Abstract
Growth and contraction of ecosystem engineers, such as trees, influence ecosystem structure and function. On coral reefs, methods to measure small changes in the structure of microhabitats, driven by growth of coral colonies and contraction of skeletons, are extremely limited. We used 3D reconstructions to quantify changes in the external structure of coral colonies of tabular Acropora spp., the dominant habitat-forming corals in shallow exposed reefs across the Pacific. The volume and surface area of live colonies increased by 21% and 22%, respectively, in 12 months, corresponding to a mean annual linear extension of 5.62 cm yr−1 (±1.81 SE). The volume and surface area of dead skeletons decreased by 52% and 47%, respectively, corresponding to a mean decline in linear extension of −29.56 cm yr−1 (±7.08 SE), which accounted for both erosion and fragmentation of dead colonies. This is the first study to use 3D photogrammetry to assess fine-scale structural changes of entire individual colonies in situ, quantifying coral growth and contraction. The high-resolution of the technique allows for detection of changes on reef structure faster than other non-intrusive approaches. These results improve our capacity to measure the drivers underpinning ecosystem biodiversity, status and trajectory.
Highlights
Coral reefs are among the most vulnerable ecosystems to climate change, owing to the thermal sensitivity of scleractinian corals[1], which bleach and can die when exposed to moderate increases in ocean temperatures
One of the most common metrics currently used for in situ studies is linear extension rate, which is often summarized as annual linear extension rate in centimeters per year for comparisons across studies[15]
Does not fully account for the complex, three-dimensional (3D) growth of corals, nor does it measure change in volume or surface area, which have an important influence on the structure and function of reef habitats for important taxa such as reef fishes[16]
Summary
Coral reefs are among the most vulnerable ecosystems to climate change, owing to the thermal sensitivity of scleractinian corals[1], which bleach and can die when exposed to moderate increases in ocean temperatures. Does not fully account for the complex, three-dimensional (3D) growth of corals, nor does it measure change in volume or surface area, which have an important influence on the structure and function of reef habitats for important taxa such as reef fishes[16] These limitations in accounting for fine-scale changes in carbonate accretion and erosion are partially overcome by explicitly quantifying changes in the weight and density of carbonate structures[17,18], but current methods require manipulating corals and are overly invasive. Recent advances in underwater photogrammetry enable 3D reconstructions from images of individual coral colonies to reef landscapes[20,21,22] They can effectively be used to monitor corals over time without disturbing or manipulating them, and measuring external structural change of corals with higher precision than existing methodologies. Given that 3D technologies are readily available to non-experts20,25, 3D reconstructions of corals can be achieved using off-the-shelf tools[20,25]
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