Spatial variation of benthic macroinvertebrates and the environmental factors influencing their distribution in Lake Victoria, Kenya

Abstract

Benthic macroinvertebrates in the Kenyan waters of Lake Victoria were sampled on a monthly basis from March 1994 to March 1995. Ekman grab samples were obtained from 10 stations representing all the major ecological zones of the lake. The aim of the study was to estimate the abundance and diversity of benthos. The highest mean (± standard error) benthic density was 15113.4 ± 3885.3 m−2 at stations 53, which was significantly different from that of all the other stations (73.5 ± 16.8 to 1625.8 ± 275.4 m−2). Oligochaeta species, which are indicators of organic pollution, dominated the benthic abundance at station 53. Benthos in this station were exposed to both organic and inorganic pollutants as evidenced by highest level of nitrates and trace metals contaminants copper, iron, manganese and aluminum in water at the bottom of water column compared to that at all the other stations. The pollutants find their way to these sites from towns of Webuye, Eldoret, and Kakagema in the catchment through runoff and rivers. The elevated benthic abundance at station 53 could be due to organic and inorganic pollutants. Shannon index values for the stations in the Winam Gulf were more than triple the values estimated for the open waters implying the gulf had better water quality than the main lake. The gulf mean (± standard error) species heterogeneity values ranged from 0.7 ± 0.2 to 1.1 ± 0.1 whereas in open waters (except station 54) the values were 0.07 to 0.2 ± 0.05. This could be attributed to mixing of water throughout the water column in the gulf ensuring high oxygen levels throughout the water column, which favoured diverse benthos to flourish. In contrast, in deep waters (ca. 40 m) of the main lake stratification occurred most time of year resulting in anoxia in the hypolimnion. Thus, only the dipterans chironomids and chaoborids and oligochates that are adapted to withstand low oxygen levels were recorded, albeit in low densities.