A quick, easy and non-invasive method to quantify coral growth rates using photogrammetry and 3D model comparisons (article)

Abstract

1. Coral growth rates vary significantly with environmental conditions and are thus important indicators of coral health and reef carbonate production. Despite the importance of this metric, data are sparse for most coral genera and species globally, including for many key reef-building species. Traditional methods to obtain growth rates, such as coral coring or staining with Alizarin are destructive and only work for a limited number of species and morphological growth forms. 2. Emerging approaches, using underwater photogrammetry to create digital models of coral colonies, are providing novel and non-invasive ways to explore colony-scale growth patterns and to address existing knowledge gaps. We developed an easy-to-follow workflow to construct 3D models from overlapping photographs and to measure linear, radial and vertical extension rates of branching, massive and encrusting corals after aligning colony models from subsequent years. 3. The method presented here was applied to measure extension rates for 46 colonies of nine coral species in the remote Chagos Archipelago, Indian Ocean. Proposed image acquisition and software settings produced 3D models of consistently high resolution and detail (precision ≤0.2 mm) and variability in growth measurements was small despite manual alignment, clipping and ruler placement (standard deviation ≤0.9 mm). Measured extension rates for the Chagos Archipelago are similar to published rates in the Indo-Pacific where comparable data is available, and provide the first published rates for several species. For encrusting corals, the results emphasize the importance of differentiating between radial and vertical growth. 4. Photogrammetry and 3D model comparisons provide a fast, easy, inexpensive and non-invasive method to quantify coral growth rates for a range of species and morphological growth forms. The simplicity of the presented workflow encourages its repeatability and permits non-specialists to learn photogrammetry with the goal of obtaining linear coral growth rates. Coral growth rates are essential metrics to quantify functional consequences of ongoing community changes on coral reefs and expanded datasets for key coral taxa will aid predictions of geographic variations in coral reef response to increasing global stressors