Marked seasonality is observed in sea-surface temperature (SST) and photosynthetically active radiation (PAR) received by shallow water corals at Phuket, Thailand (7" N latitude). This seasonality is reflected in both algal densities and algal chlorophylls (a and C?) in 4 massive species of scleractinian corals, namely Coeloseris rnayeri, Goniastrea retiformis, Porites lutea, and G. aspera. Algal numbers and algal chlorophylls are generally maximal at the end of the wet season (November) and minimal at the end of the dry season (March to May). Algal densities are reduced by up to 53 % in May compared to November in some species, whde algal chlorophylls may be up to 4-fold greater in November. Concomitant with these changes are differences in algal cell volume in 3 out of the 4 species studied and alterations in host tissue depth, with smaller algal volume and greater tissue depth at the end of the wet compared with the dry season. Over the period 1993 to 1996 algal numbers showed a significant negative correlation with monthly (30 d) and short-term (1 to 2 d) values of both SST and PAR dose, while algal chlorophylls displayed a significant negative correlation with PAR dose in the 5 d preceding collection. Rising SST and increasing PAR in the dry season are paralleled by decreasing algal numbers and algal chlorophylls on an annual basis. Recovery of algal densities and a build-up in algal chlorophylls follow reductions in SST and PAR at the end of the wet season. In one of the years, when coral 'bleaching' was widespread at the study site (May 1995), the interaction between SST and PAR resulted in an 86 % reduction in algal densities in bleached corals compared to their annual maximum density. This study highlights the importance of seasonal fluctuations in physical environmental parameters and concomitant changes in the coravalgal symbiosis. Furthermore, it emphasises the need for greater understanding of the potential effects of seasonal and inter-annual variability on the status of algal symbionts, as well as identification of critical times of the year when visible bleaching may not be obvious but when algal numbers and algal chlorophylls are low and effects on coral physiology may be far-reaching.

HPLC analysis of pigments of the symbiotic algae of the coral Goniastl-ea aspera (Verrill) showed the presence of the chlorophylls a and c2 (chl a and c,) and the carotenoids pendlnin, diadinoxanthin, diatoxanthin and p-carotene. Increased production of diatoxanthin was seen In algae extracted from the western surfaces of corals, exposed to high levels of solar radiation, during a solar bleaching event. In addition, an increase in the ratio of diatoxanthin to the total xanthophyll pool was observed in bleached tissues. together with a progressive increase in the total xanthophyll pool (relative to total chlorophyll levels) over the bleaching period. These results support a potential photoprotective function for xanthophylls in the coraValga1 symbiosis. Chlorophyll breakdown products, phaeophytin a and pyrophaeophytin a, were recognised in considerable quantities only in bleached tissues. computation of a simple chlorophyll budget showed that 45% of chl a was lost in partially bleached tissues and approximately 62 % in fully bleached tissues, indicating a very rapid photodegradation of chlorophyll 24 h after exposure to high irradiance.

The solar radiation regime on differently oriented surfaces of the shallow-water coral Goniastrea aspera is markedly different Field experiments involving rotation of small (20 cm diameter) hemispherical colonies exposed hitherto protected surfaces of the colonies to high levels of photosynthetically active radiation (PAR) whilst reducing PAR on the opposite surface. Photodamage and recovery, respectively, were recorded in the symbiotic algae using non-invasive chlorophyll fluorescence techniques. Measurements of photochemical efficiency (FJF,) indicated that the Photosystem I1 (PSII) reaction centres of algae in previously protected surfaces suffered 'chronic' photoinhibition, probably on a timescale of 3 h, as a result of exposure to elevated solar PAR. Furthermore, evidence from this study corroborates earlier findings that this photodamage is a function of high PAR irradiance for short periods rather than a similar dose accumulated more slowly. Sin~ultaneously, algae on the opposite surfaces were observed to recover their photochemical efficiency, but this recovery was at a much slower rate involving several days at reduced PAR. The photodamage was sufficiently severe to lead to the formation of visible lesions (bleached areas). Observations of coral polyp retraction and nleasurements of the protective xanthophyll pigments in the algae showed behavioural and photoprotective mechanisms being deployed to mitigate the photodamage.

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