Biological assessment of water quality and biodiversity in Rwandan rivers draining into Lake Kivu

This study aimed at assessing the biological water quality and biodiversity of Rwandan rivers draining into Lake Kivu using benthic macroinvertebrates as indicator organisms. We tested the hypothesis that biological water quality and biodiversity are negatively affected by direct human perturbations and land-use changes in the surrounding habitat. Fifty-three rivers from 29 catchment areas were investigated using the 'Average Score Per Taxon' and the 'Baur Biotic Score' as indices for assessing water quality. The Shannon index and the Pielou's evenness index were used to measure biodiversity and evenness, respectively. Multiple linear regressions were applied to test the impact of hydromorphology, physical parameters, direct human perturbations and land-use change in the surrounding habitat on water quality and biodiversity. Baur Biotic Score and Average Score per Taxon indicated strong variation in the water quality of the investigated rivers. Best biological water quality was found in the rivers south of Kibuye/Karongi. Diversity was found to be moderate, but slightly increased south of Kibuye/Karongi. Evenness was also lowest south of Kibuye/Karongi. Multiple linear regression analyses indicated that the biodiversity estimations might be affected by the sampling time, but that the assessment of water quality was not. Water velocity significantly influenced water quality as well as biodiversity. Together with pH, it has also the strongest effect on the composition of macrozoobenthos assemblages. We showed that discharge of industrial waste water in the Burehe River resulted in an alarming deterioration of water quality that demonstrates the necessity of effective wastewater treatment for a sustainable usage of water resources. Long-term water quality monitoring is highly recommended to be established in Rwanda using a combination of physicochemical, bacteriological and biomonitoring data.