Analysis of porewater extracted from coral heads from the central Great Barrier Reef

In March 1981, colonies (heads) of the massive coral, Porites lobata, were collected using SCUBA from Davies Reef and Britomart Reef, for analysis of the porewater within the coral skeletons. Coral heads, 15-30cm in diameter, were broken off, brought near the surface and transferred into large polyethylene tubs before being transferred onto a boat. Within 15 minutes to 2 hours of collection, the coral heads were taken out of the tubs and cleaved open. The living tissue and the band of endolithic algae were removed from the upper surface and blocks of about 1 litre in volume were removed from the head. All further handling of the coral blocks and porewater was carried out under nitrogen. Each block was placed inside a centrifuge designed to extract porewater. pH and dissolved oxygen concentration of the porewater were determined immediately after collection. The porewater was then transferred to acid-washed polypropylene bottles and frozen for nutrient analyses (phosphate, nitrate, nitrite, ammonia and silicate).Control blocks were prepared a month in advance by extracting blocks from coral heads, soaking them for a week in concentrated hydrogen peroxide, then immersing the blocks for a week in running seawater. Control water samples for analyses were obtained by soaking the blocks in ambient seawater for 2 hours and extracting the porewater as previously described. Ambient seawater samples were also collected and analysed. This study was initiated to determine whether the water in the interior of coral heads differed from the surrounding seawater and to investigate if evidence exists for nutrient regeneration and microbial respiration within the coral skeleton.