The Affon River is located on the right bank and in the classified forest of upper Ouémé. This river is one of the tributaries of the Ouémé River. It has a length of 152 km and a catchment area of 4320 km². The river is located in the Sudano-Guinean zone and is under the influence of the tropical climate characterized by the succession in the year of a single rainy season from April to October and a single dry season from November to March, marked by the preponderance of the harmattan. On the Affon River, 08 stations were selected after prospecting. They were chosen based on the sustainability of the water, altitude, accessibility in all seasons, depth and speed of the water [14] . Table 1 shows the characteristics of these different stations.

At each station, the measurements of physical parameters (temperature, depth,

transparency, TDS, conductivity, pH, oxygen) were carried out in situ very early in the morning between 08:00 and 12:00, before the sampling of the macroinvertebrates to avoid any disturbance of the environment. The water temperature, TDS and conductivity were determined using a portable conductivity meter (HANNA HI 99300). The pH was measured with a portable pH meter (HANNA HI 98107). A Secchi disk with a graduated rope was used to measure the transparency of the water and the depth of the stations. Water samples were made at each station in 500 ml plastic bottles decontaminated and stored in a cooler containing ice for transport to the laboratory for analysis of the dissolved substances. The determination of the concentration of dissolved elements of the water samples was carried out in the Laboratory of Hygiene, Sanitation, Ecotoxicology and Environmental Health (HECOTES) using a spectrophotometer DR 6000. Chemical parameters such as ammonium, nitrite and phosphate were respectively measured by the Nessler method [15] , the iron sulphate method and the Vanadomolybdic method with persulfate digestion.

The benthic macroinvertebrates were sampled at the 08 stations. They were taken using a Surber sampler with a 500-µm mesh. This material was placed on the bottom of the river, the opening of the net facing watercourse and the substrate is scraped a few centimeters with the hand. Twelve samples with a unit area of 1/20 m² were done per station: (08) eight on the dominant habitats and (04) on the marginal habitats as recommended by the standard IBGN and already used in the North of Benin by Abahi et al. (2018) [10] . The organisms collected in the surber sampler were spilled in labeled jars and fixed to 10% formaldehyde and then sent to the laboratory.

In the laboratory, the captured macroinvertebrates were rinsed to rid them of the formalin and were subsequently sorted station by station under a binocular dissecting microscope. After sorting, they were grouped according to their class up to the family level apart from oligochaetes, nemathelmintes, hydracarians, hydrozoans, sponges, bryozoans and nemerteans which were kept aside such as Abahi et al. (2018) [10] . The taxonomic determination was made using the following keys: Benthic macroinvertebrates of the streams of “la Nouvelle-Calédonie” by Mary (2017) [16] , Identification guide of the main benthic macroinvertebrates of freshwater from Quebec written by Moisan (2010) [17] , “Freshwater invertebrates: Systematics, biology, ecology” by Tachet et al. (2000) [18] and “Aquatic entomology” by McCafferty (1981) [19] after which macroinvertebrates were enumerated and then stored in pillboxes containing 70% alcohol.

The identified data allowed us to calculate the following metrics and indices:

Taxonomic richness (S) = number of taxa present in each station

Abundance (N) = number of individuals from a taxonomic group in each station

Relative abundance (Nr) = ratio as a percentage of the number of taxon individuals in a station to the total number of individuals of all species from all stations.

Frequency of family observation (FO) = (F_i × 100)/F_t. In such, F_i = number of stations containing the family and F_t = total number of stations studied. Three families were thus distinguished as Abahi et al. (2018) previously demonstrated. We have very frequent families (F ≥ 50%), frequent families (25% ≤ F ≤ 50%) and rare families (F ≥ 25%).

The data obtained were processed using Excel 2010 software and R3.4.4 software [20] . The taxonomic richness, the taxonomic abundance, the average values of the physico-chemical parameters were calculated per station. Parametric and non-parametric tests (test t student and test of Kruskal-Wallis) were used to evaluate the variability of the taxonomic richness of the abundance and diversity indices at the 5% threshold with the R3.4.2 software [20] . Moreover, the factorial correspondence analysis (FCA) was used for grouping the stations according to the similarity association of macroinvertebrates families. In addition, a canonical correspondence analysis (CCA) was performed using PAST statistical package [21] .

The majority of physico-chemical parameters studied except phosphates and ammonium tend to follow an altitudinal zonation. Indeed, the average values of transparency, depth and temperature recorded on the stations of the Affon River are inversely proportional to the altitude. The recorded temperatures (22.5˚C and 30.3˚C) are slightly higher than those obtained by Zinsou et al. (2016) [22] . Guigemde et al. (2003) [23] observed similar values for the Massili Basin in Burkina Faso (18.6˚C to 39.2˚C). As for the water pH values, they are between 6.5 and 8.5, perfectly within the tolerable limit (6.5 and 8.5) which characterizes aquatic life [24] . These results corroborate those recorded by Koudenoukpo et al. (2017) [25] on the Sô River in Benin and Buhungu et al. (2018) [26] on the Kinyankonge River in Burundi. The recorded conductivity and TDS values are in the range of reported values (59.8 and 238 μS/cm and 28 and 119.5 mg/l) in the Ouémé delta [22] . These values are much lower than the values reported on the Sô River [25] , but they are higher than those obtained on the Ogba River in Nigeria [27] . Mean values of conductivity and TDS increase with decreasing altitude and are within the range that characterizes natural water [24] . Regarding the values of nitrites, phosphates and ammonium, they do not follow a specific trend in relation to the altitude. The phosphate values recorded in this study (7.11 - 14.8 mg/l) are higher than those for surface waters with an average value of 0.02 mg/l [28] [29] . These observed values classify the waters of Affon River in a general state that is bad with respect to their trophic state [30] . The phosphate values obtained in this water are much higher than those reported on the Ogba River in Nigeria [27] and on the Agnéby River in Côte d'Ivoire by Diomandé et al. (2009) [31] . Phosphate is the most limiting factor and the most important for aquatic productivity whose absence could lead to the depletion of aquatic ecosystems [32] . Conversely, high concentrations of phosphate can indicate the presence of pollution and are responsible for eutrophication conditions [28] . The recorded ammonium values are lower than the values reported by Zinsou et al. (2016) [22] in the Ouémé delta. Except for the value of Tanéka 3, the values recorded are below the standard set for surface water (0.2 mg/l) [28] and WHO, which is 0.5 mg/l for drinking water [33] .

The study has identified in the Affonriver, 9755 macroinvertebrates belonging to 49 families. The observed abundance is very low compared to that reported by Agblonon Houelome et al. (2017) [8] on the Alibori River (39,718). On the other hand, it is higher than that of the upper Ouémé River, where there were 3657 individuals of macroinvertebrate [10] . As for the taxonomic richness, it is identical to that reported on the Niger River [34] and on the Agnéby River in Côte d'Ivoire [31] . While it is much higher than that obtained by Abahi et al. (2018) [10] on the upper Ouémé River. The differences observed with the study by Abahi et al. (2018) [10] in the same area are due to the Surber sampler and the sampling period. In fact, during this study, we used the Surber sampler with a 500 micrometer mesh rainy season, while Abahi et al. (2018) [10] used the Surber sampler with a 100 micrometer mesh in low water. The diversity of the macroinvertebrate community harvested in the Affon River is marked by the importance of Diptera (76.90%) dominated by the Chironomidae family which represent 73.17% of the total abundance. In Benin, the study conducted by Abahi et al. (2018) [10] on the upper Ouémé River, also revealed the predominance of Insects (85.23%), Diptera (81.65%) and Chironomidae (67.35%). Similarly, in Burkina Faso, Sanogo et al. (2014) [35] and Kabré et al. (2000) [36] reported the dominance of Chironomidae in their studies. In addition, the study revealed the low diversity of pollution-sensitive orders (Ephemeroptera, Trichoptera and Plecoptera) both in terms of richness (14 families) and in terms of numbers (1,303 individuals), which would probably reflect the poor quality of the waters in the study area. These results, which are characteristic of rivers located in anthropized zones, corroborate well with the results obtained by (Abahi et al., 2018) [10] on the upper Ouémé River, by Imorou Toko et al. (2012) [9] in the Benin cotton basin and by OrouPiami (2018) [37] on the Sota River. Most encountered families are represented by only a few individuals. But Chironomidae (Diptera) are frequently and abundantly represented at all stations. Thus, the diversity of the Diptera Order observed and especially of the Chironomidae family reveals an accumulation of nutrients in the ecosystem; consequences of intense human activities [38] [39] . Anthropogenic activities near ecosystems disturb benthic communities and contribute to reduced species richness and even species distribution [35] [40] . In addition, the correlations established between families and the physicochemical parameters show a strong positive correlation between Nemouridae and ammonium on the one hand and between Perlidae, Taeniopterygidae, Ephemeridae, Heptageniidae, Isonychiidae, Elmidae and phosphate on the other hand, where there was a strong positive correlation. These correlations show the invulnerability of these families, which are mostly pollution-sensitive families, at the doses of phosphate and ammonium measured in this study.