The most extensive and severe bleaching of coral ever recorded occurred in 1997 and 1998 as a result of anomalously high sea-surface temperatures. In the Ryukyu Islands, extensive bleaching occurred from late July to early October in 1998. A time-series transect study on a reef flat revealed the different responses of coral populations among species to this event. Coral cover was monitored just before, during, and 6 times after the bleaching occurred, along 5 fixed transects, 710 to 800 m long, across the reef flat of Shiraho Reef, Ishigaki Island, in the Ryukyu Islands of Japan. The dominant corals were Heliopora coerulea, massive Porites, and branching Porites, Montipora, Acropora, and Pavona (these latter 4 genera could not always be distinguished to species level, and are thus grouped by genus and ecoform). H. coerulea was the least susceptible to bleaching and maintained almost constant coverage before and after the bleaching. Massive Porites were susceptible to bleaching, but regained their algae after the bleaching and sustained their coverage. On the other hand, the branching Porites, Montipora, and Acropora were susceptible to bleaching and mortality was high. Coverage by branching Montipora was initially reduced by 66%, but by 2 yr after the bleaching it had recovered to pre-bleaching coverage. However, only large patches of more than 70 m along a transect, with coverage of 10 to 40%, recovered. Small patches of less than 30 m along a transect, with coverage of less than 10%, died and did not recover. The response of corals to bleaching differs according to the strategy the coral uses against bleaching; moderately frequent and severe bleaching might permit the coexistence of both types of corals on the same reef flat, as in the case of Shiraho Reef.

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To evaluate contamination of coastal sediments along Fongafale Islet, Central Pacific, a field survey was conducted in densely populated, sparsely populated, open dumping and undisturbed natural areas. Current measurements in shallow water of the lagoon indicated that contaminants from the densely populated area would only be transported for a small proportion of a tidal cycle. Acid-volatile sulfides were detected in both the intertidal beach and nearshore zones of the densely populated area, whereas these were no detection in the other areas. This observation lends support to argument that the coastal pollution mechanism that during ebb tide, domestic wastewater leaking from poorly constructed sanitary facilities seeps into the coast. The total concentrations of Cr, Mn, Ni, Cu, Zn, Cd and Pb were relatively high in all of the areas except the undisturbed natural area. The indices of contamination factor, pollution load index and geoaccumulation index were indicative of heavy metal pollution in the three areas. The densely populated area has the most significant contamination; domestic wastewater led to significant contamination of coastal sediments with Cr, Zn, Cu, Pb and Cd. The open dumping area is noteworthy with respect to Mn and Ni, which can be derived from disposed batteries.

Coral reefs have been threatened by various human and natural environmental disturbances, especially by widespread water temperature increases in 1997/1998. To understand the recovery of coral communities and shifts in their species compositions, long-term monitoring at the same location is important. Previously, we reported changes in the dominant taxa in a coral community before and after bleaching in 1998 at Shiraho Reef in the southern Ryukyus, Japan. In the present study, we continued monitoring the site for 15 yr to quantify how the coral community changed temporally and spatially. We used transect surveys and time-series aerial photographs and analysed the data with reference to seawater temperature and typhoon records. Net coral area along the transect lines increased from 1998 to 2003, but then decreased by 2008 mostly due to a decline in branching Montipora spp. The resulting net coral area was lower in 2012 than it was before the 1998 bleaching event. Aerial photographs also showed that the coral area at Shiraho Reef was similar between 1995 and 2000, but declined afterward. This decrease resulted from multiple disturbances, including bleaching events in 1998 and 2007, physical damage by 5 consecutive strong typhoons and likely inputs of sediments from heavy rain. Coral taxa reacted differently to the environmental stresses. The main change observed was a shift in the dominant taxa from branching Montipora and Acropora to Heliopora coerulea and massive and branching Porites. Those species have persisted due to high recruitment rates in H. coerulea and/or their tolerance to disturbances such as high thermal stresses, sedimentation and physical damage by typhoons.

Sessile organisms on PVC panels suspended inside and outside coral reef at Ishigaki Island, Yaeyama Islands, Japan, was monitored for 20 days just after the widespread incident of coral bleaching in September, 1998. The settlement of a total of 16 species of algae and invertebrates was recorded. Generally, Enteromorpha sp. (Chlorophyceae) dominated the panels. The percentage area cover, number of species, biomass and algal growth were significantly greater in waters outside the reef.