Primary productivity of phytoplankton in Lake Soyang

Gilson HWANG, Dong-Sup KIM, Chul-Goo KIM, and Bomchul Kim

Dept. Environmental Science, Kangwon National University, Korea

Primary production in deep reservoir ecosystems is almost entirely performed by phytoplankton. The relationship of the rate of photosynthesis and irradiance is a fundamental property characterizing the activity of phytoplankton and the availability of nutrients. In this study, primary productivity of phytoplankton in a deep reservoir, Lake Soyang, were investigated from 1986 to 1995 by the Photosynthesis-Irradiance model. Primary productivity of phytoplankton were calculated in the unit of carbon assimilation per chlorophyll a content of phytoplankton in an hour(assimilation number-AN ; gC/gChl/hr) and carbon assimilation per unit area or water volume in a day(daily primary production-APd ; mgC/㎡/day or Pzd ; mgC/㎥/day), and in yearly rate(annual primary production-APy ; gC/㎡/yr). Pzd at the surface and 5m depth ranged 20 ∼ 1,000 mgC/㎥/day and 5 ∼ 200 mgC/㎥/day, respectively. The trophic state of Lake Soyang changed from mesotrophy to eutrophy within a decade. APd in warm seasons was 0.6 ∼ 1.0 gC/㎡/day until 1989 which can be classified as mesotrophic, but it increased to the eutrophic level of 1.0 ∼ 5.0 gC/㎡/day since in 1990. During this periods, the base level concentration of total phosphorus in the inflowing water showed increasing trend from 5 mg/㎥ to 10 mg/㎥. With the increase of phosphorus concentration the annual primary production per unit area(APy) showed the increasing trends in the range of 120 ∼ 420 gC/㎡/yr during the study period.