Role of grazing by sea urchins on the persistence of a seaweed bed on a seawall in Osaka Bay; the effect of experimental density manipulation

Abstract

The effect of grazing by sea urchins was monitored in a three-year field experiment (June 2002 to June 2005) on a seaweed community established on a seawall in Osaka Bay. In the experiment, three levels of urchin density were used: one was unmanipulated (control), one from which urchins were removed manually every month (removal), and one to which urchins was added manually to twice that of the unmanipulated one (double density). The seaweed community was composed mainly of perennial red algae, such as Gelidium elegans and Chondrus spp., when the experiment commenced. At the double density of sea urchins, urchin grazing reduced these algae drastically in winter 2003, resulting in formation of large barren patches. A large annual alga, Undaria pinnatifida, occurred at a high density in the patches and flourished in the spring, followed by an increase in the density of large perennial algae, Ecklonia spp. The recruits of Ecklonia spp. grew rapidly from autumn to winter and dominated in biomass though they were damaged by typhoon from summer to autumn 2004. These results suggest that the grazing pressure of sea urchins plays an important role in the development and persistence of the seaweed community in the eutrophic waters.