The diurnal vertical migration of phytoplankton in Daechung Reservoir = 대청호 식물플랑크톤의 일주성 수직이동

Abstract

Water quality, species and cellular composition of phytoplankton with depth were investigated at two sites in Daechung Reservoir to evaluate possible causes of dominance and vertical migration of blue-green algae. In the summer of 1993, thermal stratification was very weak as a result of lower water temperature than an ordinary year. Dissolved reactive phosphorus was gradually increased with depth, but there was no significant difference among the data examined with an interval of 4 hrs. Daechung Reservoir was an eutrophic lake in which the cyanobacterium Microcystis sp. was dominant(over 95% of total phytoplankton on the basis of number). The diurnal pattern of chlorophyll a concentration in the water column was very similar to that of Microcystis sp. at Site 2. Cell carbohydrate, presumably functioning as ballast, of Microcystis sp. in the epilimnion was expressed on the basis of protein content and revealed a maximum value of 1.52 at 10 a.m. and a minimum value of 0.32 at 2 a.m. Consequently, it is likely that Microcystis sp. in the surface water to a depth of 1 m increases up to 60∼70% of total population at 10 a.m. and 2 p.m., and migrates vertically to the deeper to a depth of 4 m after 2 p.m. due to the accumulation of carbohydrates, and returns to the surface water after 6 a. m.