Spatial and temporal limnological status of Erelu Reservoir, southwestern Nigeria
Water quality monitoring is essential for determining the current condition and long-term trends of a reservoir for effective management. The spatio-temporal dynamics of the trophic lake is crucial in defining its water quality as well as biodiversity. Hence, the physico-chemical parameters and primary productivity of Lake Erelu were monitored for twenty-one months (July 2013 - March 2015) to determine its suitability for sustainable fish production. The study area was temporally and spatially stratified. Temporal stratifications covered wet (April to October) and dry (November to March) seasons. Spatially, the lake was divided into lower (LZ), middle (MZ) and upper (UZ) zones based on geographical locations. Four sampling points were randomly selected in each zone. Water samples were collected monthly from each of the sampling points following standard methods and - analyzed for variables such as biological oxygen demand (BOD), Nitrate (NO-3), Lead (Pb) and primary productivity (NPP). Mean value obtained for BOD in UZ (2.62 ± 0.25 mgL-1) and LZ (2.59 ± 0.26 mgL-1) were significantly different from MZ (2.42 ± 0.41 mgL-1), whereas no significant difference exists between wet (2.56 ± 0.28 mgL-1) and dry (2.51 ± 0.39 mgL-1) seasons. NO-3 in wet (0.66 ± 0.07 mgL-1) and dry (0.70 ± 0.05 mgL-1) differ significantly but between UZ (0.67 ± 0.07 mgL-1), MZ (0.68 ± 0.06 mgL-1) and LZ (0.67 ± 0.06 mgL-1) no significant differences occurred. Pb in LZ and UZ (1.46 ± 0.44 and 1.19 ± 0.53 mgL-1) and wet (1.07 ± 0.63 mgL-1) and dry (1.41 ± 0.52 mgL-1) were significantly different. NPP did not differ significantly between seasons and between spaces. The physico-chemical parameters revealed seasonal and spatial variations which fall within desirable limits for fish production. Also, the lake was more productive during the dry season than rainy season.
Keywords: Water quality, Heavy metals, Erelu lake, Desirable limits