For decades, the Ébrié Lagoon in Côte d'Ivoire has been the receptacle of wastewater effluent and household waste transported by runoff water. This work assesses the spatio-temporal variability of the Ébrié lagoon water quality at the city of Abidjan. The methodological approach used in this study is summarized in three stages: the choice and standardization of the parameters for assessing water quality for uses such as aquaculture, irrigation, livestock watering, and sports and recreation; the

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... weighting of these parameters using the Hierarchical Analysis Process (AHP) of Saaty; and finally, the aggregation of the weighted parameters or factors. Physicochemical and microbiological analysis data on the waters of the Ébrié lagoon for June and December of 2014 and 2015 were provided by the Ivorian Center for Anti-Pollution (Centre Ivoirien Anti-Pollution, CIAPOL), and the concentrations of trace elements in sediments (As, Cd, Cr, Pb, Zn) were used. The aggregation of standardized and weighted parameters allowed the calculation of the Water Quality Indices (WQI) by usage for each bays of the lagoon. The results show that in both 2014 and 2015, the waters of the Ébrié lagoon were generally of poor quality for the different uses examined in this study (aquaculture, irrigation, livestock watering, and sport and recreation) with an accentuation in 2015. However, some bays of the lagoon have waters of dubious to satisfactory quality. This study contributes an improved evaluation of the Ébrié lagoon waters. Hydrology 2018, 5, 32 2 of 12 development of the city of Abidjan [1], there has been increasing concern about this damage. Indeed, Côte d'Ivoire's industrial development has been heavily concentrated in the metropolitan area of Abidjan. In 2010, the National Institute of Statistics (Institut national de la statistique, INS) determined that 92.8% of the 2822 industrial establishments (agro-food, energy, chemical, textile, soap factory, an oil refinery, and many informal industries) in the country are located in the only economic capital of the country (Abidjan), and 60% of these establishments are installed around the Ébrié lagoon. The Ébrié lagoon has several bays in which untreated or insufficiently treated wastewater effluents (domestic, industrial wastewater, and so forth) or solid waste have been discharged for decades [2] . The biodegradable waste causes an intense eutrophication phenomena, especially in the low renewal areas such as bays [3] . To determine the quality of Ébrié lagoon water, many studies have been conducted since the 1980s [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] . Namely, Koné and Aka [18] showed that all the industries of the Abidjan agglomeration dumped their wastewater in the lagoon, with a total volume estimated at 12,000 m 3 per day. These inputs are likely to contain metallic trace elements (ETM) such as lead (Pb), chromium (Cr), zinc (Zn), arsenic (As), and cadmium (Cd) [16]. Kouamé et al. [19] observed a very large bioaccumulation of Cr and Pb in crab organisms (used as indicators of metal pollution) collected in the Ébrié lagoon. These studies were focused on the assessment of biological and microbiological quality, pollution by oil, metal sediment contamination, and solid waste. Additionally, the Ivorian Center of Anti-Pollution (Centre Ivoirien d'Anti-Pollution, CIAPOL) systematically and periodically analyzes the natural waters from its National Observation Network (Réseau National d'Observation, RNO) as part of its mission. Thus, the water quality of the Ébrié lagoon is periodically analyzed in situ and in a laboratory, as part of the monitoring of this water body. Based on this important existing knowledge base on the Ébrié Lagoon, it is now necessary to introduce a Water Quality Indices (WQI) approach, which summarizes large quantities of data on water quality in to simple terms of water quality (Excellent, Good, Bad, and so forth), in order to supply synthetic and comprehensive information to the decision makers and the general public. WQIs minimize the volume of data and simplify the expression of water quality status based on a number of physicochemical and bacteriological parameters [20] . The first attempts to classify water by degree of purity date back to the mid-twentieth century [21] . Horton's pioneering efforts have been followed by number of researchers; various WQIs have been formulated and used by water supply and water pollution control agencies around the world [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] . The Ébrié Lagoon has always been the particularity of Abidjan. Thus, the potential of this underutilized water body is a major challenge for the development of the city if it wants to recover its nickname "Abidjan, the pearl of lagoons". To that end, it is necessary to develop indicators for decision-makers on the quality of these waters that are simplified and easily understood to support valuation of the lagoon water body. Thus, the main objective of this study is to perform a spatio-temporal assessment of the Ébrié lagoon water quality for uses such as aquaculture, irrigation, livestock watering, and sport and recreation using a multicriteria analysis method.