.8 t yr−1. The highest mean suspended sediment concentrations in lake inputs occur during high water periods (123 mg L−1) compared to low water periods (11.2 mg L−1).