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 understand the patterns of diversity and structure along the temporal and longitudinal gradients in two tropical lotic ecosystems in the wet and dry zones of Sri Lanka. Invertebrate abundance (annual means) was in the same magnitude in the two streams (2,520 ind. m−2 in the wet zone stream and 2,940 ind. m−2 in the dry zone stream). Both streams had similar annual mean diversity levels measured 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 ± 0.09 for Yan Oya. Along the longitudinal gradient, abundance and taxa richness increased toward the lower reaches in the wet zone stream but decreased in the dry zone stream. Composition of functional feeding groups was greatly influenced by abiotic factors in the temporal gradient than in the longitudinal gradient. This was possibly due to the seasonal patterns of flow regimes, and allochthonous nutrient inputs into the streams. Hence, resource management and conservation as well as attempts of ecological assessment in tropical streams should be based not only on the in‐stream characteristics but also on the catchment properties.