Environmental controls on growth of massive Porites corals from the Great Barrier Reef

Brief description

Massive Porites colonies were collected from 29 reefs, located along the length and across the width of the Great Barrier Reef (GBR) between November 1987 and May 1992. The reefs were located within the following regions: Northern GBR (Lagoon Reef, Eel Reef, Portland Roads, Rocky Island, Night Island, Reef No. 13-055 and Reef No. 13-050); Cairns to Cape Melville (Tydeman Reef, Pipon Island, Flinders Island, Watson Island, South Petherbridge Island, Two Isles, Boulder Reef, East Hope Island, Undine Reef, Snapper Island, Batt Reef, and Double Island); Mission Beach (Stephens Island and Bedarra Island); Central GBR (Myrmidon Reef, Rib Reef and Pandora Reef); Southern GBR (Shaw Island, Reef No. 20-200, Credlin Reef, Reef No. 21-141 and Middle Percy Island). All colonies were between 0.2 and 0.5 m in height and were selected from similar environments towards the rear of the windward reef flat and on sheltered parts of fringing reefs in the lee of the islands at depths between 3 and 5 m relative to mean low water spring tide levels. Between six and 15 colonies were collected from each site. Of the 307 colonies collected, 245 were found suitable for analysis of growth characteristics. The species of coral colonies analysed were: Porites lutea, Porites lobata, Porites australensis, Porites solida and Porites mayeri.Colonies were cut in half vertically and at least 2 slices, approximately 7mm thick, were cut from the centre of each colony, dried and X-rayed. The positive X-ray prints were used to identify two tracks on each slice with clear annual density bands. Both tracks started near the origin of the colony, with one close to the vertical growth axis and the other horizontal. Skeletal density was measured at 0.25 mm intervals along each track using a gamma densitometer. The slices were dated by assuming that the high density peak closest to the outside edge had formed during the austral summer prior to collection. Each dated year represented the time between successive density peaks.From these measurements, spatial and temporal variations in three annual coral growth parameters; annual extension (linear distance between adjacent density minima (cm/y)), skeletal density (g/cm³) and calcification rate (the product of skeletal density and annual extension, g/cm²/y) were calculated. The three variables were then averaged across the two densitometer tracks for each corresponding growth year for the period 1979 to 1986, which was common to all 245 colonies.Measurements of the thickness of the coral tissue, which remained as a dark band around the outside of slices were made. Effective tissue thickness was determined by dividing tissue thickness by average annual extension to provide a measure of the time that the skeleton was covered by tissue.Sea surface temperatures (SST) were obtained from the Global Ocean Surface Temperature Atlas Plus, Version GIST 2.2 (GOSTAplus, Meteorological Office, UK). Monthly averaged data (in 1° latitude by longitude squares associated with each reef) were available for the period 1903 to 1994.Incoming solar radiation data were obtained from the International Satellite Cloud Climatology Project (ISCCP). Annual average data (in 2.5° latitude by longitude squares associated with each reef) were accessed for the period 1984 to 1990.

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