A Tale of Two Reefs: Quantifying the Complexity of Artificial Reefs and Natural Reefs Utilizing Structure-from-Motion 3D Modeling

Abstract

Anthropogenic impacts on coral reefs have stoked compounding pressures like bleaching, ocean acidification, and disease resulting in a significant global decline of coral reef cover that has weakened the resilience of adjacent coastlines to shoreline erosion. In lieu of natural defenses, coastal cities like Miami have installed artificial structures to serve as both breakwaters and refuges for coral recruitment as part of their reef mitigation initiative. However, the degree of manmade structures’ success in fulfilling these roles when compared to natural reefs is heavily debated. Photogrammetry data of three artificial and three natural reefs was collected off of the coast of Miami utilizing Structure-from-Motion methodologies to compare complexity metrics and coral coverage of each site. This approach utilizes complexity as a proxy to determine which type of habitat is more conducive to mitigating shoreline erosion and restoring coral populations. Sites were measured at geospatial scales of 1cm, 10cm, and 1m cell classes. Results revealed no significant difference between habitat types for any metric of complexity at any scale. This was an interesting finding considering the highest coral coverage was noticeably low at 30% for a natural reef, yet the low ratio of coral cover was still comparable in complexity to artificial sites. However, when artificial reefs were compared individually against the mean of combined natural reefs, a barge site indicated significant difference in complexity for all metrics on a 1cm scale, but with variable results for the other two classes depending on the metric. Coral coverage on all artificial sites was negligible, suggesting that coral recruits may exhibit site preference for natural reefs as opposed to barges or cement structures.