Trend of water quality in the Seto Inland Sea is as shown in Table 4-1. The achievement of the environmental standard of COD(chemical oxygen demand), a typical indicator of organic pollution, indicates that the water quality in the Seto Inland Sea has totally been improved by the control of total amount loaded and then 78 % of the sites achieved the standard in 1996. but it also shows no marked improvements in recent years.
However, Compared the achievement ratio(81%) in total coastal seas in Japan, it is lower. The achievement ratio in the sea zones classified as A category, which covers large parts of the coastal seas in Japan, is especially at low level (66 % in Japan and 47 % in the Seto Inland Sea). Moreover, the Seto Inland Sea has been suffering from the eutrophication problems, which caused by the abnormal reproduction of algae along as increased the nutrients.
Red tide, which is one of the eutrophication problems, occurs in broad areas in the Seto Inland Sea and 135 cases in 1997 were observed. 11 cases of the red tide appeared from March to November damaged the fishery.
Obviously, several preservation and protection measures must be completely implemented even from now on.
|
Year
|
COD
(mg/L) |
Transparency
(m) |
T-N
(mg/L) |
T-P
(mg/L) |
|
1975
|
1.6
|
6.6
|
0.30
|
0.028
|
|
1980
|
1.5
|
6.1
|
0.27
|
0.029
|
|
1985
|
1.8
|
5.5
|
0.33
|
0.029
|
|
1987
|
1.7
|
6.0
|
0.28
|
0.028
|
|
1988
|
1.7
|
6.1
|
0.29
|
0.029
|
|
1989
|
1.8
|
6.3
|
0.34
|
0.029
|
|
1990
|
1.7
|
7.2
|
0.31
|
0.027
|
|
1991
|
1.6
|
6.4
|
0.30
|
0.029
|
|
1992
|
1.6
|
6.6
|
0.33
|
0.029
|
|
1993
|
1.7
|
6.5
|
0.27
|
0.026
|
|
1994
|
1.7
|
6.7
|
0.27
|
0.027
|
|
1995
|
1.7
|
7.7
|
0.27
|
0.025
|
|
1996
|
1.7
|
7.3
|
0.31
|
0.026
|
Note: 1) after the data by Environment Agency of Japan
2) Values are mean values of the data in 18 open seas
Note: after the data by Environment Agency of Japan (December, 1997)
Figure 4-1 Trend of Achievement Ratio of Environmental Standard in COD in
the Representative Enclosed Coastal Seas in Japan
Note: after the data by Environment Agency of Japan (December, 1997)
Figure 4-2 Trend of Achievement Ratio of Environmental Standard in COD
in Each Classified Zones in the Seto Inland Sea