Evaluating Impacts of Eucalyptus Plantation on Macroinvertebirate Assemblage in the Aquatic Environment, South Western Ethiopia

Abstract

Streams are important pathways for flowing of energy, matter, and organisms through the landscape. Stream water quality monitoring program relay on the chemical, physical, biological content of water and concentration of material present in the fresh water which is affected by a wide range of natural phenomena and human influences for aquatic life support existence. River water quality and aquatic biota could be affected by many riparian and non-riparian factors. Riparian vegetation cover and type are one of the riparian factors affecting the aquatic environment. Replacement of native vegetation by exotic plant species like Eucalyptus is one of the factors that result changes in the stream hydrology, organic matter dynamics and litter quality. The main objective of this study was to investigate impacts of Eucalyptus plantation on macroinvertebirate assemblages in the selected tributaries of Gilgel Gibe River in southwestern Ethiopia. A cross-sectional study of physicochemical and macroinvertebrates composition of streams in tributaries of Gilgel Gibe River, was carried out during January to May 2015, to assess their ecological status. Water samples and macroinvertebrates were sampled from 20 sampling sites (12 sites with Eucalyptus dominated and the rest with none Eucalyptus riparian vegetation). Physicochemical parameters were measured onsite and in the laboratory. To evaluate the effect of leaf litter on water quality, leaf of Eucalyptus globules, Croton merostachyus, Ficus sure and Salix subserrata were collected from the riparian zones of the streams. The leaves were air dried. 75 l water samples were brought from the selected tributary of Gilgel Gibe River by plastic container. The air dried leaves were added to a water sample and physicochemical parameter changes were monitored May to June 2015 for one month. Nonmetric Multidimensional Scaling analysis was used to test the level of similarity between none Eucalyptus, Eucalyptus, macroinvertebrates and water quality parameters. Besides, types of vegetation, physicochemical and macroinvertebrate relationship were analyzed using canonical correspondence Analysis (CCA) and Test statics Analysis. The total of 3133 macroinvertebrate individuals belonged to 36 families and 9 orders were identified. The Most abundant orders were recorded in none eucalyptus vegetation was Ephemeroptera 44% followed by Tricoptera 16%, While in Eucalyptus vegetation stream the most abundant macroinvertebrates recorded Ephemeroptera 18% followed by Odonata 17%. The water sample contains leaf litters of Eucalyptus grandis, Croton macrostachyus, Ficus sure and Salix subserrata were affecting the physicochemical parameters. For all sampled water the pH of Salix subserrata and Eucalyptus grandis leaf litters were less than seven while the pH of Croton macrostachyus and Ficus sure were above seven. For all water samples contain these leaf litters the electrical conductivity and TP were increased from time to time, but the EC of Croton macrostachyus more abrupt than the other leaf litter decomposition. In conclusion the water quality under the streams of none Eucalyptus vegetation was positively correlated TN, pH, and TP. However, Eucalyptus plant species have to be planted in the specific area where streams are not closely found.