Comparative study of the macroinvertebrate community composition and water quality of Ona and Opa rivers , Southwestern Nigeria

Macroinvertebrate samples of Ona and Opa rivers were taken fortnightly between April and August, 2013 with the aid of a dip net of 500μm mesh size with the aim of evaluating the macroinvertebrate community composition and the water quality of the rivers. A total of 617 individuals macroinvertebrate belonging to 29 species were collected. Ona River was dominated by Chironomid larvae while Opa River was dominated by Trichoptera species. Margalef’ species richness and Shannon-wiener’s species diversity indices both revealed that Opa River is higher in terms of species richness and diversity. The physico-chemical parameters of the two water bodies showed a slightly marked variation, especially for Dissolved Oxygen, Conductivity and Total Dissolved Solids. Ona River was relatively lower in DO level but showed a higher TDS level than Opa River. The species composition and the water quality both indicated that Ona River is more stressed than Opa River due to greater impacts of anthropogenic activities which brought about the observed organically-induced pollution in the water body. INTRODUCTION All around the world, most freshwater habitats are subjected to appreciable levels of human disturbance (Saunders et al., 2002). Several environmental health stressors significantly deplete the biodiversity of aquatic ecosystems. Also, freshwater habitats and their inhabitants are among the most severely impacted and environmentally threatened because these surface waters receive and amplify abuses to the landscape, including the effects of both point sources (e.g sewage and used water) and non-point sources (e.g. erosion and agricultural leaching). As a result of these anthropogenic activities, most rivers and streams in developing countries, Nigeria inclusive, are subjected to increasing pollution load, thereby depleting the quality of such water bodies. Macroinvertebrates are important components of the aquatic ecosystem which have been used extensively to evaluate these disturbances and overall water quality of streams. Their preference as environmental indicators is based on their ability of their diversity and West African Journal of Applied Ecology, vol. 26(1), 2018: 33 48 abundance to respond to changes in a variety of environmental variables such as sediment quality, water quality, hydrological conditions, shading and biological factors (Rosenberg & Resh, 1993; Chessman, 1995; Bonada et al., 2006). Most importantly, aquatic insects are very good and most commonly used indicators of water qualities since they have various environmental disturbance tolerance levels (Arimoro & Ikomi, 2008). In Nigeria, biological richness of aquatic macroinvertebrate fauna has not been extensively documented. Also, a review of studies on the use of macroinvertebrates for assessing the environmental health status of water bodies revealed that such studies in tropical Africa are not extensive (Ogbogu & Akinya, 2001; Dobson et al., 2002; Mafuya et al., 2004; Deliz-Quinones, 2005; Arimiro & Ikomi, 2008). Ogbogu (2001) further reported that although few studies on aquatic ecosystem impairment have been done in Nigeria (e.g. Ogbeibu 1985; Ogbogu & Hassan, 1996), the number is far from being adequate. Moreover, the task of balancing the compelling demands for water use is sometimes difficult for management agencies because they lack information needed to make sound decisions, especially for fish and wildlife benefits (Ezekiel et al., 2011). Therefore, the aim of this study is to assess the water quality of Ona River, Ibadan and Opa River, Ile-Ife through the evaluation of the richness of the macroinvertebrate populations and the measurement of the physico-chemical parameters of the water bodies. This is necessary in order to determine and compare the extent of disturbance in these freshwater bodies as both sites are subjected to varying degrees of anthropogenic activities. This will further contribute to the knowledge of the use of macroinvertebrates in assessing water quality. The information gathered in this study will also serve as baseline information for management policies and for future studies on these waterbodies. MATERIALS AND METHODS Study Area The study was carried out between April and August, 2013 at Ona and Opa rivers which are located in two contiguous states in southwestern Nigeria. The Ona River (Fig. 1) (070 26`N, 0030 54`E) is in University of Ibadan, Oyo State while Opa River (Fig. 2), which drains a catchment area of 116 km and extends from longitude 040 31` to 0040 34`E and latitude 070 21` to 070 35`N in Obafemi Awolowo University, Ile-Ife, is in Osun State (Ogbogu, 2001). University of Ibadan is situated about five miles (8 Kilometers) from the centre of the city of Ibadan. Ona River has characteristic aquatic vegetation growing on its surface where some macroinvertebrates like whirligig beetles do retreat (plate 1) and it is one of the two major rivers draining the city of Ibadan. As it flows through the city, it receives a lot of waste materials from industrial, agricultural and domestic sources in both organic and inorganic forms (Adjarho et al., 2013). Andem et al. (2012) reported that the Ona river is often used as ‘latrine’. The Opa River on the other hand is the major source of water to the Opa reservoir inside Obafemi Awolowo University campus. It flows southward from the reservoir’s auxiliary spillway through a channel, crossing the major road to the campus and has a substratum that is made of large stone bed (Cobbles) at the upper portion and a sand at the lower portion (plate 2). 34 West African Journal of Applied Ecology, vol. 26(1), 2018 Fig 1: A section of the University of Ibadan showing the sampling station on Ona River Collection of macroinvertebrates Macroinvertebrates were sampled in both rivers every two weeks from April to August, 2013. A standard dip-net of 500 μm mesh size was used for the collection of the macroinvertebrates over a distance of 86 m and 69 m on Ona River and Opa River respectively, so the populations were expressed as numbers per unit area of sampling (ind/m2). These collections were supplemented by manually turning over the Amusan B. O et al Comparative study of the macroinvertebrate community composition 35 Figure 2: A section of Obafemi Awolowo University showing the sampling station (starred) on Opa River Plate 1: Ona River bed and aquatic plants growing on its surfaces. cobble-sized stones in the sampling area and scrubbing them with a brush to collect the organisms attached to them. The stones were also generally inspected for any other remaining macroinvertebrates which were removed generally by handpicking. Sampling was carried out between the hours of 8 am11am each sampling day. The essence of fixed timing for sampling is to avoid temporal and spatial variations in the quality of the water because the quality of the water and biota will