The El Nino-Southern Oscillation (ENSO) bleaching event of 1997-1998 was a severe disturbance to coral reefs in the Galapagos. Sustained sea surface temperature anomalies of 3.5-4.5°C resulted in bleaching in a reported 70-90% of all coral species by February 1998, and eventual mortality was approximately 26%. In March 1998, we sampled the algal symbiont communities (Symbiodinium spp.} from 139 samples of bleached and healthy corals, and compared them with 20 samples taken from healthy corals in June 1997, before the bleaching event. Algal symbionts were identified using restriction fragment length polymorphisms (RFLPs) in large subunit ribosomal DNA, and denaturing gradient gel electrophoresis (DOGE) of the ITS-2 ribosomal region. Samples taken before the bleaching event (1997) contained a variety of Symbiodinium in clade C, but no clade D was detected, even in the scleractinian coral Pocillopora, which commonly hosts D at other sites in the far eastern Pacific (Panama, Mexico). Clade D was also rare in samples taken during the bleaching event (1998), although they were found in a few Pocillopira that were unaffected by bleaching, as well as in two almost-dead Pocillopora sampled with the intention of identifying symbionts still remaining in bleached tissues. These results suggest that the comparative scarcity of heat tolerant symbionts in the Galapagos may explain why bleaching-related mortality was relatively nigher in the Galapagos in 1997-98 compared to elsewhere in the eastern Pacific. These findings also indicate that heat tolerant symbionts may have been present in some hosts, but at levels that were not sufficiently high to affecting survivorship following bleaching. We hypothesize that the normally cool waters of the Galapagos result in the virtual absence of heat tolerant symbionts in clade D, resulting in these coral communities being particularly susceptible to the effects of high temperature bleaching in 1997-98.
Available at: http://works.bepress.com/joshua-feingold/4/