Abstract: The tropical Asian river systems support a high array of aquatic biodiversity, especially its endemism. We choose the Nanla River in Xishuangbanna, one primary tributary of the Lancang River, as a case study to examine longitudinal patterns of macrobenthic biodiversity in July 2015. This river harbored a total of 96 taxa belonging to 5 phyla, 7 classes and 50 families. Among them, Insecta was the speciesrichest group (74 taxa), followed by Mollusca (8), Oligochaeta (9), Leech (3) and 2 others (Turbellaria and Nematoda). The dominant taxa across the whole watershed were Limnodrilus hoffmeisteri (accounting for 16.76% of total abundance), Cheumatopsyche sp. (10.60%), Polypedilum scalaenum (10.27%), Corbicula fluminea (5.01%), Bellamya aeruginosa (4.74%) and Choroterpides sp. (4.26%). Species richness, density, Margalef and Shannon diversity indices significantly decreased along the longitudinal gradient (from the upper to the lower reaches). The abundance of Oligochaeta, Chironomidae and Mollusca increased downstream at the expense of Ephemeroptera and Trichoptera. CCA analysis showed that four environmental factors (water depth and three types of substrate) and six spatial factors (PCNM14, PCNM10 and PCNM14) were the key drivers structuring the macrobenthic communities. Variance partitioning analysis indicated that the spatial factors had a stronger controlling effect on macrobenthic communities than environmental ones. This study provides useful information for the conservation of benthic biodiversity in tropical Asian rivers.

Key words: Portunus trituberculatus, biological characteristics, environmental factors, Laizhou Bay., temporal and spatial distribution