Leaf Litter Decomposition and Macroinvertebrate Assemblage in Stressed Tropical Urban Stream: Implications for Stream Ecological Integrity Assessment. In Case Of Awetu stream, Jimma, South Western Ethiopia.

Abstract

The impact of urbanization activities on streams has substantially increased in recent years. As result, most urban streams are losing their ecological integrities. Effective characterization of ecosystems integrity requires information from both structural and functional components an ecosystem. In this work we examined the ecological integrity of Awetu stream impacted by urbanization using changes in macroinvertebrate community as structural integrity measures and leaf litter decomposition as a measure of functional integrity. Macroinvertebrate derived metrics and biotic indices were used as measures of structural integrity. Leaf litter decomposition rates of exotic Eucalyptus globulus, native Salix mucronata and Syzgium guineense, and a standard cotton strip were used as ecosystem functional integrity indicator. A total of six study sites were selected along a gradient of increasing nutrient enrichment and habitat degradation in the study stream. Selected pollution indicator parameters like TSS, conductivity, BOD, SRP, ammonium and nitrate were highly increased from upper stream to downstream sites. Measures of structural integrity, biotic indices and invertebrate metrics clearly discriminated upstream sites from heavily impacted downstream sites. The invertebrate community was sensitive to nutrient enrichment and habitat quality degradations as it responded negatively to increases in nutrient concentration, changes in the riparian vegetation from native to absent or exotic, and to reduction in the habitat quality. Litter decomposition rates also were sensitive to these changes, and lowest values were observed at the upper stream sites, which were classified as having a less impacted ecosystem functioning. The functional and structural approaches used in this study gave the same results for the most impacted and unimpacted sites. The results suggests that data on both stream structure and function are important for assessing ecological integrity of impacted tropical urban streams. Therefore, the incorporation of litter decomposition as a functional measures in evaluations of tropical urban streams ecological integrity is important.