Ecotoxicological assessment of rift valley lakes in Kenya and the potential health impact on the lesser flamingo population

Abstract

The Kenyan Rift Valley lakes which include; lakes Elementaita, Nakuru, Oloidien and Crater are located in Nakuru County. The lakes host a large number of the Greater Flamingo (Phoeniconaias roseus), Lesser Flamingo (Phoeniconaias minor), Great White Pelican (Pelecanusono crotalus) and other water birds. The Lesser Flamingo congregate in vast numbers at certain times in these lakes creating major tourist attraction sites which contributes significantly to the Gross Domestic Product (GDP) of Kenya. Lesser flamingos have been classified as near-threatened due to their declining population. In recent years, there have been massive die-offs, particularly within the Lesser Flamingo population; the most recent occurred towards the end of 2013. These mass deaths have been attributed to diverse causes that either relate to the availability of food for flamingos as influenced by the salinity or toxicology of the lake waters or direct impacts upon the birds’ population. The exact cause(s) of the deaths have not been conclusively established. Environmental pollution may be a contributing factor to the die offs. This study aimed at assessing the levels of fluoride and metals which included lead, cadmium, chromium and arsenic with the aim of establishing whether they have detrimental health impact on the birds’ population directly or indirectly. This was an eco-toxicological assessment of the above lakes. The methodology involved collection of water samples (n=40), sediments samples (n=51) for ecotoxicological analysis. Live and dead Lesser Flamingos (n= 6; n=2, respectively) opportunistically collected from the lakes were also sampled. Live birds were clinically examined in the field and then euthanized for post-mortem examination. Analysis of samples for fluoride was done by use of Orion fluoride meter model EIA 940. Lead, arsenic, cadmium and chromium were analysed using Graphite Furnace Atomic Absorption Spectrophotometer (GFAAS) model-Specter AA-10 Varian. The data was entered in excel work sheet and analysed using ―Instat +‖ computer statistical package. The results of the toxicological study showed that the mean concentration (±SD) of fluoride in water samples were 55 ± 28.87, 30.75 ± 2.99, 116.9 ± 70.25, 247.9 ± 170.4 ppm for lakes Crater, Oloidien, Elementaita and Nakuru, respectively. The high fluoride levels in Lake Nakuru could be attributed to the relatively high contribution from rivers with a mean of 19.25 ± 16.26 ppm and Nakuru town sewerage (6.5 ± 0.58 ppm) in addition to the natural leachate from volcanic soil, sediment and other lakes. However this fluoride concentration in water and soil sediments differed significantly (P< 0.05) between lakes. Mean fluoride concentration in the tissues of birds varied significantly (p< 0.05) from 122.5 ± 40.93 ppm in the bones, 3.12 ± 2.45 ppm in the skin and 0.40 ± 0.21 ppm in the muscle. These concentrations were positively correlated with soil fluoride levels. Lead and arsenic were found to be in high concentration in soil sediments in all four lakes while chromium and cadmium were in low concentration. The concentration (±SD) of lead in the soil sediments were 212.5 ± 44.25, 567.3 ± 46.12, 430.1 ± 122.1, 273.7 ± 67.12 ppb while that of arsenic were 409.3 ± 243.9, 512 ± 66.96, 354.4 ± 294.9, 265.5 ± 290.8 ppb for lakes Crater, Elementaita, Nakuru and Oloidien, respectively. Soil sediments analysed from the inflow of the Nakuru sewerage drain (1754 ± 22.81 ppb) and rivers to Lake Nakuru (1129 ± 107 ppb) had the highest concentration of lead. The concentration (±SD) of lead in the flamingo tissues were 11.09 ± 22.69, 141.6 ± 37.45, 1.89 ± 1.89, 17.36 ± 21.67, 24.59 ± 30.79, 16.68 ± 8.83, 13.9 ± 8.54 ppb while that of arsenic were 10.2 ± 2.72, 6.82 ± 2.57, 5.24 ± 2.27, 10.43 ± 3.25, 19.12 ± 4.68, 16.92 ± 6.1, 12.45 ± 3.87 ppb for muscle, bone, brain, heart, kidney, liver and lungs, respectively. Arsenic, cadmium, chromium and lead were also observed in bird tissues. Metals in the Lesser Flamingo tissues were below the toxicological levels that are reported in literature to be harmful, except lead which was above the level recommended by the US Environmental Protection Agency. It was concluded from the results that the toxicants detected were not the main cause of the die-offs and other factors may be involved. It is recommended that more data should be collected in order to conclusively determine the cause of the die-offs and population decline in the Lesser Flamingo.