The seasonal changes and composition of phytoplankton of the Choonsan reservior were studied from November 1990 to October 1991. 195 taxa which belong to 65 genera, 150 species, 40 varieties and 1 forms were identified. Species number during summer monthes was higher than that of other seasons. Standing crops of phytoplankton were varied from 7.8 × 10 exp (5) cells/ℓto 2.2 × 10 exp (6) cells/ℓ The dominant species were Cryptomonas erosa and Trachelomonas uolvocina. Chlorophyll a contents of phytoplankton was varied from 1.36 ㎍/ℓto 127.46 ㎍/ℓ. The biotic indices of phytoplankton were very similar among three sites. Saprobien-system by index of species diversity suggests that the Choonsan reservior belongs to the oligosaprophic.

A study on phytoplankton community was carried out from June, 1992 to May, 1993 at selected stations in Hoedong Reservoir. The phytoplankton are identified as 176 taxa including 5 phylum, 67 genera. The important species in this reservoir were Asterionella formosa, C. meneghiniana, Dictyasphaerium pulchellum, Fragilaria crotonensis, Melosira distans, M. granulata, M. granulata var. angustissima, M. granulata var, angustissima f. spiralis, Micractiniurn pussillum, Microcystis aeruginosa, Pandorina morum, Pediastrum boryanum, P. duplex, Peridinium sp., Scenedesmus quadricauda, Synedra acus, S. rumpens and S. ulna. The causative species of water bloom were identified as Microcystis aeruginosa, Trachellomonas hispida, Ceratium hirundinella, Peridinium sp., Melosira italica, Staurastrum dorsidentiferum var. ornatum in the area. During the study periods standing crops of phytoplankton were maximum in August, 1992 and minimum in December, 1992. The species dominance index and diversity index were ranged 24.7-99.9, 0.001-3.06, respectively.

Phytoplankton flora of Chinyang Lake and Hapchun Lake were studied. A total of 161 species(Chinyang Lake, 135 species; Hapchun Lake, 80 species) were identified. The major species of two lakes were Melosira granulate var. angustissima and M. italica. As the bloom causing species, Microcystis aeruginosa, Ceratium hirundinella, and Peridinium sp. were identified. The chlorophyll a concentration of Chinyang Lake was higher than Hapchun Lake.