ALBERT

Description: Coral reefs represent major accumulations of calcium carbonate (CaCO3). The particularly labyrinthine network of reefs in Torres Strait, north of the Great Barrier Reef (GBR), has been examined in order to estimate their gross CaCO3 productivity. The approach involved a two-step procedure, first characterising and classifying the morphology of reefs based on a classification scheme widely employed on the GBR and then estimating gross CaCO3 productivity rates across the region using a regional census-based approach. This was undertaken by independently verifying published rates of coral reef community gross production for use in Torres Strait, based on site-specific ecological and morphological data. A total of 606 reef platforms were mapped and classified using classification trees. Despite the complexity of the maze of reefs in Torres Strait, there are broad morphological similarities with reefs in the GBR. The spatial distribution and dimensions of reef types across both regions are underpinned by similar geological processes, sea-level history in the Holocene and exposure to the same wind/wave energetic regime, resulting in comparable geomorphic zonation. However, the presence of strong tidal currents flowing through Torres Strait and the relatively shallow and narrow dimensions of the shelf exert a control on local morphology and spatial distribution of the reef platforms. A total amount of 8.7 million tonnes of CaCO3 per year, at an average rate of 3.7 kg CaCO3 m-2 yr-1 (G), were estimated for the studied area. Extrapolated production rates based on detailed and regional census-based approaches for geomorphic zones across Torres Strait were comparable to those reported elsewhere, particularly values for the GBR based on alkalinity-reduction methods. However, differences in mapping methodologies and the impact of reduced calcification due to global trends in coral reef ecological decline and changing oceanic physical conditions warrant further research. The novel method proposed in this study to characterise the geomorphology of reef types based on classification trees provides an objective and repeatable data-driven approach that combined with regional census-based approaches has the potential to be adapted and transferred to different coral reef regions, depicting a more accurate picture of interactions between reef ecology and geomorphology.

Description: A high-resolution LiDAR digital elevation model was developed to investigate the geomorphic features and surface roughness of a coral reef in the Southern Great Barrier (One Tree Reef). Given that there are few data sets of equivalent resolution the focus of this research was to detail the change in surface roughness expression over multiple spatial scales. Data were collected 8 October 2018 using a Riegl VQ-820-G topo-bathymetric LiDAR combined with a Riegl Q680i-S topographic scanner and a Canon EOS 5Dmk4 DSLR on a small research aircraft (Diamond Aircraft ECO-Dimona). The whole One Tree Reef area was covered twice using two different pulse rate settings for the VQ-820-G, viz. 284kHz and 522kHz. This measurement strategy ensured maximum spatial resolution (at 522kHz) and maximum depth penetration (at 284kHz). All LiDAR data was processed to a 0.25 m cell-size DEM using a combination of Riegl proprietary software, ARA-developed software, the RAPIDLASSO LAStools utilities, Bayesmap's StripAlign™ utility and Global Mapper V 20. Processing included human-machine interactive selection and confirmation of valid bathymetric points. The imagery from the DSLR was mosaiced at 0.14 m cell size using the AgiSoft PhotoScan© (now Metashape) Software package and overlaid onto the LiDAR point cloud. The relative error of the LiDAR point cloud was ± 0.1 m. Two methods of characterising surface roughness were applied to the LiDAR DEM: the vector ruggedness measure (VRM) and the Multiscale Roughness (MR) tool from WhiteBox Tools. The VRM was applied over filter radii of 8, 20, 100 and 400 cells. The MR approach was conducted between filter radii of 1 – 1500 cells (≈ 0.5 to 750 m) with step intervals of 1. MR analysis revealed the roughness signatures of the geomorphic coral reef zones defined by Roelfsema et al. (2018, doi:10.1016/j.rse.2018.02.005). Geomorphic zones with similar roughness signatures were combined to produce roughness equivalent habitats. Roughness equivalent habitats are regions with similar roughness signature that do not necessarily form geographical contiguous areas. Data contained in this repository are: 1. The LiDAR DEM as a geotiff; 2. Roughness magnitude and scale geotiffs computed using the MultiscaleRoughness tool from Whitebox Tools and 3. The ESRI shapefiles for the roughness equivalent habitats. Further details of the results and analysis of the LiDAR DEM can be found in Harris et al. (2023, doi:/10.1016/j.geomorph.2023.108852).