Date of Award
Diploria labyrinthiformis, Montastrea annularis
Through analysis of hermatypic coral incremental growth features, it is possible to determine the response of recent corals to their environments. In this way I have attempted to obtain a tool not only to gain knowledge of recent growth processes but also for reconstructing aspects of past environmental conditions associated with fossil coral growth. The procedure has been first to determine time relationships and characteristics of coral skeletal density bands and to next use banding for investigations into areal growth rate patterns, coral population age distributions, and coral-climate interactions.
Density band couplets, visible X-radiographically in medial sections of coral skeletons, are demonstrated to be formed annually for the corals Diploria labyrinthiformis from Bermuda and Montastrea annularis from Jamaica through radiochemical analysis for 228Ra and 210Pb. Density, width, and mass relationships of annual and subannual bands have been quantified by microdensitometer analysis of coral X-radiographs. Band width varies inversely with density and positively with band mass. The width of the low density (LD) band portion is greater than the width of the high density (HD) portion and contains greater mass of CaCO3. From observation of banding characteristics at the coral growth surface, the date of collections, and measurements of the width of the outer band portion, I conclude that the HD band for M. annularis from Jamaica, Barbados, and Bermuda forms during the late summer and fall months in response to warmest water temperatures. For Bermuda, Diploria spp. the LD band appears to be forming during spring and early summer. More detailed study is required for confirmation of season of band portion formation and for assignment of controlling environmental variables.
Areal patterns of mean growth rate and variability for collections of Diploria spp. corals from various stations on Bermuda reefs are best explained by wind energy distribution over the Bermuda platform. Protected areas show highest mean growth rates while mean growth is low at most exposed locations. Lowest growth variability on the reef margins corresponds to regions exposed to highest wind and hence wave energy, while more protected areas have higher growth variability.
Comparison of coral populations from inside Castle Harbor, an area extensively dredged in the early 1940's, with populations from external reef areas has revealed that dredging, through probable increased resuspension, sedimentation, and turbidity, has been severely detrimental. Data on coral abundance, species distributions, coral growth patterns, and age distributions of both living and dead coral assemblages all support this conclusion as do detailed coral age and abundance measurements from transects established inside and outside the harbor.
Chronologies of yearly growth of Bermuda corals are demonstrated to be similar through visual and correlation methods as well as by analysis of variance. Comparisons of coral band width chronologies with recorded time series of environmental parameters reveals best matches with Bermuda South shore corals and air temperature where, with respect to long term averages, a negative relationship is evident. This result is best explained by the interpretation that coral growth is actually positively related to nutrient supply which is greater in colder years through increased upwelling. A minor component of coral growth may be positively related to temperature per se. Regression analysis reveals that it is possible to reconstruct temperatures using a coral master chronology (although a sufficient number of large old corals were not obtained in this study for reliable temperature reconstruction.
The banding characteristics of samples of Barbados M. annularis from the recent and each of three raised reefs (Barbados I, II, and III dated at 82,000, 105,000, and 125,000 yrs. B.P.) indicates that average growth rate and variability was lower in the 105,000 yrs. B.P. sample. To the extent that climate controls coral growth it would appear that the climate during the 105,000 yrs. B.P. high sea stand was significantly different from that of the Recent. A possible explanation is that sunlight was both lower in intensity and less variable than during either the present or the times of Barbados I and III.
Richard E. Dodge. 1978. The Natural Growth Records of Reef-Building Corals. Doctoral dissertation. Nova Southeastern University. Retrieved from NSUWorks, . (2)