Abstract
Reef structural complexity provides important refuge habitat for a range of marine organisms, and is a useful indicator of the health and resilience of reefs as a whole. Marine scientists have recently begun to use ‘Structure from Motion’ (SfM) photogrammetry in order to accurately and repeatably capture the 3D structure of physical objects underwater, including reefs. There has however been limited research on the comparability of this new method with existing analogue methods already used widely for measuring and monitoring 3D structure, such as ‘tape and chain rugosity index (RI)’ and graded visual assessments. Our findings show that analogue and SfM RI can be reliably converted over a standard 10-m reef section (SfM RI = 1.348 × chain RI—0.359, r2 = 0.82; and Chain RI = 0.606 × SfM RI + 0.465) for RI values up to 2.0; however, SfM RI values above this number become increasingly divergent from traditional tape and chain measurements. Additionally, we found SfM RI correlates well with visual assessment grades of coral reefs over a 10 × 10 m area (SfM RI = 0.1461 × visual grade + 1.117; r2 = 0.83). The SfM method is shown to be affordable and non-destructive whilst also allowing the data collected to be archival, less biased by the observer, and broader in its scope of applications than standard methods. This work allows researchers to easily transition from analogue to digital structural assessment techniques, facilitating continued long-term monitoring, whilst also improving the quality and additional research value of the data collected.
Highlights
The physical structure of coral reef habitats is a strong determinant of the abundance and diversity of many reef-associated organisms (Graham & Nash, 2013; Darling et al, 2017)
Changes to reef structure can be ecologically relevant at a range of scales according to the reef’s associated organism’s body size; even centimetre-level changes in habitat can be important to reef community structure on a local scale (Nash et al, 2013)
Reef structure was recorded within varying visual grades of habitat complexity over a square 10 Â 10 m (100 m2) reef area following the method described by Polunin & Roberts (1993), with grades ranging from zero to four, corresponding to: 0 = no vertical relief; 1 = low and sparse relief; 2 = low but widespread relief; 3 = moderately complex; 4 = very complex with numerous caves and fissures
Summary
The physical structure of coral reef habitats is a strong determinant of the abundance and diversity of many reef-associated organisms (Graham & Nash, 2013; Darling et al, 2017). Changes to reef structure can be ecologically relevant at a range of scales according to the reef’s associated organism’s body size; even centimetre-level changes in habitat can be important to reef community structure on a local scale (Nash et al, 2013). Typical monitoring budgets tend to be restrictive, so coarse visual or analogue methods that combine practicability with low cost—such as ‘tape-and-chain rugosity’ (Risk, 1972), broad qualitative visual estimation (Wilson, Graham & Polunin, 2007) or depth measures (Dustan, Doherty & Pardede, 2013)—are most commonly used. Structural assessments conducted using such methods tend, to be limited in scale due to SCUBA time restraints as they rely on researchers being in the water and are time-consuming to complete (Knudby & LeDrew, 2007; Harborne, Mumby & Ferrari, 2012)
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