Estimating three-dimensional surface areas on coral reefs

One of the main obstacles for biological assessments of coral reefs over large spatial scales is the ability to link data obtained at the laboratory scale to spatially large data sets. This is particularly the case when trying to assess the ecological function of microbial processes following dramatic large-scale events such as mass coral bleaching. To be able to infer ecological function of field corals from laboratory measurement standardised to surface area it is imperative to be able to measure the actual surface area of corals in-situ. There have been several approaches proposed to estimate the three-dimensional surface area of field corals. While these have been shown to be reliable for simple coral growth forms, large degrees of error are introduced when applying them to complex growth forms. This paper refines a technique for calculating the three-dimensional surface area based on the projected surface area, with errors associated with complex growth forms reduced to < 5%. Once developed, the simple mathematical relationship (called the surface index) can be used to estimate the three-dimensional surface area of field corals from photograph or video imagery, allowing physiological parameters of corals determined at the sub-colony scale to whole colony and spatially large data sets of coral reefs. The effectiveness of using laser scanning techniques to derive three-dimensional images of corals is also discussed.