Along the Kansai International Airport islands, which are two artificial contiguous islands in the eastern area of Osaka Bay, a large-scale sloped seawall suited for seaweed bed development was constructed for the first time in Japan. The degree of coverage and areas of seaweed vegetation were monitored from 1989 to 2010. However, the change in standing stock of seaweed was not followed in the monitoring program. We conducted a new survey to convert the seaweed coverage data to standing stock for evaluation of the ecological functions of seaweed beds formed on the seawalls of the airport islands. The result showed that the total surface coverage of the airport island seawalls in 2010 was approximately 2.5 times higher than in 1999 but the total standing stock in 2010 was only approximately 1.4 times higher than in 1998. This is thought to be attributed to the significant decrease of Ecklonia that had been growing thickly near the first airport island, caused by reduced current speed and wave flows due to the second island. The annual production from the surface area of seawalls in 2010 is estimated at 44,680 kgC/y, 2,992 kgN/y, and 181 kgP/y, respectively. We estimate that 18% of the annual carbon absorption of seaweed beds that was lost in Osaka Bay in the past is absorbed by the sloped seawall area near the airport islands. The annual production from the slightly sloped seawalls (per unit area) in terms of carbon, nitrogen, and phosphorus is approximately 9, 7, and 8 times higher than that of vertical seawalls, respectively. In terms of the cost of constructing a sewage treatment facility, we estimate that converting vertical seawalls to sloped seawalls for a distance of 1 km would yield benefits of 8.2 million yen per annum in terms of nitrogen treatment amount and 4.5 million yen in terms of phosphorus treatment amount.
