This work presents recent data on the physico-chemical properties and the trophic status of Kpassa reservoir at eighteen locations from August 2014 to February 2015. Seventeen physico-chemical parameters were measured and data obtained were statistically analyzed. The descriptive statistics showed their variations (minima-maxima) as follows: pH (4.65 -7.30), temperature (25.1˚C -29.9˚C), dissolved oxygen (1 -5.79 mg/L), oxydability (0.32 -10.88 mg O 2 /L), electrical conductivity (55 -77 μS/cm −1 ), TDS (76 -94 mg/L), turbidity (15.70 -274.40 NTU), transparency (0.24 -1.55 m), suspended matter (3 -92 mg/L), total phosphorus (0.25 -1.90 mg/L), orthophosphate (0.08 -0.61 mg/L), nitrate (undetected 5.50 mg/L), nitrite (undetected 0.79 mg/L), ammonia (undetected 0.36 mg/L), chlorophyll a (7.20 -2334.6 µg/L), silica (4.34 -15.67 mg/L) and N/P ratio (0.08 -42.62). These parameters were mainly influenced by agricultural activities and climatic conditions. The restriction or the removal of the use of chemical fertilizers in agricultural activities in Kpassa reservoir basin was recommended. The highest values of Pearson and Spearman correlations were observed across TDS and conductivity (0.967; 0.951), turbidity and oxydability (0.924; 0.665), turbidity and chlorophyll a (0.884; 0.663), turbidity and suspended matter (0.982; 0.793), suspended matter and transparency (−0.781; −0.819), suspended matter and nitrate (−0.813; −0.839), suspended matter and oxydability (0.919; 0.602) and suspended matter and chlorophyll a (0.879; 0.656). Carlson's trophic state index (TSI) values varied between 36 and 66 showing that Kpassa reservoir was eutrophic in August and mesotrophic to oligotrophic during the other months of the sampling period. ANOVA and Kruskal-Wallis test indicated that there were no significant differences between sampling stations.

L’étude portait sur l’évaluation de la qualité de l’eau du lac Ahémé au moyen des communautés planctoniques. Cette étude a été initiée suite aux travaux d’assainissements du plan d’eau menés par l’Agence pour le Développement intégré de la zone Economique du Lac Ahémé et de ses Chenaux (ADELAC). Les échantillonnages se sont déroulés en juillet 2020 sur quatre points définis sur le lac. L’inventaire et l’évaluation de la biomasse ont été réalisés grâce aux observations microscopiques et clés d’identification des taxons. L’inventaire des espèces a révélé 22 espèces de phytoplancton appartenant à 04 classes et 13 espèces de zooplancton réparties en 03 groupes. Il ressort de l’évaluation de la biomasse phytoplanctonique que les Diatomophycées présentent la plus forte densité (862 individus/L) et les Chlorophycées la plus faible (3 individus/L). Quant au zooplancton, les Copépodes sont plus abondants (118 individus/L) et les Cladocères les moins (3 individus/L). L’indice planctonique (3 – 15.25) a montré que le lac est oligotrophe au niveau des quatre sites échantillonnés. Selon la grille de qualité des eaux, l’oligotrophie traduit une bonne qualité de l’eau du lac. Par comparaison aux études antérieures, la qualité de l’eau s’est améliorée suite à l’assainissement opéré. Cette avancée doit être préservée à travers des actions de gestion durable du lac. The study concerns the evaluation of the water quality of Lake Ahémé by plankton communities and was initiated following the sanitation works carried out by the Agency for the Integrated Development of the Economic Zone of Lake Ahémé and its Channels (ADELAC). The work was done in July 2020 on four defined points on the lake. The inventory and the evaluation of the biomass were carried out thanks to microscopic observations and identification of taxa’s keys. The species inventory revealed 22 phytoplankton species belonging to 04 classes and 13 zooplankton species divided into 03 groups. The evaluation of the phytoplankton biomass shows that Diatomophyceae have the highest density (862 individuals/L) and Chlorophyceae the lowest (3 individuals/L). As for the zooplankton, Copepods are the most abundant (118 individuals/L) and Cladocerans the least (3 individuals/L). The planktonic index (3 - 15.25) showed that the lake is oligotrophic at the four sampled sites. According to the water quality grid, oligotrophy reflects good water quality. Compared to previous studies, the water quality has improved following the sanitation operation. This progress must be preserved through actions to protect the lake.

The study was carried out to assess the physico-chemical parameters, the trophic state and the N:P ratio of the Kpassa reservoir. The reservoir was studied at six sampling sites over the period going from August 2014 to May 2016. Water transparency was determined in situ with the secchi disk SDMO QUINIOU model F043SE. Chlorophyll a, NO 3-, NO 2-, NH 4 + , PO 4 3and total phosphorus were analyzed using the standard methods. Measures were made with the HACH DR2800 spectrophotometer. The trophic state was assessed using the Carlson's trophic state index (TSI). The N:P ratio was evaluated using NO 3-, NO 2-, NH 4 + and PO 4 3concentrations. The physico-chemical parameters investigated were: transparency (0.25-1.92m), chlorophyll a (8.2-233.46µg/L), nitrate (undetected-4.73mg/L), nitrite (undetected-0.23mg/L), ammonia (undetected-0.21mg/L), orthophosphate (0.03-1.04mg/L) and total phosphorus (0.1-3.2mg/L). The trophic state index (TSI) values oscillate between 38 and 59. The majority of the values are characteristic of mesotrophic conditions. Kpassa reservoir is eutrophic only in August 2014 at the whole studied stations. N:P ratio values vary between 0.22 and 60.46 showing that eutrophication in Kpassa reservoir is limited by phosphorus and nitrogen. Some physico-chemical parameters values are upper than the WHO and Ifremer safe limit making the reservoir water unfit for human consumption at the concerned sites and during the concerned period. It is recommended that the national company of Benin water (SONEB) sensitize the population on the rational use of the fertilizers and the adoption of water control technics in the farm. Washing and crockery activities should be also forbidden along the reservoir river. The trophic state of the reservoir seems improved compared to previous studies results. According to the N: P ratio values, measurements to be implemented for eutrophication management in Kpassa reservoir should emphasize on the control of the flow of nitrogen and phosphorus compounds.