Structural Complexity of Coral Reefs in Guam, Mariana Islands

Abstract

The complexity of tropical reef habitats affects the occurrence and diversity of the organisms residing in these ecosystems. Quantifying this complexity is important to better understand and monitor reef community assemblages and their roles in providing ecological services. This study employed structure-from-motion photogrammetry to produce accurate 3D reconstructions of eight reefs in Guam and quantified the structural complexity of these sites using seven terrain metrics: rugosity, slope, vector ruggedness measure (VRM), multiscale roughness (magnitude and scale), plan curvature, and profile curvature. The relationships between terrain complexity, benthic community diversity, and coral cover were investigated with generalized linear models. While the average structural complexity metrics did not differ between most sites, there was significant variation within sites. All surveyed transects exhibited high structural complexity, with an average rugosity of 2.28 and an average slope of 43 degrees. Benthic diversity was significantly correlated with the roughness magnitude. Coral cover was significantly correlated with slope, roughness magnitude, and VRM. This study is among the first to employ this methodology in Guam and provides additional insight into the structural complexity of Guam’s reefs, which can become an important component of holistic reef assessments in the future.

Coral reefs in transition: Temporal photoquadrat analyses and validation of underwater hyperspectral imaging for resource-efficient monitoring in Guam

“…Extensive coral mortality and increases in turf algae, cyanobacteria, and bare substrate in 2019 were a result of the combined pressure of unprecedented heat stress and the cumulative effects of the bleaching and low tide events in 2013, 2014, 2016, and 2017, as well as the ENSO-triggered extreme low tides in 2014 and 2015 [ 84 ]. Despite the significant decline in live coral cover, the structural complexity of reefs at Lafac Bay remained high (average rugosity of 2.81 and slope of 44 degrees) [ 115 ], indicating that the reef had not ‘flattened’ or eroded yet [ 28 ]. Examples of coral recovery following acute or recurrent disturbances have been reported for the tropical Pacific [ 37 , 99 ].…”

“…Site selection is an important consideration when conducting underwater hyperspectral surveys, as estimates of benthic cover and community composition have been confounded on reefs of low macrobenthic species richness, with high structurally complexity, and situated along steep slopes [ 87 ]. However, despite the significant decline in coral cover, Lafac Bay is still structurally complex [ 115 ]. As such, underwater hyperspectral imaging is capable of surveying reefs with considerable terrain complexity.…”