Diversity and composition of macroinvertebrates along river Rwizi in the Mbarara city catchment area.

Abstract

Wherever they occur, river systems are part of the primary means of disposal of wastes, especially industrial effluents and sewage discharges from surrounding communities. Surrounded by a large and rapidly growing population, river Rwizi has suffered anthropogenic disturbances whose impact on the integrity of the system is a subject of concern. However, studies on how the constituent components of the recent degradation are affecting the biological organisms are largely lacking. This study investigated the diversity and composition of macroinvertebrates as indicators of how the ongoing degradation of river Rwizi is affecting its biological resources. Macroinvertebrates in water and sediment were sampled from six sites (two sites each from the upstream, downstream and midstream) using a scoop net following a modified sampling protocol adopted from the Tanzanian River scoring system (TARISS).. The identified macroinvertebrates were analyzed for taxon diversity using the Shanon-Weiner index . Data on physicochemical parameter variations between the sites was tested using oneway ANOVA. Simple linear regression was used to determine the relationship between macroinvertebrate abundance and the physical and chemical parameters. The results revealed that eleven families of macroinvertebrates were recorded in the study area belonging to six orders. The river was dominated by phyla Arthropoda (90.91%) and Mollusca (9.09%), while no species of phylum Annelida was recorded. Family Gerridae was the most abundant. Generally, the downstream had the highest abundance of macroinvertebrates, followed by the upstream whereas the midstream had the least abundance of macroinvertebrates. The mean abundance of macroinvertebrates was significantly higher in the water column compared to the sediment. The wet season had higher abundance of macroinvertebrates compared to the dry season. This study revealed that the downstream had higher vegetation cover and minimal agricultural activities and human settlements which could have resulted in higher support for the abundance of macroinvertebrates. The study therefore recommends that a more comprehensive and longer study should be done to assess the impacts of other constituents of pollutants on the river. Also, the residents should be sensitized about the impacts of their activities on the river.