Assessment of the effect of anthropogenic activities on the ecosystem health of river Malaba using macroinvertebrates as bioindicators
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River Malaba, located in Eastern Uganda is a transboundary water resource shared by Uganda and Kenya, whose water flows to River Nile through Lake Kyoga. The river currently faces increasing environmental disturbance; especially from human activities such as; agriculture that includes crop farming and livestock grazing, sand mining, disposal of solid wastes, and factory effluents from the distillery and the upcoming phosphate factory, affecting its physico-chemical characteristics and subsequently the biological characteristics of the river. These in turn have negatively impacted on the river water quality hence diminishing its benefits to the riparian people. This study investigated the macroinvertebrates abundance, diversity and distribution in relation with the physico-chemical characteristics such as dissolved oxygen, pH, electrical conductivity, turbidity, total phosphorus, total nitrate-nitrogen, and temperature of the river. This is because macroinvertebrate species respond differently to different stressors in the assessment of water quality, hence their importance in biomonitoring of rivers. Water and sediment samples were collected from the five selected Sites; M0, M1, M2, M3, and M4 along the river to determine the physico-chemical characteristics and the macroinvertebrate abundance, diversity and distribution respectively. Most of the chemical variables, i.e., electrical conductivity, total phosphorus, total dissolved solids were significantly different (P < 0.05) among the sampling sites. Total nitrogen and nitrates were not significantly different (p > 0.05, one-way ANOVA) among the sampling sites. The pH of the various sampling sites revealed that the river had slightly alkaline water (8.13 – 7.78), but differences among the sites were not significant (P > 0.05, one-way ANOVA). A total of 580 individuals belonging to 11 taxonomic groups were encountered in the selected sampling sites of River Malaba; Diptera (43.3%), Ephemeroptera (27%), Collembola (14.4%), Hemipteran (7.81%), Trichoptera (4.0%), Coleoptera (1.86%), Odonata (1.7%), Plecoptera and Decapoda (0.2%) including Hirudinea and Oligochaeta. There was a decrease in taxa diversity downstream with 11 taxa in sampling Sites M0 and M1, followed by Site M2 with 6 taxa and lastly in Site M4 with 1 taxon. Generally, this study revealed that the physico-chemical characteristics of the river affect the distribution of macroinvertebrates. The study recommends that riparian communities minimize discharge of pollutants, to maintain health of the river. Authorities should restrict the public from planting crops a few meters to the river banks, but rather encourage them to conserve the river bank vegetation to reduce on sedimentation into the river.