An Epidemic Zooxanthellae Expulsion During 1983 in the Lower Florida Keys Coral Reefs: Hyperthermic Etiology

Jaap, W

Extensive reef coral zooxanthellae expulsion occurred from Key Largo to Dry Tortugas, Florida, during September 1983. Coral bleaching was intensive between Pelican Shoal and Sand Key Reef off Key West. Coral discoloration extended to depths exceeding 14 m but was especially severe in shallow (1-2 m) spur and groove habitats. Approximately 75-95% of all Millepora complanata and Palythoa caribaeorum were bone white, but most colonies remained viable. Affected M. complanata (bladed fire coral) retained the ability to inflict pain from dactylzooid nematocysts. Some individuals (5 to 107.) had fine algal growth indicating death on all or parts of their skeletons. Although 15 species of cnidarians, principally Scleractinia, were affected, some species (Madracis mirabilis, Porites porites, Montastraea cavernosa, Dendrogyra cylindrus) appeard to be immune. A quantitative sample at Eastern Samba Reef on 6 October documented 11 species and 209 colonies; M. complanata comprised 32.5% of all colonies. Transmission electronmicrographs did not reveal epidemic pathogenic organisms in affected coral tissues. Warm, calm weather prior to the expulsion was conducive to elevated seawater temperature. A seawater thermograph deployed off Marquesas Key recorded temperatures of 32.3C during the period. A seven month post-event survey revealed that most hermatypic organisms had recovered normal color. Survival of hermatypic cnidarians was high. Twenty quadrats sampled during and seven months after the event were compared using classification analysis (Czekanowski's quantitative coefficient); mean similarity value among all quadrats was 0.69, and 12 of 20 quadrats had similarity values > 0.70. Values obtained using this technique in similar habitats generally ranged from .4 to .9 annually. The event is an example of a disturbance of intermediate frequency and magnitude controlling shallow reef communities.