Response of Millepora alcicornis (Milleporina: Milleporidae) to two bleaching events at Puerto Morelos reef, Mexican Caribbean.

Two naturally occurring colonies of Millepora alcicornis were monitored during 1997 and 1998, both years in which this species bleached in the Mexican Caribbean. One colony (HL) was naturally exposed to a high light environment and another nearby colony (LL) was exposed to 5.9 times lower light levels due to shadowing by a pier. For 10 days in August 1997, seawater temperatures in the surrounding reef lagoon rose up to 1.5 degrees C above the 6-year August average. The HL colony bleached to white during this period, whereas, the LL colony remained dark-brown colored. The HL colony recovered its normal dark-brown coloration (reversible bleaching) within several weeks, during which time the seawater temperatures returned to average. The following year, for 10 days, seawater temperatures rose up to 3 degrees C above the 7-year August average and both colonies bleached to white and neither colony recovered (irreversible bleaching). Both colonies were rapidly overgrown by algae and hydroids and, as of June 2003, no recovery has taken place. Prior to the 1997 bleaching, experiments using solar radiation showed that the quantum yield of photosystem II charge separation of branches from HL and LL colonies were affected for several hours by exposure to ultraviolet radiation (UVR, 280 to 400 nm), but recovered by the same evening, suggesting that UVR does not have long-term effects on photochemistry in M. alcicornis. In situ effective quantum yield of photosystem II charge separation (deltaF/Fm') measurements before the 1998 bleaching event indicate that both colonies were healthy in terms of the physiological status of their endosymbionts. During and after the 1998 bleaching event, both colonies showed a reduction in deltaF/Fm' and consequently an increase in excitation pressure on photosystem II. The data suggest that temperature is not the only factor that causes bleaching and that solar radiation may play an important role in coral bleaching.