Phytoplankton succession at the dam site and stream inlet site in Lake Soyang

Eun-Joo Lee, Jae-Ok Kim, Bomchul Kim, Kyu-Song Cho

Dept. Environmental Science, Kangwon National University, Korea

Phytoplankton community and environmental factors were surveyed from 1984 to 1997 at the dam site and a stream inlet site, Sangkullli, in a deep reservoir, Lake Soyang. Because of the dendritic shape horizontal mixing of water is limited and longitudinal gradient of plankton community and environmental factors are observed. Nutrients and chlorophyll a concentrations are higher at the stream inlet site than the central part of the lake. Peridinium bipes (a dinoflagellate) was the dominant species in 1980's which was replaced by Anabaena spp. (cyanobacteria) since 1990. The change of phytoplankton community coincided with the trend of eutrophication cuased by increase of fishfarming. Phytoplankton density showed peak in 1990 and since then it showed fluctuation depending on the fish production and storm runoff. When Peridinium was dominant, phytoplankton density was much higher at the inlet site becuase phototactic dinoflagellates aggregate toward the plunging point of inflowing stream. Dominant species in winter is ditoms, Asterionella spp., both at the dam site and inlet site, and it did not change with the change of trophic state.