Exploratory analysis of field derived data from in situ aquatic net-enclosure mesocosm and control stations, and the considerations for experimental design aquatic mesocosm were investigated in Togyo Reservoir at Chulwon, Korea. Physico-chemical parameters were measured and subsamples were collected weekly from surface layer of the each station from 5 November 1994 to 15 April 1995. Changing patterns for weekly fluctuation of phytoplankton biomass and relative dominant species within net enclosure(station N) were similar to the 2 control stations(Cl and C2), although nutrient concentrations had slightly different value at each stations during the study periods. Dominant species of phytoplankton changed dramatically from a Chlorococaloid(coccoid form) in autumn to dinoflagellate, Peridinium inconspicuum in spring at all stations. It has suggested that the field test for in situ aquatic net-enclosure mesocosmis successful for the ecological risk assessment. Trophic state of Togyo Reservoir according to its physicochemical parameters, phytoplankton biomass and species composition could be regarded as mesotrophic.