Gastropod distribution in Lakes George and Edward, Uganda, relative to copper and cobalt levels

Abstract

Published data show that Lake George, Uganda, has a poorer gastropod fauna than Lake Edward, to which it is connected through the Kazinga Channel. We conducted a study in 2001 to test whether this could be linked to copper (Cu) and cobalt (Co) loading of Lake George, resulting from previous mining activities just north of the lake. Snail and/or sediment samples collected along a transect from north of the mining site, through the runoff area from the stockpiles, Lake George, the Kazinga Channel to the northern part of Lake Edward were examined for Cu and Co content. Copper concentrations in sediment samples ranged from 3 to 855 ppm and Co from 6 to 481 ppm, being highest in the area closest to the mining activities and where the Nyamwamba River enters Lake George. About 54% of the total Cu was extractable in 20 mM EDTA, suggesting a large pool of bioavailable copper. The snail fauna was more diverse than expected, totalling 10 species, and was associated primarily with shore sampling sites, where sediment was firmer than in deeper water. Except for two species in one location, no snails were found where sediment levels of Cu were above 23 ppm and Co levels above 21 ppm, suggesting the existence of a threshold level limiting gastropod occurrence. Although the occurrence of snails was negatively related to Co level, there was no correlation between concentrations of Cu and Co in snail tissue and those in the sediment.

Keywords:

• bioavailability
• mining
• tissue concentration