关键词: 氮添加, 土壤呼吸, 微生物生物量, 土壤酶活性, 草甸草甸, 刈割, 生长季

Abstract: To evaluate plankton community structure and eutrophication status in Dongting Lake, water quality and the current situation of plankton community structure were investigated inAugust 2017. There were 24 sampling points, which distributed in “Sankou”, “Sishui” and other inflow river cross sections, lake regions and outlet sections. The results showed that phytoplankton in Dongting Lake had a total of 75 species belonging to 53 genera from six phyla. The average algae density was 458.3 × 10⁴ cells·L^-1, and average biomass was 0.92 mg·L^-1 (ranging from0.06-3.64 mg·L^-1). Cyanophyta and Bacillariophytadominated in Dongting Lake, which made the lake a Cyanophyta Bacillariophyta lake.Aulacoseira granulata and Cyclotella comensis were the dominant species in “Sankou” while Aulacoseira granulata and Microcystis aeruginosa dominated in “Sishui”. Aulacoseira granulata, Microcystis aeruginosa, Merismopedia tenuissimaand Phormidium tenue were dominant species in lake regions. In terms of zooplankton, 20 genera from four phyla were identified, with total density from 5.0 to 3.4×10³ ind·L^-1. Dominant zooplankton varied with “Sankou” (Difflugia), “Sishui” (Difflugia and Tintinnidium) and lake regions (Difflugia and Polyarthra). To evaluate the eutrophication level, we used three assessment methods, including comprehensive nutritive index, Shannon index and Margalef index. All indices indicated that the lake was under eutrophication. Compared with other freshwater lakes such as Taihu and Chaohu, Dongting Lake was positioned at the lower trophic level and had relatively lower algal density. However, the trend of historical eutrophication implied that the water quality status in Dongting Lake was not optimistic. As a result, more attention should be paid to water body of Dongting Lake and endeavor on ecological remediation is critically needed.

Key words: microbial biomass, growing season., soil enzyme activities, mowing, meadow steppe, nitrogen addition, soil respiration