Abstract:
Oluwa River is used for artisanal fishing, ferrying and domestic activities in Ilaje Local Government Area (ILGA), Ondo State. The river receives domestic and agricultural wastes, which could impact adversely on the water quality and resident biota. Inspite of anthropogenic activities, there is dearth of information on the limnology of the river. This study was carried out to investigate physico-chemical parameters, diversity, abundance and distribution of plankton, Benthic Macro-Invertebrates (BMI) and fish fauna of Oluwa River.
Surface water, plankton and BMI samples were collected monthly from June, 2014 to November, 2015 at six purposively selected stations along the river. Water temperature and transparency were measured in situ while Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), pH, Total Solid (TS), chloride, alkalinity and conductivity were determined according to APHA methods. Plankton samples collected with plankton net (mesh size, 55µm), were identified and counted microscopically. Benthic macro-invertebrate samples collected with Van-Veen grab (surface area, 0.6m2), were sieved (mesh size, 0.5 mm), sorted, identified, and counted macroscopically. Fish samples collected monthly from the landing centre of fishermen were identified and counted. All identification including pollution-indicator species were done using standard identification keys. Species diversity was determined with Shannon Weiner Index (H). Data were analysed using descriptive statistics, student’s t-test, ANOVA, Pearson’s correlation coefficient and Principal Component Analysis (PCA) at α0.05.
Water temperature was 27.6±0.3oC; transparency, 3.9±0.3 m; DO, 2.8±0.1 mg/L; BOD, 2.22±0.1 mg/L; TS, 58.7±3.1mg/L, chloride, 27.4±0.7 mg/L; alkalinity, 20.2±0.3 mg/L and conductivity, 41.0±3.0 mg/L. Dissolved oxygen had significant spatial variation while transparency, conductivity and BOD had significant seasonal variation. Dissolved oxygen was below the limit (4 mg/L) recommended as suitable for aquatic life by NESREA. Conductivity correlated positively with pH (r = 0.99) and Transparency (r = 0.7). The PCA showed high positive loadings for chloride (0.3) and alkalinity (0.3). Among the four classes of phytoplankton encountered, Chlorophyceae was the least abundant (50.2%). Pollution-indicator phytoplankton encountered were Asterionella, Fragilaria, Melosira, Navicula, Nitzschia, Oscillatoria, Phacus, Closterium and they accounted for 22.9% of phytoplankton population. Zooplankton was dominated by rotifers (86.7%). Mean species diversities of 3.88±0.2 and 3.62±0.3 were recorded for phytoplankton and zooplankton taxa respectively. Seventeen species of BMI were recorded with the gastropod, Pachymelania aurita (12.9%) dominating while the insect larva, Chaoborus sp. (3.1%) was the least dominant. Pollution-indicator macro-invertebrates recorded included Melanoides tuberculata (8.0%), Chironomus larvae (4.5%) and Nereis diversicolor (4.5%). Thirty-four fish species were encountered of which the Family Clariidae (29.0%) was the most abundant while; the least abundant was Malapteruridae (1.0%).
The abundance of pollution indicator species coupled with low dissolved oxygen are indication that Oluwa River is under pollution stress which calls for adequate management
Keywords: Physicochemical characteristics, Benthic macro-invertebrates, fish, Pollution Indicator, Oluwa river.
Word count: 464
