Skeletal micro-density, porosity and bulk density of acroporid corals

Abstract

Users of the buoyant weighing technique to measure growth in scleractinian corals have often assumed a specific gravity of coral skeletal material equivalent to that of pure aragonite (2.94 g cm −3). Previous studies have demonstrated that this assumption is not valid for some taxa, although data are very limited. In this study, density of the carbonate skeleton, here termed micro-density, is determined for eight species of Acropora corals from three sites in Australia, using Archimedean principles. Micro-density ranged from averages per species of 2.781 to 2.873 g cm −3. Significant differences were recorded between some species, and between some colonies of the same species, but micro-density was relatively constant within a colony and for a single species between sites. Micro-density was below the value for pure aragonite by 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 deviation from the specific gravity of pure aragonite are the variable inclusion of low-density organic material into the skeletons, and/or variation in the chemistry and micro-architecture of the skeleton. Skeletal bulk density and porosity were calculated for branch tips from a range of Acropora species. Precision of the porosity measurement for dried specimens was reduced in the initial trials by the potential for coral skeletons to retain air in skeletal voids, introducing errors into the measurement of skeletal matrix volume. A modification of existing techniques to calculate porosity on dried samples is recommended, including 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 same parameter measured from image analysis of branch sections. The use of Archimedean principles to determine density is not new to coral biology, and yet there has been no published standard methodology. The methods described permit precise and accurate quantification of bulk density, micro-density and porosity, using basic laboratory equipment, which may provide a means of investigating the effects of environmental alterations such as elevated dissolved nutrient levels.