Abundance and Distribution of Macroinvertebrates of the Affon River in Bénin

This study focuses on the biodiversity of macroinvertebrates of the Affon River. To reach this goal, eight stations were sampled and physical parameters such as temperature, pH, conductivity, transparency, depth, total dissolved solids (TDS) and dissolved oxygen were measured. Chemical parameters such as ammonium nitrite and phosphate were measured in the laboratory. We identified 9755 macroinvertebrates belonging to 4 classes, 14 orders, and 49 families. Chironomidae were the most abundant family whereas other sensitive insect’s orders such as Ephemeroptera, Trichoptera, and Plecoptera were rarely found, suggesting a poor water quality of the Affon River. The principal components analysis yielded three groups of stations: the first group (Tenéka 2 and 3) characterized by high values of ammonium and phosphate and low values of conductivity and TDS; the second group (Tanéka 1 and Kolokondé 1) with low values of pH, transparency, ammonium and phosphate; and the third group of stations (Kolokondé 2, Kpébouko1, Kpébouko 2, and Affon) marked by high values of conductivity, TDS, transparency, depth and temperature. This study is a crucial step for any management and monitoring of the Affon River.


Introduction
Rivers are among the most complex and dynamic ecosystems [1]. They are extensively diversified and play a crucial role in the conservation of biodiversity.

Study Area and Sampling Stations
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 km 2 . 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.

Sampling of Macroinvertebrates
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

Macroinvertebrates Identification
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 [18] and "Aquatic entomology" by McCafferty (1981) [19] after which macroinvertebrates were enumerated and then stored in pillboxes containing 70% alcohol.

Data interpretation
The identified data allowed us to calculate the following metrics and indices: We have very frequent families (F ≥ 50%), frequent families (25% ≤ F ≤ 50%) and rare families (F ≥ 25%).

Statistics Analysis of Data
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].

Water Quality of the Affon River
The analyzed results of the physico-chemical parameters of the Affon River waters are shown in Table 2. It indicates that the lowest temperature value of 22.5˚C was recorded at Tanéka 1 (upstream) and the highest 30.3˚C at Kpébouco 2. Depth and transparency increased from upstream to downstream. Thus, the maximum values were measured at Tanéka 1 while the minimum values were recorded at the Affon. The pH ranged between 6.5 and 8.5, with the minimum value recorded at Tanéka 2 while the maximum value was observed at      Figure 3).
Among the fourteen orders, Diptera, made up of 76.90% of the total population, were the most dominant. Then come Trichoptera, Odonata and Ephemeroptera with 9.06%; 5.06% and 4.25% of the total number of individuals harvested respectively. The remaining orders: Oligochaetes, Coleoptera, Heteroptera, Nemathelminthes, Gastropods, Hydracarina, Lepidoptera, Achaetes, Plecoptera and Hydracarians are the marginal communities with relative abundances lower than 02% (Figure 4).   dominant in this watercourse, followed by Hydropsychidae (Nr = 9.03%). About 95.92% of captured families have a relative abundance of less than 3%.

Taxonomic Composition of Macroinvertebrate Classes 1) Class of Insects
The class of insects was very rich with 9501 individuals, 39 families and 08 orders. This class is dominated by Diptera containing 9 families and 7502 individuals, or 78.96% of the insect richness. They were followed by Trichoptera and Odonata with 2 families, 884 individuals; and 4 families, 494 individuals, thus constituting 9.30% and 5.20% of the insect richness respectively. Ephemeroptera, Coleoptera, Heteroptera, Lepidoptera and Plecoptera represent respectively 4.37%; 1.07%; 0.94% and 0.12%; 0.04% of the insect richness.

2) Class of Worms
The worms are made up of three orders, namely the order of the Achetes

Evolution of Richness and Taxonomic Abundance per Station
The highest taxonomic richness (30 families) was obtained at Tanéka 1 which is upstream while Tanéka 3 has the lowest taxonomic richness (6 families). As for the highest abundance, 2940 individuals or 30.14% of the total number of macroinvertebrates inventoried were obtained at Kolokondé 2 while Tanéka 3 was the station with the lowest abundance (43 individuals), or (0.44%). In addition, values of richness and abundance showed significant differences between stations ( Figure 6).

Physico-Chemical Quality of Affon River
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   [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].

Composition and Distribution of Macroinvertebrates
The study has identified in the Affonriver, 9755 macroinvertebrates belonging to 49 families. The observed abundance is very low compared to that reported by 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.

Conclusion
The study of the water quality of Affon River reveals that the main physi-