Longitudinal variation of benthic macroinvertebrate communities in two contrasting tropical streams in Sri Lanka.

Abstract

The benthic macroinvertebrate fauna and main abiotic parameters were studied to understandthe patterns of diversity and structure along the temporal and longitudinal gradients in twotropical lotic ecosystems in the wet and dry zones of Sri Lanka. Invertebrate abundance (annualmeans) was in the same magnitude in the two streams (2,520 ind. m−2in the wet zone streamand 2,940 ind. m−2in the dry zone stream). Both streams had similar annual mean diversity levelsmeasured as Shannon diversity (Eswathu Oya (wet zone) = 2.11; Yan Oya (dry zone) = 2.07),with a mean annual evenness (Pielou evenness) of 0.56 ± 0.14 for Eswathu Oya and 0.60 ± 0.09for Yan Oya. Along the longitudinal gradient, abundance and taxa richness increased toward thelower reaches in the wet zone stream but decreased in the dry zone stream. Composition offunctional feeding groups was greatly influenced by abiotic factors in the temporal gradient thanin the longitudinal gradient. This was possibly due to the seasonal patterns of flow regimes, andallochthonous nutrient inputs into the streams. Hence, resource management and conservationas well as attempts of ecological assessment in tropical streams should be based not only on thein-stream characteristics but also on the catchment properties.