Luminescent lines in massive Porites corals from the Great Barrier Reef as proxies for mainland runoff

Brief description

Massive Porites colonies were collected from 1 reef in the Coral Sea and 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 Coral Sea (Ashmore Reef); 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 or 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 and 232 similar sized colonies were used for luminescence analysis. 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. Coral growth characteristics had been recorded from these slices using X-ray and densitometry in a previous study and slices were dated using the annual density banding patterns and gamma density traces. Each dated year represents the time between successive density peaks.Slices were viewed under UV light in a darkened room. Luminescence was assessed along 2 tracks close to the vertical growth axis and 1 horizontal track. For each slice and year the appearance of the luminescent lines was graded visually into one of four categories: 0 = no visible line, 1 = faint luminescent line, 2 = moderate luminescent line and 3 = intense luminescent line. A luminescence index was derived for each year that 2 or more colonies from the same reef were analysed. The index for a particular year and reef was the sum of the number of luminescent lines in each category multiplied by the value of that category, divided by the total number of colonies.Daily river flows were obtained from the Queensland Department of Natural Resources for the 9 major rivers that drain into the GBR lagoon: Normanby River, Daintree River, Barron River, North Johnstone River, South Johnstone River, Tully River, Herbert River, Burdekin River and Fitzroy River.A 3-D hydrodynamic model of the movement of freshwater river plumes from the Burdekin River was used to provide daily salinity variations at Rib and Pandora Reefs for the period 1966-1986. Measures of salinity derived for each reef were: average annual salinity; minimum daily salinity; number of days with salinity < 35 ppt; sum of daily salinity values < 35 ppt; and average of daily salinities < 35 ppt. Reefs were classified as inshore, midshelf and offshore. In addition, the linear distance (km) between each reef and the nearest mainland was estimated and expressed as a percentage of the shelf width at that latitude. The average water depth between each reef and the nearest point on the mainland was calculated from depths at taken at approximately 225 m intervals from a bathymetry data model for the GBR.

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