Defining multi-scale surface roughness of a coral reef using a high-resolution LiDAR digital elevation model

A decline in coral reef surface roughness may indicate that a coral reef ecosystem is approaching functional collapse. This is because surface roughness underpins many critical ecosystem metrics, such as, live coral cover and high fish biomass. Yet, we understand very little about how surface roughness changes at ecosystem scales due to the limited spatial extents of coral reef ecological surveys. Here, we investigate the surface roughness of a coral reef across multiple spatial scales on a high-resolution Light Detection and Ranging (LiDAR) derived digital elevation model (DEM, 0.25 m cells). We found that complex features dominated regions with high roughness (e. g., spur and groove systems on the reef slope) and increased the average roughness of geomorphic zones that are otherwise flat and featureless (e.g., lagoonal patch reefs in the sandy deep lagoon). Each geomorphic zone had a unique signature with high values of roughness usually expressed over finer spatial scales and, conversely, lower values of roughness observed over broader spatial scales. We allocated each geomorphic zone into one of four roughness equivalent habitats (REHs 1-4) that share similar surface roughness properties but are not necessarily geographically contiguous regions. These results identify fine scale features and geomorphic zones that are important for providing roughness to coral reef systems, such as spur and groove systems and the fore-reef slope respectively. They also suggest that morphodynamic processes operating at broader spatial scales influence the physical structure of coral reef ecosystems. The quantification of coral reef surface roughness is becoming increasingly important due to the wider availability of high (<1 m) and hyper (<0.1 m) resolution DEMs. Our approach and findings here can be used to provide greater surface textural information for common coral reef geomorphic zones and aid future management and research efforts, such as, monitoring of coral reef ecosystem response to environmental change and reef restoration and adaptation programs.