Undaria pinnatifida Habitat Loss in Relation to Sea Urchin Grazing and Water Flow Conditions, and Their Restoration Effort in Ogatsu Bay, Japan

Abstract

This study investigated the mechanism of the loss of Undaria pinnatifida beds in Ogatsu Bay, Japan. Wave heights at the outside of the bay were 1 to 2 m over the course of study. In the outer areas of the bay with high water velocities, more than 14.5 ± 3.4 cm sec -1 , U. pinnatifida grew densely and sea urchins were scarce. However, in some outer areas with lower velocities, less than 7.8 ± 2.3 cm sec -1 , U. pinnatifida grew sparsely where the aggregation of sea urchin was found. In contrast, in the inner areas of the bay with calm water having velocities of 2.4 to 4.6 cm sec -1 , the density of sea urchin was high and the U. pinnatifida beds disappeared. These results indicated high water velocities in the outer bay areas prevent the grazing by sea urchins. Disappearance of U. pinnatifida in the inner bay areas seemed to be caused by the high grazing pressure of sea urchins in calm water velocity conditions. We also performed a U. pinnatifida restoration effort to reduce the effects of the grazing pressure by sea urchins in the barren grounds in the inner areas of the bay. Artificial buoyed reefs were designed to prevent the migration of sea urchins by being detached from the bottom and allowed the recovery of U. pinnatifida and other non-encrusting macroalgae. Although there were some losses of transplanted U. pinnatifida partly caused by the withering after the reproductive maturation period, Saccharina japonica and other macroalgae were naturally recruited and increased due to the inhibition of migration by sea urchins using the buoyed reefs. In contrast, the formation of barren community remained at the area grounded to the bottom allowing the migration of urchin in the inner bay. Overall, our restoration efforts using the artificial buoyed reef, although not ideal, resulted in the success of the recovery of macrolagal habitats in the sea urchin -dominated barren grounds by the reduction of grazing pressure of sea urchins.

Sea urchin barrens as alternative stable states of collapsed kelp ecosystems

“…On the east coast of Japan, sea urchins Strongylocentrotus nudus caused a phase shift from kelp forest (Undaria pinnatifida and Laminaria japonica) to barrens in Ogatsu Bay in the 1990s, and prevented kelp regrowth for over 11 yr (Tamaki et al 2005(Tamaki et al , 2009. Along the west coast of Hok kaido, Japan, sea urchins were documented overgrazing the kelp forest in the 1930s and formed extensive coralline barrens by the 1960s (Matsunaga et al 1999, Fujita 2010, Graham 2010.…”

Effect of Water Flow on Grazing by the Sea Urchin (<i>Strongylocentrotus Nudus</i>) in the Presence of Refuge Habitat

“…This velocity that decreased the feeding rate by half was almost equivalent with the water flow in which sea urchin, S. nudus, did not approach algal communities and macroalgal habitats have remained in the field [4].…”

“…In a laboratory study using a flume tank, the feeding activity of the sea urchin, S. nudus, markedly reduced in the peak velocity exceeding 0.3 m/s [15]. In field surveys, however, there are many areas where S. nudus do not approach algal communities and macroalgal habitats have remained with water velocities less than 0.3 m/s, even which were around 0.1 − 0.2 m/s [4]. The difference in the water velocities required to prevent grazing by sea urchins might be attributed to changes in their behaviors by having access to refuge habitats or not.…”

“…Wave-induced benthic water flow is well known to inhibit the feeding behaviors of sea urchins [4,14,15]. High water velocities prevent the active grazing of sea urchins and restrict their distributions by dislodgment.…”

“…They provide suitable habitats for many commercial fishes and benthic animals [1,3,4]. However, subtidal macrophyte habitats around the world have declined due to human pollutants [5,6], overgrazing of sea urchins [4,[7][8][9] and herbivorous fishes [10], as well as natural disturbances [11][12][13]. Sea urchins are important agents of disturbance and are frequently regarded as proximate determinants of community structure and abundance of macroalgal habitats [14].…”