1Kansai Airport Research Institute, Chuo, Osaka, 540-0012, 2Kansai International Airport, Co. Ltd., Izumisano, Osaka, 549-0001, 3Graduate school of agriculture, Kyoto University,
Seaweed production rates were estimated at an intact and two manipulated urchin-dominated seawall communities in Osaka Bay for three years from 2002 to 2005. Three experimental sections were made at a depth of 2 m, in which the densities of sea urchins were kept at three levels: ‘removal’ (1.3-4.8 ind./m2), ‘control (intact)’ (11.7-22.3 ind./m2), and ‘double density’ (30.0-59.6 ind./m2). Monthly feeding rates of sea urchins were estimated to be 5 to 20 g-dry weight/m2/month at ‘removal’, 40 to 80 g-dry weight/m2/month at ‘control’, 100 to 250 g-dry weight/m2/month at ‘double density’.
On the other hand, the biomass of seaweed community was maintained at a stable level within the range of 300 to 800 g-dry weight/m2 regardless of density of sea urchins. The amount of seaweeds grazed by sea urchins could be regenerated by the remaining thalli immediately, because the growth rates of seaweeds were much higher. Thus, seaweed production rates were larger where there was a higher density of sea urchins, resulting in the maintenance of seaweed community even at 2000 g-wet weight /m2 (60 ind./m2). The major reason for the coexistence of high density of sea urchins and seaweed community is supposed to be the eutrophicated nature of the area.