Effects of elevated water temperature on coral bleaching and survival during El Nino disturbance events

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Marine Biology and Fisheries

First Committee Member

Peter W. Glynn, Committee Chair


In the Galapagos Islands, Ecuador, two species of deep-dwelling (15 m depth) corals (Psammocora stellata and Diaseris distorta) survived elevated sea water temperatures that killed 95% of the coral living in shallow (2 m depth) water. These elevated temperatures occurred during the severe El Nino-Southern Oscillation of 1982-1983. Their survival was due to several contributing factors: (1) Psammocora was more resistant to bleaching than the shallow water coral, Pocillopora, (2) Diaseris was able to survive extended periods ($>$180 d) in the bleached state, and (3) the deep water habitat where Psammocora and Diaseris were studied experienced periodic cooling that may have mitigated El Nino-induced surface water warming.Psammocora is widely distributed throughout the Galapagos archipelago, but Diaseris can only be found at the study site at Devil's Crown, Floreana Island. These two free-living corals form a dense aggregation 100 m east of Devil's Crown in the Psammocora/Diaseris community. Monospecific aggregations of Diaseris can be found to the east and southeast, within several hundred meters of this site. The Psammocora/Diaseris community supports a large number of associated organisms, whereas the Diaseris community is devoid of macroscopic associates.Mean Psammocora growth rates were significantly (p $<<$.001) higher (11.2 mm y$\sp{-1}$) during moderate warming associated with the 1991-1992 El Nino compared to 1990-1991 (6.0 mm y$\sp{-1}$), a cooler year.Water temperatures at the Psammocora/Diaseris community (15 m depth) were significantly cooler (in all 22 monthly comparisons) and usually more variable (in 12 of 22 monthly comparisons) than those at the site of a former Pocillopora reef (2 m depth) within Devil's Crown. Water temperatures fluctuated in a diurnal cycle in shallow water, and followed a semi-diurnal period in deep water.Significant decreases in zooxanthellae number, and concentrations of chlorophyll a and c were observed in Diaseris during two experiments simulating El Nino sea water warming. Psammocora also demonstrated significant decreases in chlorophyll a and c, but did not bleach, and regained pigment during a 30 d recovery period. Though significant responses were observed in only one experiment in Pocillopora, this species exhibited 22% mortality in one experiment and 50% mortality in the other.Sediment cores documented the continued presence of Psammocora and Diaseris at the study site through a long period of carbonate sediment accumulation.


Biology, Ecology; Biology, Oceanography

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