Causes of fish kill in a natural water purification system

An incident of fish kill occurred in the detention pond of a natural water purification system. The objective of this study is to reveal the cause of the incident. Novel measurements unprecedentedly employed in this study include: SOD to measure sediment oxygen decay; PCR to find sediment microbial activity. Algal bloom was the main reason for the serious fish kill. Particle size analysis demonstrated that large particles were settling in the upstream region. Chlorophyll A, COD, SS, phosphorus and E coli were following the stream direction. The end of the measuring point, the detention pond, demonstrated the serious eutrophication which leads to diurnal pH and DO fluctuations which induce anoxic condition at night, which induced the fish kill. PCR and microbial optical investigation demonstrated that blue green algae were the predominant species. The results of this study can serve as reference for other natural water purification systems worldwide, particular for high luminance and temperature climate, particularly in tropical regions