Effects of nutrients on the water quality and the algal community of river Nyakomisaro-Riana, Kisii, Kenya

Abstract

Water quality changes affect the ecological functioning of riverine ecosystems as well as their ability to support organisms and sustain environmental values. The study eighteen months study was conducted from May 2013 to October 2014 to determine the changes in water quality and algal community. Water samples for nutrient and algae composition were collected in triplicates monthly at five sampling sites along the Nyakomisaro-Riana River and laboratory analyses were conducted by following the standard methods described by the American Public Health Association (APHA). Data was oorganized, described and analysed with One-Way Analysis of Variances (ANOVA) and bivariate regression, using the Microsoft Excel 2010 and Minitab software’s and the significance of ddifferences considered at the p-value of  0.05. Tukeys pairwise multiple comparisons were used for post hoc analysis. The level of Total Phosphorous (TP) exhibited a general increasing upstream - downstream trend with an overall mean of 250.99±25.31 µgL-1 and a range of 2.59 - 1281.40 µgL-1. The Soluble Reactive Phosphorous (SRP) concentrations ranged between 1.66 - 632.00 µgL-1 with a spatial mean of 57.24±8.59 µgL-1 within a range of ., while Total Nitrogen (TN) concentration had an overall mean of 996.49±128.60 µgL-1 and ranged between 4.76 - 4332.33 µgL-1. The lowest mean TN concentration (664.87±90.49 µgL-1) was recorded at site NK1 while the highest mean (1349.27±146.48 µgL-1) was recorded at site R1. Nitrate -Nitrogen [NO3-N] concentration ranged between 0.18 - 742.90 µgL-1 with an overall mean of µgL-1 142.28±16.40 µgL-1. Regression analysis showed a positive correlation between Nitrate nitrogen and distance downstream (b = 15.05, a = 112.12, R 2 = 0.40). Out of 135 algal species recorded, 113 had a low percentage occurrence of less than 30%. The algal community comprised of seven genera of diatoms, six genera of chlorophyceae and cyanophyceae and three genera of euglenophyceae. The percentage algal abundance indicated the existence of a climax community,the blue green algae (Microcystis aeruginosa) with a dominance of 34%. This suggested that nutrient loading into Nyakomisaro-Riana River is high. The study recommends that regular monitoring of water quality parameters and algal community changes should be conducted to control the nutrient discharges into the river.