It is transparency that seasonally changes most. 

Transparency decreases as more suspension material exists such as soil particles and organic materials suspending in water.

It is also important to reduce the influx of such suspension materials containing nitrogen or phosphorus 　in order to improve the water quality of Akanoi Bay.

It is COD, an inidcator of organic pollutants, that hardly shows improvement among the monitored items.

There are a lot to be understood why COD does not improve and the causes and the countermeasures need to be studied.

COD is the amount expressed in concentration of oxygen consumed to chemically oxidize organic material in water.