COMPARISONS OF SKELETAL DENSITY VARIATIONS IN PORITES FROM THE CENTRAL GREAT-BARRIER-REEF

Forty colonies of Porites, representing four species, were collected from three reefs along an inshore-offshore transect in the central Great Barrier Reef. A gamma densitometer was used to measure variations in density along three tracks across a slice of skeleton cut from a central growth axis of each colony. Twenty five colonies were selected for analysis on the basis of the density profiles and X-radiographic images of the slices. Several parameters concerning skeletal density and growth were recovered from each densitometer profile. These parameters described various features of the annual density banding pattern and annual growth rate of the coral colonies. An increase in average density of coral skeletons from the inshore to shelf-edge reef was accompanied by a decrease in annual growth rate. The mass of skeleton deposited per unit area per year (annual calcification) was highest in corals from the inshore reef and least in corals from the shelf-edge reef. Significant differences in the three growth parameters were found between reefs but not between species. No convincing evidence was found for common patterns of density and growth variation amongst colonies from different reefs or amongst colonies from the same reef. Indeed, no statistically significant common patterns were found amongst the three densitometer profiles across each skeletal slice. The appearance of density bands on X-radiographs of skeletal slices shows that the different densitometer tracks across a single slice should present information in common.