Estimating 3-dimensional colony surface area of field corals

Colony surface area is a critical descriptor for biological and physical attributes of reef-building (scleractinian, stony) corals. The three-dimensional (3D) size and structure of corals are directly related to many ecosystem values and functions. Most methods to estimate colony surface area have been limited to laboratory settings and cannot be used for field corals. Photographic methods for digital 3D reconstruction were applied here to determine the accuracy of three different approaches for estimating colony surface area of field corals from simple underwater measurements. The approaches include a volumetric size-class method, a hemispherical surrogate and a suite of log–linear models generated from stepwise multiple regression analyses of digitally-reconstructed colonies. For each approach, surface area values were calculated from field measurements of colony size and the accuracy was determined by comparison with digitally-derived values for the same colonies. Accuracy varied among approaches; log–linear models (12% difference) were most accurate, followed by the hemispherical surrogate (17% difference) and size-classes (40% difference). The log–linear and hemispherical surrogate approaches are potentially applicable to at least nine common coral species. The photographic reconstruction method, although time-consuming and not intended for routine application, was shown by comparison with laser-scanned images to provide a highly accurate method for determining 3D colony surface area.