Researchers at the National Fisheries University in Busan investigated the daily changes in dissolved oxygen levels and plankton species in a freshwater fishery in August 1972. They found that on clear days, ponds with abundant plant-based plankton and fish had varying oxygen levels throughout the day, with the highest levels in the afternoon and the lowest in the early morning. On rainy and windy days, ponds with many fish and rich plant-based plankton had the highest oxygen levels during the day and the lowest in the early morning. They identified 13 genera of plankton, with Cyanophyta and Chlorophyta being the most common. Sudden water discoloration events due to the death of plant-based plankton were observed, likely caused by high water temperatures and intense sunlight leading to excessive oxygen release and carbon dioxide depletion. Oxygen levels below 0.2ppm were associated with fish mortality in the ponds.