The massive coral species Porites lobata from upwelling and non-upwelling environments on the Pacific coast of Panama was exposed to experimentally elevated water temperature and ambient UV radiation to assess the response of the coral-zooxanthella symbiosis. Our experiment demonstrated
that elevated water temperature in the range of 30 to 31°C caused bleaching in P. lobata, whereas no significant effect on the coralzooxanthella symbiosis was observed from exposure to ambient UV radiation. Corals maintained under experimental conditions for 31 d showed a significant
decrease in zooxanthellae density and chlorophyll concentration as a function of elevated water temperatures. Changes in the concentrations of chlorophylls a and c2 per zooxanthella were observed only when corals from the upwelling environment were exposed to high
water temperatures. Also, corals from the upwelling environment bleached earlier and more severely than those from the non-upwelling area. Corals returned to ambient conditions showed complete recovery in zooxanthellae density and chlorophyll concentration after 30 d. Bleaching and mortality
responses of P. lobata indicate a lower threshold to sea warming conditions in coral populations from upwelling Gulf of Panama sites compared to those from the non-upwelling Gulf of Chiriquí. Coral bleaching during the 1997–98 ENSO occurred under similar temperature/time
conditions as those examined experimentally in this study, namely 30–31°C per 30 d.
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