Long-term trends in water quality, water quantity and biodiversity at Lake Nakuru, Kenya
Routine in situ and laboratory measurements of pH, Conductivity, DO, temperature, alkalinity, salinity, TSS, TDS, Secchi depth and nutrient levels have been carried out at 13 sampling sites in Lake Nakuru (Kenya), influent streams and urban effluent since 1993. Mean values for 8 lake sampling sites are: pH (10.14), Temperature (27.3°C), Conductivity (38.8mS/Cm), DO (9.21mg/l, 83% saturation), Secchi depth (10.3Cm). Mid-lake sampling at 19 sites showed the presence of a thermocline of upto 4 °C gradient (surface to bottom). DO levels (>2m/l) remained within the tolerance thresholds for A. fusiformis and the fish (S. grahami). Trace metal concentrations in Lake Nakuru water and sediment as determined by X-ray fluorescence spectroscopy, varied over an order of magnitude for each of the metals for dry sediment and water samples. In 1995, results (ppm) ranged as follows: chromium (8.6-155), zinc (44-629), lead (4-102), copper (4.5-94.8), arsenic (1.5-3) and Nickel (1.5-16.5), mercury (1.5-3), selenium (1.5-8.8), titanium (748-14,367). In 1995 pesticide residues by Gas Liquid Chromatography (ppb) were: DDT (3.3-19.2), DDD (5.2-9.6), DDE (7.8-8.6), g- BHC (3.4-5,470) and dieldrin (1-29.1). River and sewage channel sediment revealed presence of heavy metal and pesticide residues. Mean Lake Depth (1992-2003) was 1.01m (range <0.1 to 4.5m). Lake volume ranged from 1.0 m3x107 to 18 m3x107. The long-term areal precipitation over the catchment is 940mm/annum. Isohyetal analyses show a decrease of rainfall towards the lake. The proportion of annual precipitation over each sub-basin was as follows: Njoro: 29%, Nderit: 23%, Makalia: 17%, Ngosur: 17%, Larmudiac: 6%, and Lion Hill: 8%. Annual runoff volume from each sub-basin is as follows: Njoro: 39%, Ngosur: 23%, Makalia: 21%, Nderit: 13%, Lion Hill 3%, and Larmudiac, 1%. Only 3.2% of the annual rainfall is available to recharge the basin. The highest evaporation coincides with periods of highest radiation and temperature. Evaporation is always higher than precipitation.
- Conference Papers