SKELETAL MICRO-DENSITY, POROSITY AND BULK-DENSITY OF ACROPORID CORALS

Users of the buoyant weighing technique to measure growth in scleracti nian corals have often assumed a specific gravity of coral skeletal ma terial equivalent to that of pure aragonite (2.94 g cm(-3)). Previous studies have demonstrated that this assumption is not valid for some t axa, although data are very limited. In this study, density of the car bonate skeleton, here termed micro-density, is determined for eight sp ecies of Acropora corals from three sites in Australia, using Archimed ean principles. Micro-density ranged from averages per species of 2.78 1 to 2.873 g cm(-3). Significant differences were recorded between som e species, and between some colonies of the same species, but micro-de nsity was relatively constant within a colony and for a single species between sites. Micro-density was below the value for pure aragonite b y 2-6%, indicating that variation in this parameter must be accounted for when comparing growth and porosity measurements of coral skeletons obtained from buoyant weight methods. Possible explanations for the d eviation from the specific gravity of pure aragonite are the variable inclusion of low-density organic material into the skeletons, and/or v ariation in the chemistry and micro-architecture of the skeleton. Skel etal bulk density and porosity were calculated for branch tips from a range of Acropora species. Precision of the porosity measurement for d ried specimens was reduced in the initial trials by the potential for coral skeletons to retain air in skeletal voids, introducing errors in to the measurement of skeletal matrix volume. A modification of existi ng techniques to calculate porosity on dried samples is recommended, i ncluding soaking in acetone to displace air from skeletal voids and to remove waxy residues, and vacuum infiltration by water. Skeletal bulk density of Acropora branch tips ranged from 0.86 to 1.68 g cm(-3) and porosity ranged from 33 to 70%. There was a good correlation between skeletal porosity calculated using displacement techniques and the sam e parameter measured from image analysis of branch sections. The use o f Archimedean principles to determine density is not new to coral biol ogy, and yet there has been no published standard methodology. The met hods described permit precise and accurate quantification of bulk dens ity, micro-density and porosity, using basic laboratory equipment, whi ch may provide a means of investigating the effects of environmental a lterations such as elevated dissolved nutrient levels. (C) 1998 Elsevi er Science B.V. All rights reserved.