Abstract: In order to understand the status of water quality in Maqu section at the upper reaches of Yellow River, the plankton community composition was investigated from April to May 2006. Twenty three species (genera) and 5 phylum of phytoplankton were observed in the whole section. The density of the phytoplankton was 32 ×104〖KG-*2〗-〖KG-*7〗54×104 cells·L-1, averaging 197×104 cells·L-1, and the biomass of the phytoplankton was 0069〖KG-*2〗-〖KG-*7〗0464 mg·L-1, averaging 0228 mg·L-1. The highest density was in Qihama, and the highest biomass was in Maqu Bridge. At the same time, 10 species (genera) and 2 phylum of zooplankton were observed. The density of the zooplankton was 670〖KG-*2〗-〖KG-*7〗1273 cells·L-1, averaging 88156 cells·L-1, and the biomass of the zooplankton was 0044〖KG-*2〗-〖KG-*7〗0089 mg·L-1, averaging 0064 mg·L-1. The composition of plankton in Maqu section was mainly of cold-water type, and the phytoplankton diversity index ranged from 2 to 3. These results suggested that in Maqu section, the water quality was of slightly polluted, the richness and evenness of plankton were lower, and the composition and structure of plankton community were instable, being easily disturbed by external environment.

Key words: Even-aged pure stands, Self-thinning rule, Density change, Genetic algorithms