Factor Analysis of Seasonal Fluctuations in Water Quality and Fish Mortality Events within Shallow Eutrophic Water under Freezing Conditions

Fish mortality was investigated in a shallow, constantly stored-water-type regulating pond located in a cold region in mid-March 2018 after ice had melted. In general, fish mortality occurs when water temperatures increase; however, in this case, it occured under freezing conditions. In this study, we attempted to understand the actual water quality by referring to the results of water quality surveys conducted throughout the year and the factors leading to the accident. The studied pond is eutrophic with treated wastewater. Phytoplankton proliferated during the non-freezing season by consuming nutrients in the treated wastewater, and organic matter derived from the phytoplankton accumulated as bottom mud. In early March 2018, the dissolved oxygen (DO) concentration in the bottom layer was almost 0 mg L, while it was also under 1.2 mg L in the upper layer. Concurrently, NH4-N leached out from the reduced bottom mud, and its concentration in the bottom layer increased to 25.4 mg L. The pH of the bottom layer increased to 8.96, and the free ammonia concentration was estimated to be about 3.0 mg L. These results indicate that the low DO and elevated NH4-N concentrations under freezing conditions directly caused this fish mortality incident.