Bioassessment of water quality using benthic macroinvertebrates as bioindicators in Akagera river and wetland freshwater ecosystem

Abstract

Every living organism needs fresh and clean water to survive. Particularly benthic macroinvertebrates are critically affected by changes in water quality. Despite their central role in hosting aquatic organisms and valuable services to human being, freshwater ecosystems are critically affected by habitat loss for aquatic macroinvertebrates and degradation mainly due to human activities such as agriculture and different sources of pollutants. The main purpose of this study is to assess the quality of Akagera river and wetland freshwater ecosystem using macroinvertebrates as bioindicators. I assessed the distribution and abundance of aquatic macroinvertebrates in the rivers that enter Akagera and Akagera river itself that forms the upper Akagera river and wetland freshwater ecosystem where this study was conducted. Data were collected within four main sites including two sites on Akanyaru and Nyabarongo rivers, and two sites on Akagera river and surrounding wetland for benthic macroinvertebrate assemblages and physico-chemical parameters including pH value, conductivity, temperature, and water speed. Riparian land use types were also recorded with focus on natural vegetation and agriculture. Collected macroinvertebrates were conserved in labeled containers with 96% ethanol solution and taken to the Zoological Collection located in the Center the Centre of Excellence in Biodiversity and Natural Resource Management and later identified to the family level using the identification keys. A total of 1920 specimens were collected, of which 1865 macroinvertebrates were identified and grouped into 5 classes, 10 orders, and 21 families. The results revealed that Akagera river and the surrounding wetland are dominated by macroinvertebrates that are highly tolerant to water pollution. The most commonly found families were Dytiscidae (55.07 %, N = 1027), Veliidae (14.64%, N=273), Belostomidae (9.71%, N=181), Grynidae (6.33%, N=116) and Psauridae (6.22%, N=116). A few individuals were found that are considered as moderately tolerate pollution. These are Hydranaenidae (0.05%, N=1), Nemouridae (0.05%, N=1), Paleonomidae (0.16%, N=3), Perlidae (0.11, N=2) and Naididae (0.21%, N=4). Regarding physico-chemical parameters, the recorded average pH value averagely varied between 6.7 and 7.8, the conductivity varied between 122.1 and 138.8 (µS/cm), water speed varied between 0.3 and 0.6m/s, while temperature varied between 21 and 22 oC. The land use within the sampled area is dominated by agriculture mainly cropland and sugarcane plantations on Nyabarongo and Akanyaru rivers. Natural vegetation was the most common land cover on Akanyaru river and some patches on Akagera river. vii The upper Akagera river and wetland freshwater ecosystem seems to be polluted. The benthic macroinvertebrates found during this study indicated the dominance of highly tolerant macroinvertebrates and few of very low tolerant benthic macroinvertebrates.