BLEACHING PATTERNS OF 4 SPECIES OF CARIBBEAN REEF CORALS

Bleaching of reef corals, involving loss of symbiotic algae (= zooxant hellae), loss of algal pigments, or both, has been linked to temperatu re stress. In this study the effects of high temperature and light on zooxanthellae living in the Caribbean reef corals Montastrea annularis , M. cavernosa, Agaricia agaricites, and A. lamarcki were studied. Pie ces of coral colonies were incubated at ambient seawater temperature ( 26 degrees + 1 degrees), and at 30 degrees, 32 degrees, and 34 degrees C. Symbiotic algae from M. annularis, a species of coral from the for ereef that commonly bleaches, showed the following sequence of events when exposed to natural light at 32 degrees C: loss of photosynthetic potential measured as fluorescence yield, corresponding reduction of b oth oxygen production per zooxanthella and P:R (photosynthesis:respira tion) ratio, and subsequent reduction in density of algae in relation to surface area of the coral. These parameters were not signifcantly r educed and no deaths occurred for M. annularis or any other coral spec ies maintained at 26 degrees or 30 degrees C. However, the sequence of events was condensed to less than 24 h when M. annularis was subjecte d to 34 degrees C seawater, except that there was little if any reduct ion in algal density before tissue-sloughing and death occurred betwee n 10 and 24 h. Loss of significant amounts of chlorophyll a per alga w as not evident for any corals except those maintained at 34 degrees C longer than 10 h. In contrast, symbiotic algae in M. cavernosa, a spec ies that rarely bleaches in nature, showed only slight reductions in p hotosynthesis and fluorescence yield, and no significant loss of algal cells or chlorophyll a, when maintained in seawater at 32 degrees C f or 2 days. Thus zooxanthellae in M. cavernosa appeared to be less affe cted by sublethal high-temperature stress. Similar contrasting pattern s of bleaching were seen in zooxanthellae from the plating coral Agari cia lamarcki, which often bleaches during the late summer and fall, co mpared with zooxanthellae from A. agaricites, a coral which bleaches l ess frequently. In addition, M. annularis exposed to sublethal high te mperatures and ambient light bleached faster than those kept in dimmer light, supporting past field observations suggesting that light energ y is an important component of bleaching in nature. When M. annularis was exposed to different wavelengths of natural light at 32 degrees C, the fluorescence yield declined more quickly in the presence of highe r energy UV-A and blue light than with other photosynthetically active radiation. Natural levels of UV-B had little effect in this study. Th ese data suggest that the patterns of bleaching seen in nature may be at least partially explained by different tolerances of the symbiotic algae in the corals, and that light plays a significant role in bleach ing.