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Solar irradiation has been cited as a possible cause of bleaching in corals, either acting alone or in conjunction with other environmental factors. However, evidence of a solar involvement in naturally occurring bleaching is s t d largely conjectural. We have recorded a particular type of naturally occurring bleaching damage at intertidal sites at Phuket, Thailand for a number of years which has a strong directional component. Use of tidal data, sun track analysis, and solar irradiance measurements have enabled us to show that this bleaching directly corresponds to sun altitude and azimuth. Our work has shown that for the massive coral Goniastrea aspera, bleaching is induced during periods of subaerial exposure with high sun altitude and irradiance. Furthermore, on-site measurements of solar irradiance mitigate against the biologically damaging effect of shorter wavelength ultraviolet radiation (UVR) as a major causative factor. Desiccation, heating, or photochemical reactions by photosynthetically active radiation (PAR) (400 to 700 nm) remain possible candidates for further investigation

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Microatolls, those coral colonies with dead, flat tops and living perimeters, result from a restriction of upward growth by the air/water interface. The principal growth direction is horizontal and is recorded in the internal structure, though fluctuations in water depth can influence the surface morphology producing a terraced effect. The morphology of the basal surface of the colony is controlled by the sand/water interface such that the thickness of the coral records the depth of water in which it lived. In open water at the margin of reefs in the Northern Province of the Great Barrier Reef, tall-sided uneven-topped microatolls live, whereas, on the reef flats in rampart-bounded moats and ponds, thin flat-topped and terraced microatolls are abundant. Because water in moats can be ponded to levels as high as high water neaps (1.6 m above datum at Cairns) and still have daily water replenishment, microatolls on reef flats can grow to levels 1.1 m higher than open-water microatolls (which grow up to a maximum elevation of low water springs, i.e. 0.5 m above datum). This imposes a major constraint on the use of microatolls in establishing sea level history. The two factors controlling pond height during one sea stand (relative to the reef) are tidal range (which governs the height of high water neaps) and wave energy (which governs the height of ramparts which enclose moats). Dating and levelling fossil microatolls exposed on the reefs show that 4000 years (a) B.P., high water neaps was at least 0.7 m higher than it is at present.