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.
