Ecological status of a newly impounded sub-saharan reservoir based on benthic macroinvertebrates community (Burkina Faso, West Africa)

In West Africa, particularly in Burkina Faso, the aquatic ecosystems are under human pressures. Therefore, for long term management of these ecosystems, the managers need baseline data for regular assessment of ecological health condition. In this regard, the structure of the benthic macroinvertebrate community was investigated and the relationship between the environmental variables and biotic indices of the Samandéni reservoir from January to December 2018, after one-year impoundment was explored. The macroinvertebrates were collected with a hand net (25 × 25 cm 2 coverage area and 500 μ m mesh) according to the multi-habitat sampling method. Key environmental variables such as temperature, conductivity, dissolved oxygen, pH and total dissolve solids (TDS) were measured in situ . High diversity of macroinvertebrates was reported and thirty-four (34) taxa were determined belonging to eight (8) orders dominated by insects (79.41% of all taxa). Good ecological quality of the reservoir was testified by the presence of several sensitive taxa like Ephemeroptera and Trichoptera. The trends of environmental variables also reflect good habitat conditions. The results revealed a strong and negative relationship between some abiotic variables and biotic indices. The finding of this study is of big importance for sustainable management of the Samandéni Reservoir.


INTRODUCTION
The key global challenge in the 21 st century is to maintain the supply of clean water and other aquatic ecosystem services that are of benefit to human's well-being, without affecting biodiversity and ecosystem processes that reinforce their sustainability (Pawlowsky et al., 2018). The effect of climate changes on hydrological and biological dynamics is poorly documented in developing countries (Taniwaki et al., 2017). Therefore, studies on the health of inland aquatic ecosystems are crucial for each nation which must supply water both qualitatively and quantitatively to meet the increasing demands of local populations. Many authors have demonstrated that monitoring water quality by using biological approaches, especially macroinvertebrates communities has greater advantages compared to physical and chemical approaches (Jun et al., 2012;Nyamsi et al., 2014). While *Corresponding author. E-mail: bance605@gmail.com. Tel: +226 70644887.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License physical and chemicals measurement required expensive and specialized equipments, and do not provide longterm predictions of environment changes. But benthic macroinvertebrates are recognized as very good indicators, and are the most commonly used (Nyamsi et al., 2014;Kaboré et al., 2016a, c;Gerami et al., 2016;Arimoro and Muller, 2010). This is due to: (a) their sedentary nature, which facilitates spatial analyses of pollution effects, (b) the sensitivity of many common species are well-documented for different types of pollutants (Myers et al., 2011), and (c) the long life cycles of some species, which make them more cost-effective tools to trace pollution effects over longer periods (Heino and Peckarsky, 2014). Habitats degradation, land uses, hydromodification and water pollution can be monitored by assessing benthic macroinvertebrates community structure, and indicators species (Flores and Zafalara, 2012;Kambwiri et al., 2014;Kaboré et al., 2016a;Patang et al., 2018). A large number of biotic metrics/indices have been developed in different countries (Chi et al., 2017;Borja et al., 2013;Dedieu et al., 2015;USEP, 2018;Tampo et al., 2020). However, in developing countries, and particularly in Western African region, the use of biological organisms for monitoring aquatic ecosystems health is still less documented (Kaboré et al., 2016a;Tanon et al., 2020). In Burkina Faso, few studies have addressed the ecological status of the reservoirs. The studies on macroinvertebrates are those of Guenda (1986), Kaboré (2016) and Kaboré et al. (2016aKaboré et al. ( , 2018 that have assessed riverine ecosystems health, and those of Koblinger and Trauner (2013) and Sanogo et al. (2014) on benthic invertebrates' diversity in standing waters. Kaboré et al. (2016aKaboré et al. ( , 2018, Koblinger and Trauner (2013) and Sanogo et al. (2014) mentioned that macroinvertebrates are good indicators of ecosystem conditions (Reyjol et al., 2012;Selvanayagan and Abril, 2016), because they respond to environment disturbances due to human activities. Burkina Faso's last big reservoir is Samandéni, built in 2017 and classified as a Ramsar site in October 2020 (https://rsis.ramsar.org/ris/2439). The dam and associated wetlands provide water for electricity, agriculture and human consumption, while the sparse population plays a major role in enabling the maintenance of biodiversity levels. The vegetation consists of wooded savannah and open forests, combined with dense semi-deciduous gallery forests with trees of up to 40 m in height. The site hosts internationally threatened tree species like the kosso Pterocarpus erinaceus and the mahogany Afzelia Africana and Khaya senegalensis. It also features a diverse range of mammals, waterbirds, fish and reptiles. Minoungou (2020) recorded 40 species of fish, dominated by seven species (Sarotherodon galilaeus, Coptodon zillii, Oreochromis niloticus, Marcusenius senegalensis, Synodontis schall, Schilbe intermedius and Brycinus nurse) (Minougou et al., 2020). Globally threatened mammals such as the Hippo (Hippopotamus amphibious) and the African elephant (Loxodonta Africana) also feature. The increasing human population downstream of the site, increased spread among local communities of waterrelated diseases, dispersal of hippopotamus herds, impacts of climate change and the lack of stakeholder consultation in its management and development are among the more urgent threats facing the natural resources in the site (https://rsis.ramsar.org/ris/2439). The sustainability of local socio-economic activities reposes on better management of this important reservoir. The present study aims to provide a first database on his benthic macrofauna, one year after its impoundment. The specific objectives were to: (1) evaluate the composition and diversity of benthic macrofauna in the reservoir and (2) explore the relationship between environmental variables and biotic indices as well as describe ecological status using macroinvertebrate community.

Study area
In Burkina Faso, the climate is characterized by highly irregular rainfall patterns with marked differences between wet and dry seasons which lead to highly chronic water scarcity and episodes of severe drought. The country relies much on reservoirs to produce food (maize, cereals, fruits, vegetables, fish, meat, etc.) for local and national consumptions and sustain socio-economic development in rural area. Following this, a new reservoir named Samandéni reservoir was created in 2017. This study was undertaken in this reservoir, located in the upper Mouhoun catchment (a sub-catchment of the Volta River), with an area of 20,980 km 2 (DGIRH, 2001). The data were collected from January to December 2018 in five sampling stations ( . The Badoville and Dioufoulma stations are characterized by low human activities, with natural vegetation in the floodplain (e.g. shrubs and trees) typical to the area. Banzon, Sikorola and Sadina stations are characterized by intense activities, such as crops farming, banana plantation, and livestock in the riparian zone. In order to assess the dynamic of the benthic community in the reservoir, four sampling campaign were conducted from January to December 2018. Unfortunately, Dioufoulma, Banzon and Sikorola stations are prohibited during periods of heavy rain by the authorities for safety reasons.

Water variables measurement
At each sampling station, the key water variables such as pH, temperature (°C), electrical conductivity (µS cm -1 ), dissolved oxygen (DO) (mg/L) and total dissolved solids (TDS) (ppm) were recorded in-situ using a portable multi-parameter device (Hanna Instrument, HI9829) between 9 and 11 a.m. Each variable was measured in triplicate at each sampling station. Immediately after water variables measurement, the macroinvertebrates were collected.

Benthic macroinvertebrates sampling
The macroinvertebrates were sampled with a hand net (rectangular opening: 25 cm x 25 cm, mesh size: 500 µm) according to the multihabitat sampling approach described in Moog (2007). At each station, a pooled sample, consisting of 20 sampling units was taken from all microhabitats available in proportion to their coverage. The habitats were mainly composed of macrophytes (emerged and submerged plants), sediment (sand, mud, litter) and coarse substrate. Samples were fixed in ethanol (90%), and transported to the laboratory for detailed examination. Prior to sorting out the organisms, samples were sieved and the animals were sorted using a binocular microscope. All taxa were identified to the lowest taxonomic level as possible using taxonomic manuals and keys (Tachet et al., 2003;Moisan and Pelletier, 2008).

Data analysis
The mean and standard deviations, as well as extreme values (minimum and maximum) were calculated for all physico-chemical variables. We used the Kruskall-Wallis test (p<0.05) to analyse the variation of physico-chemical variables between sampling stations.
Species occurrences, number of taxa, EOT taxa, relative abundance of some groups (Molluscs, Chironomids, Hemipterans Coleopteran) per station, Shannon-Wiener index (H') and Equitability were estimated to evaluate invertebrates' diversity and abundance. The relative abundance is the percentage of individual particular taxon by total number of all taxa. The Shannon-Wiener index (H') (Shannon and Wiener, 1949) and the Equitability (Piélou, 1969), are the common diversity indices which provide information about community composition, rarity and commonness were calculated using respectively formulas (1) and (2).
where Ni: number of individuals of a given taxa and N: total number of individuals.

Ethical statement
According to the authors, the research was conducted according to ethical standards.

Taxa abundances
In total, 2804 individuals of macroinvertebrate were recorded. Overall, the Order of Diptera was the most abundant (28%), followed by Ephemeroptera (25%) and Hemiptera (22%) (Figure 2). In Badoville and Dioufoulma, the Diptera (35.61%) were the most dominant group followed by the Ephemeroptera (29.49%). While in Sadina, the Ephemeroptera (31.48%) were the most abundant group followed by Diptera (22.66%). The results reveal that the Hemiptera and Diptera were well represented in Sikorola and Banzon. Figure 3 showed the variation of metrics (taxa richness, EOT-taxa, % of Chironomidae, % of Molluscs, % of Coleoptera, % of Hemiptera, Shannon-Weiner diversity and Equitability) used to analyse benthic invertebrate's diversity. We found that the indices: total taxa and EOTtaxa, show the same variation. The high values of diversities were found in Badoville and Sadina, while the low values of diversities and EOT-taxa were recorded in Dioufoulma and Banzon (Figure 3a, d). Figure 3b and c show that the percentages of chironomids, molluscs, hemipterans and beetles vary from station to station. In stations where the percentage of Chironomidae is high, that of molluscs is low (Figure 3b). Similarly, in stations where the percentage of hemipterans is high, that of beetles is low (Figure 3c).

Correlations between biological and environmental data
Spearman correlation between metrics and physicochemical variables (Table 3) indicated that conductivity  taxa (Spearman correlation r=-0.97, p< 0.05) and total number of taxa (r=-0.90, p< 0.05) while water pH was highly and positively correlated to EOT-taxa (r=0.87, p< 0.05). The significant positive correlation was also detected between the temperature and hemipterans (r=0.8, p< 0.05). We found that chironomids abundances  Table 3. Correlation Matrix of Water Variables and Biological Index marked with an asterisk (*) =statistically significant (p<0.05).
was highly and strongly correlated with EOT-were significantly and positively correlated to total dissolved solids (Table 3).

DISCUSSION
Water quality parameters are key factors that influence the life of living organisms in water bodies (Pardo et al., 2012;Hussain and Pandit, 2012). Other studies have underlined the importance of physical-chemical variables for biomonitoring in tropical streams (Thorne and Williams, 1997;Lakew and Moog, 2015). Indeed, authors have demonstrated that the intense land use in riparian zone can deteriorate the quality of water physical and chemical variables (Kaboré et al., 2018;Meulenbroek et al., 2019). In this study, the values of the main physicochemical variables are similar to those obtained by Kaboré et al. (2018) in the reference sites of Burkina Faso. That can be explained by the fact that the riparian zone of the reservoir is slightly impacted by human activities such as agriculture and livestock, hence the good physical and chemical condition of the Samandéni Reservoir. This data can be used as baseline status data to monitor the quality of reservoir habitat. The diversity obtained (34 taxa) is higher than that obtained by Sanogo et al. (2014) in three reservoirs located in the same Mouhoun catchment. The high diversity of macroinvertebrates found in the present study can be explained by the habitat heterogeneity and suitable conditions due to lower human pressures in the riparian zone. Our results are similar to those reported by Camara et al. (2012) and Kaboré et al. (2016a), who have demonstrated that areas are created to preserve natural resources and wilde animals. According to Kaboré et al. (2016a), Moog (2007), and Selvanayagam and Abril (2016), the variation of the structure of macroinvertebrate community is influenced by the habitat conditions. Here, the lower diversity observed in Dioufoulma, Banzon and Sikorola could be explained by anthropogenic activities such as vegetable crops farming (e.g. using fertilizers and pesticide) in the riparian areas. In contrast, the good ecological condition of Badoville and Sadina stations can justify the hight diversity recorded here.
Our findings showed that insects are the most abundant. That could be explained by the large ability of this group to survive in different types of habitats (Tachet et al., 2010;Mereta et al., 2011), and can also adapt to many feeding strategies in water colunm. This agrees with previous studies on sub-Saharan areas that have proven that the tropical fresh water, harbors a high diversity of macroinvertebrates dominated by insects (Adandedjan et al., 2011;Edia et al., 2013;Sanogo et al., 2014;Kaboré et al., 2016a;Sirima et al., 2017). The presence of macroinvertebrate groups such as Ephemeroptera, Trichoptera and Odonata, recognized as sensitive to pollution, indicates that the Samandéni Reservoir has good ecological conditions (Wahizatul et al., 2013;Kaboré et al., 2016a, b). Other authors (Suleiman and Abdullahi, 2011;Sharma and Chowdhury, 2011) argued that the range of habitats and water chemistry offers basis for a high diversity of freshwater macroinvertebrates. In the same trend, many studies showed that, key sensitive taxa can decline with a decrease of water quality, which is mainly caused by human activities (Flores and Zafalara, 2012;Elias et al., 2014;Morris et al., 2014). The functional composition of benthic communities is linked to the supply and persistence of particular resources taken up by aquatic food webs and should be responsive to any changes affecting the latter (Merritt and Cummins, 2006). The low electrical conductivity of the reservoir water testifies to its low mineralization due to the weakness of organic matter. The opposite variation in the abundance of Chironomidae and Molluscs may be due to competition between the two groups for habitat and food supply. Indeed, organic matters are used by other Chironomids to build cases. Also, it contributes to improve algae and macrophytes proliferation in water bodies, which constitute an important food resource for mollusks proliferation. In the sites, the variation of the abundances of other macroinvertebrate groups, such as Beetles and Hemiptera reveal that the two groups do not share the same food resources. Thus, according to Kaboré et al. (2016a) and Masese et al. (2014), the Beetles are herbivoredetritivore, while most of Hemipterans are predators. The diversity of taxa sensitive to pollution are parameters most often used to assess the quality of an aquatic environment (Kaboré et al., 2016a;Masese and Raburu, 2017). The same variations observed for the indices: total diversity and EOT taxa in the stations, could be explained by the fact that they reveal the same information on the ecological state of the site. Total-taxa and the EOT index would be more suitable for evaluating the quality of stagnant water than the EPT taxa, which have been recognized as very good indicators of riverine ecosystems integrity (Myers et al., 2011;Kaboré et al., 2016a;Masese and Raburu, 2017;Jerves-Cobo et al., 2017). The absence of plecopterans may be due to the fact that the water in the reservoir is stagnant. The Odonata are considered as important indicators of environmental conditions, as they inhabit both aquatic and terrestrial habitats during their life cycle, and therefore may better reflect disturbance in riparian buffer modification (Goertzen and Suhling, 2013;Oliveira-Junior et al., 2015;Miguel et al., 2015;Monteiro-Júnior et al., 2015). Because the Odonates are predators, according to Stoks and Córdoba-Aguilar (2012) and Nedjwa et al. (2018), their presence can express good diversity of macro-fauna in the water. Here we found also a strong and negative correlation between the electrical conductivity and EOTtaxa. Indeed, the high conductivity is often an indicator of water pollution because it is likely to be affected by different riparian land use types (DeTroyer et al., 2016;Kaboré et al., 2016aKaboré et al., , c, 2018Meulenbroek et al., 2019).

Conclusion
This study provides the first inventory data of macroinvertebrates in Samandéni Reservoir. It harbors the high diversity of macroinvertebrates dominated by insects. We found that EOT-index could be a good indicator of ecological status of standing water bodies, and could be used as monitoring basis tool for the management and conservation of big fisheries ecosystems, particularly in Burkina Faso.