Abstract
The supply of drinking water is a vital challenge for the people who live on the African continent, as this continent is experiencing strong demographic growth and therefore increasing water demands. To meet these needs, surface water resources are becoming increasingly mobilized because underground resources are not always available or have already been overexploited. This situation is the case in the region of Abidjan in the Ivory Coast, where the drinking water deficit is a growing problem and it is therefore necessary to mobilize new water resources to ensure the supply of drinking water. Among the potential resources, local managers have identified a freshwater lagoon, Lagoon Aghien, That is in close proximity to the city of Abidjan. With the aim of enhancing knowledge on the ecological functioning of the lagoon and contributing to the assessment of its ability to provide drinking water, several physical and chemical parameters of the water and the phytoplankton community of the lagoon were monitored for 17 months (December 2016-April 2018) at six sampling stations. Our findings show that the lagoon is eutrophic, as evidenced by the high concentrations of total phosphorus (>140 μg L-1), nitrogen (1.36 mg L-1) and average chlorophyll-a (26 to 167 μg L-1) concentrations. The phytoplankton community in the lagoon is dominated by genera typical of eutrophic environments including mixotrophic genera such as Peridinium and by cyanobacteria such as Cylindrospermopsis/Raphidiopsis, Microcystis and Dolichospermum that can potentially produce cyanotoxins. The two rainfall peaks that occur in June and October appeared to be major events in terms of nutrient flows entering the lagoon, and the dynamics of these flows are complex. Significant differences were also found in the nutrient concentrations and to a lesser extent in the phytoplankton communities among the different stations, especially during the rainfall peaks. Overall, these results reveal that the quality of the lagoon’s water is already severely degraded, and this degradation could increase in future years due to increasing urbanization in the watershed. These results therefore raise questions about the potential use of the lagoon as a source of drinking water if measures are not taken very quickly to protect this lagoon from increasing eutrophication and other pollution sources.
Highlights
The African continent is experiencing a rapid growth in human population [1], and the freshwater ecosystems of this continent are subjected to growing anthropogenic pressures that could potentially impact their water quality
This freshwater lagoon is located in the northeastern part of the Abidjan district (Fig 1) and has until recently been used for multiple purposes by the local population living on its banks
As expected under tropical climate conditions, strong rainfall variations were found during the monitoring period (Table 1), and the two rainfall peaks occurred in June and October 2017
Summary
The African continent is experiencing a rapid growth in human population [1], and the freshwater ecosystems of this continent are subjected to growing anthropogenic pressures that could potentially impact their water quality. Except for a restricted number of large lakes mainly located in East Africa (Lake Victoria, Lake Kivu, etc.) [e.g. 2, 3], there are very limited data available on the water quality and ecological status of most lentic ecosystems in tropical Africa. From these data, it seems that growing eutrophication of these ecosystems [e.g. 4–7] and increasing pollution by pharmaceutical products [8] and other pollutants, such as organochlorine pesticides [9] and heavy metals [10], are occurring. This freshwater lagoon is located in the northeastern part of the Abidjan district (Fig 1) and has until recently been used for multiple purposes by the local population living on its banks
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