Abstract
The choice of ecological monitoring methods and descriptors determines the effectiveness of a program designed to assess the state of coral reef ecosystems. Here, we comparer the relative performance of the traditional Line Intercept Transect (LIT) method with three methods derived from underwater photogrammetry: LIT on orthomosaics, photoquadrats from orthomosaics, and surface analyses on orthomosaics. The data were acquired at Reunion Island on five outer reef slopes and two coral communities on underwater lava-flows. Coral cover was estimated in situ using the LIT method and with LITs and photoquadrats digitized on orthomosaic. Surface analyses were done on the same orthomosaics. Structural complexity of the surveyed sites was calculated from digital elevation models using three physical descriptors (fractal dimension, slope, surface complexity), and used to explore their possible influence in coral cover estimates. We also compared the methods in terms of scientific outputs, the human expertise and time required. Coral cover estimates obtained with in situ LITs were higher than those obtained with digitized LITs and photoquadrats. Surfaces analyses on orthomosaics yielded the lowest but most the precise cover estimates (i.e., lowest sample dispersion). Sites with the highest coral cover also had the highest structural complexity. Finally, when we added scientific outputs, and requirements for human expertise and time to our comparisons between methods, we found that surface analysis on the orthomosaics was the most efficient method. Photoquadrats were more time-consuming than both in situ and digitized LITs, even though they provided coral cover estimates similar to those of digitized LITs and yielded more than one descriptor. The LIT in situ method remains the least time-consuming and most effective for species-level taxonomic identifications but is the most limited method in terms of data outputs and representativeness of the ecosystem.
Highlights
The advent of SCUBA diving in the second part of the twentieth century facilitated direct observations of underwater marine ecosystems and prompted the development of coral reef survey techniques (Goreau, 1959; Loya, 1972; Riedl, 1980; Dahl, 1981)
In comparison Line Intercept Transect Method (LIT) on orthomosaics and photoquadrats from orthomosaic methods yielded significantly lower coral cover and there was no significant difference in coral cover estimates between these two last methods
Coral cover estimates done at outer reef slope sites SAL, SBL and TRC were higher than the two sampling surfaces (40m2 × 3 and 150m2 × 3) but lowest than LIT in situ method
Summary
The advent of SCUBA diving in the second part of the twentieth century facilitated direct observations of underwater marine ecosystems and prompted the development of coral reef survey techniques (Goreau, 1959; Loya, 1972; Riedl, 1980; Dahl, 1981). Photogrammetry enables the quantitative monitoring of physical (e.g., structural complexity: slope, fractal dimension, surface complexity) and biological features (e.g., cover of benthic communities, colonies size and abundance) of ecosystems over time (e.g., Storlazzi et al, 2016; Fukunaga et al, 2019; Price et al, 2019; Carlot et al, 2020) These new techniques and methods are likely to become new standards for reef surveying in the coming years (Obura et al, 2019; D’Urban et al, 2020) notably with new solutions helping to automate image analysis such as the widely used machine-learning CoralNet tool, which estimates of coral cover are highly comparable to those generated by reef experts (Williams et al, 2019) and other artificial intelligence applications (e.g., González-Rivero et al, 2016; Hopkinson et al, 2020; Mohamed et al, 2020). We summarize our comparisons of practical aspects of traditional and novel reef assessment methods with regard to project objectives, available manpower and financial resources
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
