An assessment of the effectiveness of the use of benthic insects as bio-indicators of water pollution in selected rivers of Lusaka district

Abstract

Presently in Zambia, physico-chemical methods are employed to monitor water quality of the country’s rivers and other freshwater bodies. These conventional methods have not only proved to be expensive, but also tell us very little about impacts of pollution on aquatic organisms. With this realization comes the need to develop cheaper monitoring methods for freshwater bodies. Biomonitoring of pollution in freshwater bodies using live organisms called bio-indicators is one such cheaper alternative method. This study investigated the effectiveness of using benthic insects as bio-indicators of water pollution in selected rivers of Lusaka district, as an alternative water quality monitoring method. The study hypothesized that there was; no significant differences in pollution levels due to human activities between rivers within and those surrounding Lusaka district; no significant differences in benthic insect macro-invertebrate diversity and richness among rivers outside Lusaka district boundaries and those within the district; no relationship between levels of water pollution and benthic insect macro-invertebrate diversity and; and no benthic insect macro-invertebrate groups that were sensitive to water pollution in Lusaka District rivers that could be used as bio-indicators of water quality. Benthic insect macro-invertebrates were collected from six different rivers in and around Lusaka using a dip-net. These were enumerated and identified. Water samples were collected at each sampling area and analysed for physico-chemical parameters. Benthic insect macro-invertebrate diversity and evenness was determined using the Shannon-Weiner Diversity Index (H). The relationship of benthic insect macro-invertebrate diversity and richness with levels of water pollution was determined by correlating the family richness and family diversity to physico-chemical water characteristics determined for the selected sampling areas on each river. A total of 40 different insect macro-invertebrate families were collected. There was no significant differences (p ≥ 0.05) in the family richness of the benthic insect macro-invertebrates in the rivers of Lusaka. Further, there were no significant differences (p ≥ 0.05) in pollution levels of the rivers within the city, and those outside the city. Nine benthic insect macro-invertebrate families had significant positive or negative correlations with physico-chemical parameters and hence were selected as bioindicators of water pollution in Lusaka District. Bioassay results showed the that less pollution tolerant macro-invertebrate species had high mortality levels in the river sites classified as ‘Disturbed’ whilst showing reduced mortality in river sites classified as ‘Good’ and ‘Natural’. This proved that there are benthic insect macro-invertebrate groups that are sensitive to water pollution in Lusaka district freshwater bodies that could be used as bio-indicators of water quality.