2 An assessment environment.cdr

The Odaw River is formed from several streams running from the Aburi Mountains in the Eastern Region of Ghana and ending up in the Korle Lagoon in Accra. Human activities have modified the course of this channel resulting in changes in water quality. This study was therefore conducted to investigate the extent of impact on macro-fauna communities by comparing the results with similar studies carried out in previous years. Faunal abundance and the similarity of macro-fauna among sampling stations for each season were respectively investigated by one-way analysis of similarity (ANOSIM) and cluster analysis all based on Bray-Curtis similarity index of species abundance. The Odaw River was found to be highly polluted, showing a pollution gradient from upstream to downstream, the upstream being the least polluted. Shannon-Wiener diversity index upstream was 0.53–2.00, midstream 0.64–1.41 and downstream had the least ranging from 0–0.04. Sensitive taxa of Dytiscidae, Hydropsychidae and Libellulidae were found only upstream from Obommirem to Buade. The mid portion of the Odaw catchment area were dominated by Oligochaeta, Naedidae and Lymnaea while the lower portion was dominated by the Chironomidae. There was no seasonal variation in the physico-chemical parameters analysed except for phosphate which showed significantly higher concentrations in the dry season than in the rainy season. The distribution of invertebrates did not also show seasonal variation in abundance between sampling stations suggesting good adaptation of prevailing species to small changes in water quality. Midstream and lower stream portions of the catchment area showed increased pollution over the years when previous studies were compared. This study shows that prevailing macro-fauna can be used to characterize sites of differing water qualities. 1 1 1 3 Ansa , Sakyi Asmah , Acheampong and Lamptey West African Journal of Applied Ecology, vol. 25(2), 2017: 17–29. chemical pollutants including heavy metals (De Jonge et. al., 2008). In Ghana, there has been some effort to document the ecology, composition, spatial and temporal distribution and diversity of fish and macro benthic invertebrate communities of lakes, rivers, and lagoons (Hynes, 1975; Thorne et al., 2000; Aggrey-Fynn et al., 2011; Armah et al., 2012; Kyerematen and Gordon, 2012; Baa-Poku et al., 2013; Aheto et al., 2014). One such river system that has received some attention is the Odaw River system which flows through the capital city Accra, from the Aburi highlands, in the Eastern Region of Ghana. The Odaw River is used mainly as a source of domestic water supply upstream and midstream. Extraction of water from the river for commercial purposes such as car washing and cement block manufacturing is also not uncommon. Downstream in the Greater-Accra Region, particularly in the Airport Residential Area, it is an important source of water and nutrients for vegetable crop production. Previous investigations on this channel focused mainly on the physico-chemical characteristics of the river and particularly on the adjoining lagoon, the Korle Lagoon (Biney and Amuzu, 1995). Some bio-monitoring surveys have also been carried out on the river. Thorne et. al. (2000) investigated the macro-invertebrate communities found in some portions of the river. Baa-Poku et al. (2013) also focused on the impact of urban e ff luen ts on macroinver tebra te communities in one of its polluted tributaries, the Nima Creek. The study seeks to evaluate potential changes in the ecology of macro-fauna, particularly the benthic fauna of the catchment area of the Odaw River over the years, in relation to its present use. Materials and methods The study area The Odaw catchment area runs from the o Aburi Mountains (latitude 5 52’N) down o to Accra (latitude 5 32’N) where it discharges into the Korle Lagoon. The o river is located within longitudes 0 10’W o and 0 15’W and consists of six streams namely; Ntare, Obommirem, Dakobi, Buade, Onyasia and Odaw, as well as two other streams running from East Legon and from behind Shangri-la hotel (Fig. 1). Thirteen sampling stations were selected along the catchment area and these were categorized as upstream, midstream downstream based on categorization adopted by previous studies in the Odaw catchment area (Nana-Amankwah et al., 1995; Thorne et al., 2000; Baa-Poku et al., 2013). Samples were collected from upstream: Obommirem (OB), Agyemadi (AG), Ntare (NT), Dakobi1 (DK1), Buade (BU), and Dakobi 2 (DK2), midstream: Atomic (AT), Okponglo (OKP), Secaps (SEC) and downstream: Alliance Francais (AF), Kawukudi (KKD), Achimota (ACH), and Korle (KO). The rivers and streams at these locations were all less than 1.0 m deep in both dry and wet seasons and have varied banks ranging from a vegetative cover of shrubs and herbs to bare banks. From Achimota to the Korle Lagoon, the channels consist of concrete drains. Water quality sampling Samples for water quality analysis were collected in 1 litre polyethylene bottles. 18 West African Journal of Applied Ecology, vol. 25(2), 2017 E. D. O. Ansa, R. L. Sakyi, et al.: An assessment of environmental conditions 19 Fig. 1. Map showing the Study Area Samples were collected both in the dry (November to February) and the rainy (March to August) seasons. Analyses were carried out for the following physicochemical parameters: temperature, pH, conductivity, turbidity, Biochemical Oxygen Demand (BOD), dissolved oxygen (DO), total suspended solids 2(TSS), nitrate (NO ), nitrite (NO ), 3 2 ammonia as nitrogen (NH -N), and 4 phosphate (PO ). BOD, TSS, nitrate, 4 nitrite, ammonia nitrogen and phosphate concentrations were determined by dilution, gravimetric, hydrazine reduction, direct nesslerization and stannous chloride methods respectively as outlined in Standard Methods (APHA, 1998). pH, dissolved oxygen, turbidi ty and conductivity were measured using WTW 340, WTW Oxi 330, Hach 2100p and W T W L F 3 4 0 p o r t a b l e m e t e r s respectively. Sampling of fish and macro-invertebrates Sampling for fish was done in the rainy and dry seasons using cast nets of 1.2m radius as outlined by Stein et al. (2014). Sampling of macro-invertebrates was done by means of a handnet of 500mm mesh size over a defined area. Four replicates per sampling station were used and selection of sampling stations was based on ease of access to the shoreline, and also taking into consideration point sources of pollution. Where rocks existed, they were washed to dislodge attached fauna, which were transferred into containers. Animals retained by net were also washed into containers. Samples were then preserved with 4% formalin in the field. Samples were further processed in the laboratory, and organisms identified up to the family level with the help of taxonomic keys (Thorp and Covich, 2010; Umar et al., 2013). Statistical analysis The Shannon-Wiener diversity index (H’) (Eq.1), (Clarke &Warwick, 1994), was used to compare the macroinvertebrates of this study with those of studies conducted by other authors over the years, while the Simpson diversity index (Ludwig and Reynolds, 1988) was used to compare the seasonal differences in diversity of macro-invertebrates sampled during this study. The Shannon Weiner diversity index (H’) is given by, (Eq.1) where P is the proportion of individuals i belonging to the ith species and n is the total number of species The Simpson diversity index (l) is given by, ë = Ón (n -1)/N(N-1) (Eq. 2). i i where N = total number of individuals and n = total number of individuals in species i. i Simpson’s diversity index l takes values from 0 to 1 and is inversely proportional to the wealth of species (Hellawell, 1978). Brower and Zar (1977) indicated that in comparing two communities with very small differences in l, the reciprocal of l gives a better measure of diversity. To evaluate the influence of seasonal variations on physico-chemical variables, a Welsh unequal variances t-Test was conducted. The influence of seasonal variation on species abundance at the various sampling stations was assessed 20 West African Journal of Applied Ecology, vol. 25(2), 2017