Abstract: In April 2017, there was been an abnormal phenomenon of the “dark brown-water” in Lin Hekou Reservoir of Lanhe River, a tributary of the Hanjiang river basin, which caused panic within local residents. The water quality and the algae community structure were investigated to explore the mechanism for the “dark brown-water” in the reservoir. It was shown that the reservoir was eutrophicated and reached a heavy extent of algae bloom, and the dominating species were Bacillariophyta, Cyanophyta and Cryptophyta, of which, Bacillariophyta accounted for 90.4% of the total phytoplankton in terms of density; with respect to biomass, Bacillariophyta, Cryptophyta and Pyrrophyta were the top three phylum, and Bacillariophyta accounted for 92% of the total phytoplankton biomass. The predominant genera of diatom were Achnanthes exigua and Synedra spp., among which, Achnanthes exigua (98.2%) showed the highest abundance of dominance. Being eutrophic state of the Lin Hekou Reservoir, diatom blooms may be the main cause of “dark brown-water” and the diatom blooms are related to the factors of nutrient level, weather and hydrology conditions. With the aim of water safety of Central Line Project of South-to-North Water Diversion, catchment management and reduction of nutrients into reservoir are crucial for decreasing the risk of algae blooms in the channel-reservoir of Hanjiang river basin.

Key words: Hanjiang river basin, Lin Hekou Reservoir, dark brown water, diatom blooms, influencing factors