The reef morphology and the changes in coral community at Yamakawa

Reef morphology

Yamakawa site is located in an inlet and is protected from both northerly and southerly seasonal wind. The reef structure resembles that of the back reef lacking distinctive reef crest and spur-and-groove system. Reef rock extending from the beach inclines gently toward the reef front and then become steep at 3m depth. The bottom substrate deeper than 7m is coral rubble. The visibility tends to be low throughout a year due to turbid water.

Changes in coral community

Information on the status of coral communities at Yamakawa site before 1988 is unavailable, but a series of survey on the Crown-of-Thorns Starfish was conducted in early 1970s at a patch reef 250m south west of Yamakawa. According to the report, more than 90% of coral community comprised of tabular Acropora had been killed by the starfish predation in 1973. It is very likely that coral community of Yamakawa, even if it existed, also suffered predation in early 1970s in the same manner as the neighboring reefs.
In 1988, branching Acropora communities having the coverage of 29-89% were reported to occur on the reef flat of Yamakawa. There was also diverse coral communities comprised of Poritids and Faviids on the reef slope, although their cover was relatively low (13% at most). However, these coral communities completely disappeared between 1989 and 1991 because of repeated infestation of the Crown-of-Thorns starfish. Consequently the benthic communities of Yamakawa site shifted from hard corals to soft corals such as Sarcophyton and Lobophytum. Many juvenile Acropora colonies that attached on coral rubble in 1994 had died after the bleaching event. Since then coral cover remained below 3% until 2003. The average coral cover at the depth of 3m and 6m estimated by photo-transect survey in 2006 was 5.4% and 7.0%. The average cover increased to 13.3% and 17.7% in 2012, but this was due to the partial recovery of branching Porites community.