On August 6 (Tuesday), NIFS observed ‘hypoxic water mass’ in the Cheonsu Bay of west coast of Korea, causing concerns over damages to aquaculture organisms. 

*Hypoxic water mass refers to those in which dissolved oxygen concentrations were less than 3mg/L, causing damages to fisheries by interrupting respiratory activity of fish and shellfish. 

On August 5, NIFS observed hypoxic water mass with 1.99mg/L of dissolved oxygen concentration at the bottom layer of the northern Cheonsu Bay through its on-site observation utilizing the research vessel (R/V). 

The hypoxic water mass occurred in six years since 2018. The main cause was analyzed that as the temperature increased rapidly starting from July, it caused the thermocline between surface-bottom layers to be stronger, making seawaters in the two layers hard to mix. 

As of today, the distribution of hypoxic water mass, still in its early stage, covers some parts of the Cheonsu Bay, but is projected to expand to the areas around the Bay as the high water temperature in it is expected to last for the time being. 

When water temperature in the bottom layer of basins, where seawater circulations are not smooth, reaches 15℃~16℃, the microorganisms in the sediment actively decompose organic matters. In the process, the microorganisms rapidly consume the dissolved oxygen, generating hypoxic water mass.