Abstract:
Erelu reservoir is the main source of water supply for domestic, agriculture and fishing activities in Oyo town. The reservoir receives inflow of materials from the catchment where various anthropogenic activities occur which could impact its ecosystem. Despite the socio-economic importance of Erelu reservoir, there is paucity of information on the reservoir limnology, which is important for its management. This study was conducted to determine the physico-chemical parameters, occurrence and abundance of macro-invertebrate fauna in Erelu reservoir.
Surface water, macro-invertebrates and sediment samples were collected monthly from June, 2013 to May, 2015 at seven selected stations on the reservoir. Water temperature and transparency were measured in-situ using standard methods. Dissolved Oxygen (DO) and Biological Oxygen Demand (BOD) were measured with standard kits. Total Alkalinity (TA), Primary nutrients (phosphate and nitrate) and turbidity were determined using APHA methods. Lead, iron, chromium, zinc and calcium were determined using Atomic Absorption Spectrophotometry. Aquatic insects were collected using sweep nets (250 and 500 µm mesh sizes). Benthos and sediment samples were collected with van-Veen grab (surface area 0.6m2), benthic samples were sieved with 0.5mm sieve, sorted, identified with standard identification guides and counted. Species diversity was determined using Margalef (d) and Shannon Wiener Index (H’). Data were analysed with descriptive statistics, ANOVA, Pearson’s correlation coefficient and paired t-test at α0.05.
The physicochemical parameters of the reservoir water were temperature 28.50±0.12oC; transparency 64.82±1.80 cm; turbidity 10.79±1.28 NTU; DO 8.36±0.11mg/L; BOD 1.18±0.11 mg/L; TA 98.23+2.70 mg/L and Calcium ion 17.11±0.54 mg/L. Phosphate (0.14±0.03 mg/L) and nitrate (0.84±0.08 mg/L) values were within NESREA (3.5mg/L and 10mg/L, respectively) guideline limit for drinking water while turbidity (5mg/L) was not. Temperature (27.52+0.32oC), TA (104.71+11.34 mg/L), Fe (0.62+0.16 mg/L), Zn (1.43+0.71 mg/L) and Cr (0.54+0.30 mg/L) in wet season were significantly different from dry season, (30.04+0.14oC; 96.20+7.50; 0.77+0.22; 0.62+0.14; 0.19+0.07 mg/L, respectively). The transparency, BOD and lead significantly varied across sampling stations. Lead, iron and chromium were lower in water (1.01+0.06; 0.59+0.04; 0.26+0.03 mg/L, respectively) than sediment (23.79+1.72; 20.63+2.25; 11.51+1.54 mg/kg, respectively, an indication of the sediment acting as the favourable sink for the metals. Fifty six species (Aquatic insects=38, Benthos=18) of macro-invertebrates were identified. Four orders of aquatic insects encountered were Hemiptera (92.06%) > Coleoptera (7.61%) > Odonata (0.33%), while the least was Diptera (0.01%) across all stations. Among the eighteen species of benthic macro-invertebrates encountered, the pollution tolerant species: Melanoides tuberculata (61.61%) and Potadoma moerchii (14.51%) had the highest percentage occurrence, an indication of the pollution status. Transparency showed significant positive correlation with Hemiptera (r=0.88), while phosphate had positive relationship with Capitellida (r=0.94), which indicated that the parameters had positive effect on the abundance of the organisms. Diversity values recorded for aquatic insects and benthic macro-invertebrates were (H’=0.64; d=0.54) and (H’=1.06; d=0.50), respectively.
Turbidity and heavy metal concentrations exceeded the recommended limits in Erelu reservoir. A high percentage of pollution tolerant species suggest poor water quality, hence, the need for the introduction of appropriate management methods.
