Abstract: An investigation on the trophic status and phytoplankton in Feilaixia reservoir in flood- and dry season of 2000, 2001 and 2002 showed that in the three years, total phosphorus (TP) concentration had a sharp decrease, while total nitrogen concentration (TN) had no remarkable variation. The species number of phytoplankton increased from 29 in 2000 to 99 in 2001 and 2002, among which, Chrolophyceae and Bacillariophyta contributed most, with an increase of 34 and 27, respectively. The phytoplankton abundance was ranged from 13.4×10⁴ cell·L^-1 to 41.6×10⁴ cell·L^-1, being the lowest in 2000, and the highest in 2001. In the flood season of 2000, the phytoplankton abundance was mainly contributed by cyanobacteria and dominated by Pseudanbeana spp., while in the dry season of this year, the abundance was mainly contributed by diatoms and dominated by Melosira granulate. In the flood season of 2001 and 2002, Chlorophyta and Cyanophyta had high abundance, and became dominant groups together with diatoms. Cyanobacteria was dominated by Psudoanbeana spp., Dactylococcopsis sp. and Gloeocapsa sp. Chrolophyceae was dominated by Chlamydomonas spp. and Dictyospharium pulchellum, and Bacillariophyta was dominated byCyclotella menighiniana andSynedra spp., while in the dry season of these two years, Melosira granulata and Melosira varians were predominated.

Key words: Oilfield exploitation, Drainage projects, Environmental risk, Fuzzy comprehensive assessment