Water quality trends and input loads to Lake Nakuru

Water quality trends of Lake Nakuru, Kenya and its feeder streams were monitored in 2001-2004. Water samples were obtained from eleven stations in the lake and five points on inflowing streams. The items monitored were physico-chemical and biological components. The drainage basin was characterized by recurrence of dry spells throughout the period with water losses in the lake exceeding the amount received to cause a fall in water volume. The lake water salts content rose – conductivity and alkalinity reached 58 ms/cm and 5,200 mg/l respectively. PH was more or less constant (10.00 – 10.36). Daytime oxygen concentration in the mid-lake system rose (to greater than 20 mg/l) while remaining low in the inflow portions of the lake. On one occasion, the lake suddenly deoxygenated resulting to fish kills. Silt and organic loading were intensive in the north and south, contributing significantly to a decline in water transparency (secchi depth, 13 cm or less). Total phosphates varied significantly (4.39-9.128 mg/l). Of the nitrogen forms, only ammonia and total nitrogen were detected in appreciable amounts. The phytoplankton biomass was high and was mainly dominated by populations of Arthrospira fusiformis (mean, 158-2924 coils/ml). Various strains of Anabaenopsis spp. with Coccoids also occurred at low biomass. The only exception was in 2003 and 2004, when they developed into a dense bloom that was followed by flamingo mortalities. The northern portion had very low phytoplankton content while in the south, the phytoplankton community was one with relatively higher numbers of zooplankters and Anabaena spp. The yearly input load of the lake was 40,000 tons with Rivers Makalia and Njoro contributing significantly to the load. These results suggest eminent change of the lake ecosystem, which could arise due to human impacts from catchment degradation and urbanization.

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