A review of hydrodynamics and water quality of Lake Nokoué: Current state of knowledge and prospects for further research

Abstract

A systematic literature review was undertaken to identify the significant changes that have affected the hydrodynamics and the water quality of Lake Nokoué (Benin, West Africa) since 1960 and to emphasise the relationship between both. It appeared that the construction of the port of Cotonou in early 1950 permanently opened Lake Nokoué to the Atlantic Ocean and subsequently its retention time has considerably decreased from more than one year to about 40 days. During low water periods from December to mid-May, the hydrodynamics of the lake is influenced by tidal variation. This leads to a significant increase in the salinity of the lake from freshwater to brackish water with an average salinity of 16 ppt. During high water periods, freshwater inflows from tributaries invade the lake and keep the sea water out. Inorganic nutrient concentrations in the lake increase from the low water to the high water period, partially as a result of inputs from the lake tributaries. Yet, to date, the other external sources of nutrient loads in the lake, as well as the internal nutrient loads of the lake, have not been quantified. Therefore an important management question is whether the control of external nutrient loads alone is sufficient to significantly reduce the eutrophication problems of Lake Nokoué. This implies that further research on the lake could investigate the components of the nutrient budget including the influence of seasonal water hyacinth death during low water periods (caused by increased salinity), on the internal nutrient loads. Urgent management actions should consider a continuous spatio-temporal monitoring of the system in order to generate long time series data which will help to model the relationship between hydrodynamics and water quality and to understand the linkage of both with the biodiversity occurring in Lake Nokoué and more generally in tropical coastal lakes.