PAST AND RECENT LAKE EUTROPHICATION EVIDENCED BY MICROFOSSIL (OVER 160 YEARS) DIATOM SUCCESSION IN SEDIMENTS, LAKE XINGYUN (SOUTHWEST CHINA)

Eutrophication, disappearance of aquatic vegetation and fish introduction are important disturbances of aquatic ecosystems, especially in plateau lakes, which are generally considered to be very vulnerable. This study investigated the effects of eutrophication, aquatic vegetation and fish introduction on the diatom community of Lake Xingyun by examining the changes of microfossil diatom assemblage. Results showed that diatom assemblages changed with increasing trophic level, decreasing aquatic vegetation and fish introduction. A mesotrophic, alkaliphilous species, Aulacoseira ambigua, was dominant in Lake Xingyun before 1950, which indicated the lake was mesotrophic in the period. Epiphytic diatom species (Achnanthes minutissima, Denticula sp., Gomphonema pumilum) presented high relative concentration between 1950-1980, which indicated aquatic vegetation had a higher cover in this period. The number of diatom species varied from 20 to 66, and to 52 with the change of aquatic vegetation from the past to present. Epiphytic diatoms except Achnanthes minutissima disappeared with the decrease of aquatic vegetation after 1980. Since 1980, with the sharply rising of TP and TN concentration, mesotrophic species (Aulacoseira ambigua) disappeared and eutrophic species (Cyclostephanos dubius, Fragilaria crotonensis, Aulacoseira granulata, Nitzschia palea) were dominant. It indicates the lake has evolved towards eutrophication after 1980. Our study indicated that change of trophic status and aquatic vegetation are important driving factors affecting diatom community composition in Lake Xingyun, but that fish introductions have also been an additional factor.