Photogrammetric analysis reveals distinct topographic signatures for stony corals

Abstract

Structural complexity is well understood to influence the biodiversity, productivity, and ecosystem functionality of coral reefs through niche diversification. Digital surface models (DSMs) created through photogrammetric techniques enable a more accurate reef characterization through DSM-derived metrics such as terrain variability, slope, and curvature. Massive reef-building corals are highly susceptible to stressors and are typically replaced by weedier corals post-disturbance that decelerate calcification rates and result in reef flattening. Therefore, generating topographic footprints of stony corals with these structural metrics is critical to advance our understanding of how disturbances might alter reef function. We utilized structure-from-motion photogrammetry to create orthomosaics and DSMs of five pinnacle reefs in the Dry Tortugas, collected as part of Florida Fish and Wildlife Conservation Commission’s Coral Reef Evaluation and Monitoring Program (CREMP). These data products were employed to investigate fine-scale (1 cm) differences between the most abundant coral genera using six terrain metrics: topographic position index, surface area to planar area ratio, slope, profile curvature, maximum curvature, and minimum curvature. This study found that when divided into functional groups (i.e., massive reef-builders vs. weedy opportunistic species), stony corals were topographically distinct with varying levels of contribution to the reefs’ overall structural complexity. Massive corals such as Orbicella , Colpophyllia , and Montastraea had the highest relative abundance and also provided the highest structural complexity. Specifically, they had the highest topographic position index and slope values, and positive profile curvature, all of which together are indicative of rugose, steep, and convex forms. We also found that within the massive reef-builders, there was a topographic distinction between star corals ( Orbicella , Montastraea ) and brain corals ( Colpophyllia , Pseudodiploria ) based on their differing growth morphologies. Of the weedy corals, topography was much more variable, with Porites having topography similar to the massive species. Using this information, we can begin to create “topographic signatures” for corals, which could be utilized in large-scale mapping efforts to efficiently characterize benthic communities or serve as the foundation for machine-learning capabilities to automate species identification in DSMs and determine how a potential loss of reef-building corals will impact reef functionality and ecosystem services moving forward.