Chlorophyll-A concentrations in relation to water quality and trophic status in the Weija reservoir from 2005 – 2008

Abstract

The Weija Reservoir has a high algal content which has posed water quality problems in the reservoir for a long time. Chlorophyll-a has been determined in water samples from the surface (0–10 cm) of the Weija Reservoir, collected at 3 different sites in the Reservoir from 2005 to 2008 at bi-monthly intervals. Chlorophyll-a concentrations were measured to determine its impact on the water quality of the reservoir, assess levels of eutrophication in the reservoir, and suggest ways of mitigating pollution in the reservoir. Chlorophyll-a concentrations in the reservoir were found to be high, ranging from 1.00 – 7.46 mg/m3 in 2005, 8.75 – 114 mg/m3 in 2006, 2.69 – 60.7 mg/m3 in 2007, and 18.4 – 106 mg/m3 in 2008, respectively. The values indicate high algal biomass in the reservoir since water bodies with low levels of nutrients (e.g. oligotrophic lakes) have low levels of chlorophyll-a (< 2.5 mg/m3) whereas waters with high nutrient contents (especially those classed as eutrophic) have high levels of chlorophyll-a (5-140 mg/m3). These levels of chlorophyll-a were then used to determine the trophic status of the Weija Reservoir. Indications are that the Weija Reservoir is a hypereutrophic lake. Chlorophyll-a concentrations showed positive correlation with Total Phosphate (r = 0.21), but negative correlation with ammonium concentrations (r = -0.24). Chlorophyll-A concentrations also showed moderately high correlations with DO (r = 0.54), TSS (r = 0.60), and COD (r = 0.60), at the 95 % level. The high chlorophyll-a concentrations could impair the aesthetic values as well as contaminate the reservoir waters, and this may be one of the reasons why water from the Weija Reservoir has unpalatable taste, consequently, compounding the high cost of potable water production at Weija.