Impact of heavy metal pollution on the composition and abundance of benthic macroinvertebrates along Nariobi river, Kenya

Abstract

In this study the heavy metal pollution of a perenial tropical river, Nairobi River, was investigated over a period of six months from January to June 2001. Nairobi River has its source in an intensively settled and farmed area and flows through Nairobi City, the capital city as well as the industrial hub of Kenya. The river collects run-off from urban and agricultural areas, effluent from residential and industrial sectors as well as discharge from sewage treatment plants.

The concentration of calcium, magnesium, zinc, copper, cadmium, lead and manganese were determined in both water and sediment samples using a Varian Spectra AA-10, Single Beam Atomic Absorption Spectrophotometer (AAS). Benthic macroinvertebrates were sampled using a grab sampler or kick and sweep method and classified to family level. Their diversity was calculated using the Shannon-Weinner Diversity Index.

Calcium concentration ranged from 10.86 to 21.36 Mg/l in water and 1831 to 6181 Mg/Kg in sediments. The mean concentration of Magnesium ranged between 1.64 and 2.84 Mg/l in water and 456 and 2350 Mg/Kg in sediments. Sediment samples had higher heavy metal concentrations than water samples. The concentration ranges in Mg/l were lead (0.03-0.08), zinc (0.02-0.36), copper (0.05-0.2), cadmium (0.00-0.02), and manganese (0.49-1.83) in water. In sediments the ranges in Mg/Kg were lead (38.83-221.17), zinc (126.33-307.00), copper (7.03-100.17), cadmium (1.00-2.33) and manganese (1598.33-4322.83). Heavy metal concentrations were higher than the WHO recommended standards for drinking water and therefore not safe for human consumption. The major benthic macroinvetebrate families found in the river were Chironomidae, Hydropsychidae, Baetidae, Simuliidae and class Oligochaeta. The diversity index (H1) of benthic macroinvertebrate decreased downsteam with a range of 0.204-2.054.

Dissolved Oxygen (DO) and water pH were the major factors influencing the distribution of benthic macroinvertebrate in the river explaining about 44% and 22% variation in diversity respectively. Zinc in sediment was the only metal pollutant that showed a significant negative influence on the diversity of macroinvertebrate explaining 12% of the reduction in diversity. Lead in water had a negative influence of about 7%, while Cadmium in water had a positive influence of about 16% on the abundance of benthic macroinvertebrates.