Search

Photogrammetry is a common tool for evaluating ecosystem-scale questions on coral reefs due to the ability to measure complex structures in situ. This technique is also increasingly being used at smaller scales to collect growth and morphometric data about individual coral fragments in manipulative experiments. However, there are substantial uncertainties in data quality and interpretation and limited reporting of the parameters useful for standardization across studies. There is a need to characterize the capabilities of photogrammetry as applied to coral fragments, to establish validation metrics for reporting, and to determine sources of variation in measurements to refine and improve methods. Here, we used fragments of two common reef-building corals (Montipora capitata and Porites compressa) and known size standards to evaluate accuracy and precision and present suggested validation metrics. We also used a tiered experimental design to evaluate sources of error in a photogrammetry workflow in a manipulative experiment using a purpose-built multi-camera workstation. We demonstrate extremely high accuracy (R2 > 0.999) in determining the surface area and volume of known objects, as well as very high precision (average CV < 0.01) in coral measurements during tests of reproducibility. These outcomes show the utility of fragment photogrammetry for experimental coral reef science and present suggested validation metrics and approaches that can help standardize data evaluation and interpretation in any application of photogrammetry to coral fragments.

Corals are habitat-forming organisms on tropical and sub-tropical reefs, often displaying diverse phenotypic behaviors that challenge field-based monitoring and assessment efforts. Symbiont chlorophyll (Chl) is a long-recognized indicator of intra- and inter-specific variation in coral’s response to environmental variability and stress, but the quantitative Chl assessment of corals at the reef scale continues to prove challenging. We integrated field, airborne, and laboratory techniques to test and apply the use of reflectance spectroscopy for in situ and reef-scale estimation of Chl a and Chl c2 concentrations in a shallow reef environment of Kāne‘ohe Bay, O‘ahu. High-fidelity spectral signatures (420–660 nm) derived from field and airborne spectroscopy quantified Chl a and Chl c2 concentrations with demonstrable precision and accuracy. Airborne imaging spectroscopy revealed a 10-fold range of Chl concentrations across the reef ecosystem. We discovered a differential pattern of Chl a and Chl c2 use in symbiont algae in coexisting corals indicative of a physiological response to decreasing light levels with increasing water depth. The depth-dependent ratio of Chl c2:a indicated the presence of two distinct light-driven habitats spanning just 5 m of water depth range. Our findings provide a pathway for further study of coral pigment responses to environmental conditions using field and high-resolution airborne imaging spectroscopy.

Coral reefs are threatened by climate change, overfishing, and pollution. Artificial reefs may provide havens for corals, both to escape warming surface waters and to assist in the geographic migration of corals to more habitable natural reef conditions of the future. The largest artificial reefs have been generated by nearly 2000 shipwrecks around the world, but the coral diversity on these wrecks is virtually unknown. Ship size and hull material, location relative to natural reef, time since sinking, ocean currents, and water depth may affect coral diversity. As a test of the biodiversity capacity of very large sunken structures relative to surrounding natural reef, we carried out technical diver-based surveys to quantify genus-level coral diversity on 29 warships sunk in Bikini Atoll and Chuuk Lagoon. We also assessed whether ship length, as an index of substrate availability, and water depth, as an indicator of light and temperature, can serve as predictors of coral diversity. We surveyed a total of 9105 scleractinian corals. The total number of genera identified at Bikini was 34, and at Chuuk it was 51, representing 67% and 72% of genera found on natural reefs at Bikini and Chuuk, respectively. Ship length, but not water depth, was positively correlated with relative abundance and richness at the genus level. Our results suggest that very large wrecks can serve as havens for reef-building corals with a broad genetic diversity, expressed at the genus level, commensurate with corals found on neighboring natural reefs. The role of large artificial reefs could include protecting coral biodiversity from warming surface waters.