A Non-destructive Method to Create a Time Series of Surface Area for Coral Using 3D Photogrammetry

Daniel D Conley,Erin N R Hollander

doi:10.3389/fmars.2021.660846

Abstract

The wax dip method typically used to determine the surface area of corals for data normalization is destructive, rendering the collection of time series for such data impossible. With recent advancements in photogrammetric technology, it is now possible to collect these data in a non-destructive manner at very high levels of accuracy. This photogrammetric method using Agisoft’s Metashape is compared to the standard wax-dip method using both objects of known surface area and objects of unknown surface area. Objects of known surface area (i.e., objects that have surface areas that can be calculated using geometrical formulas) were estimated with a similar degree of accuracy with the Photogrammetry (PG) method (R2 = 0.9922, slope = 0.9835) as with the wax-dip method (R2 = 0.9872, slope = 1). A single factor ANOVA confirmed that there was no significant difference between measurements from the three methods of geometrical calculation, wax dipping, or photogrammetry for objects of known surface area. This paper describes the methods for rapidly collecting surface area data of small to moderately sized coral nubbins in a laboratory setting and characterizes the relationship between buoyant weight and surface area over time for the coral species Stylophora pistillata. Finally, two predictive models are proposed to estimate surface area from weight in air measurements.

Highlights

Surface area (SA) is an extremely important parameter in benthic coral ecology (Dahl, 1973), and it is commonly used as a metric to normalize oxygen production and consumption measurements, and other biomass-dependent data
The wax dip estimates of branching corals similar to the ones in this study were only 57% accurate compared to the computed tomography scan (CT) method (Naumann et al, 2009)
When Naumann et al (2009) compared surface areas for various species of coral derived with different methods, the wax dip method did not compare favorably with the CT method, which was assumed to produce the true value (Naumann et al, 2009)

Summary

Introduction

Surface area (SA) is an extremely important parameter in benthic coral ecology (Dahl, 1973), and it is commonly used as a metric to normalize oxygen production and consumption measurements, and other biomass-dependent data. This is due to photosynthesis and respiration occurring in the surface tissues of the coral. Most hermatypic corals have a thin layer of tissue that closely corresponds to the shape of the coral’s skeleton (Veal et al, 2010) For these reasons, SA is a better normalizing parameter than one like buoyant weight (BWt) (Dodge et al, 1984). While BWt is a good parameter for estimating health, growth, and calcification, due to the complex growth forms and differences in density exhibited by many corals, BWt may not be indicative of the SA and is not the preferred normalizing parameter (Dodge et al, 1984)

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