Intra-annual timing of density band formation of Porites coral from the central Great Barrier Reef

Abstract

40 colonies of Porites, representing four species, were collected within a 2-wk period from three reefs of the central Great Barrier Reef. X-radiographs were made of 6–7-mm slices cut from each colony. Three, reasonably distortion-free, tracks across the density banding were identified for each slice. Density was measured along each of the tracks for each slice by gamma densitometry. Four observers identified the density characteristics at the youngest edge of each slice. This was done for both the X-radiographs and the density profiles. The edge represented the date of colony collection: all colonies were collected in November. Based on the density profiles, the observers generally agreed that this was when most of the colonies were in the process of, or had just completed, high density band formation. A considerable scatter in dating was evident between observers. Agreement between observers averaged 60% of the total number of possible agreements for the density profile method but only 27% for the X-radiograph method. Positions of fluorescent lines, resulting from dated river flood events, relative to the annual density banding pattern were examined in colonies from two of the reefs. The fluorescent lines were formed in the first 5 months of the year and were found to occur mainly in the fall in the annual density cycle from high density to low density skeleton formation. Positions of the fluorescent lines were measured on both sides of the skeletal slices. Positions of the fluorescent lines were displaced up to 10 mm through the 6–7 mm slice thickness. Density banding characteristics were also identified for the top edge (youngest skeleton) of 18 coral cores collected at various times of the year throughout the Great Barrier Reef. In general, data from these cores support the model in which high density skeleton in Porites from the Great Barrier Reef forms in summer and low density skeleton forms in winter. Exceptions to this model were found.