Cays of the Great Barrier Reef - Technical Report to support monitoring of reef islands in the Great Barrier Reef
Duce, Stephanie, Muecke, Holly, Joyce, Karen, and Smithers, Scott (2024) Cays of the Great Barrier Reef - Technical Report to support monitoring of reef islands in the Great Barrier Reef. Report. James Cook University, Townsville, QLD, Australia.
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Abstract
Cays are small, low-lying reef islands composed of biogenic sediments produced on their host reef platforms and deposited by waves. The inextricable links between cays, coral reef ecosystems and the physical and climatic processes that form and maintain them make them highly dynamic and complex systems. Cays on the Great Barrier Reef are many and varied and support critical ecosystem services such as seabird and turtle rookeries. Many are also culturally significant and/or support high value tourism. Monitoring geomorphic change on cays is necessary to understand them, how they change over different timeframes, and how they are likely to change into the future. Hence, monitoring is crucial for their sustainable management. The aims of this project were to:
1. Audit all relevant, available cay literature and data and consolidate it in a central repository. 2. Develop a preliminary cay classification scheme based on key geomorphological and environmental parameters for priority cays. 3. Develop a drone-imagery based monitoring methodology to enable ongoing documentation of cay changes and improve the quality of data, the temporal frequency of monitoring and the spatial coverage of cays within the Great Barrier Reef Marine Park estate.
This report describes the audit of accessible literature, datasets and imagery relevant to understanding the geomorphological condition, processes, and dynamics of the 233 coral cays on the Great Barrier Reef. More than 400 documents were reviewed, and relevant information was systematically extracted and stored in a series of databases. Aerial photograph and satellite imagery of cays, available through Queensland Globe, was also reviewed, and the details of suitable imagery for assessments of cay geomorphology and historical change were recorded.
This report defines and summarises the geomorphological and other attributes needed to describe, quantify, and monitor coral cay morphology and change. It describes the creation of a comprehensive spatial database, including the digitization of the shorelines of all 233 cays from the best available imagery creating a valuable baseline dataset of cays. Twenty-two cay attributes were identified and extracted or calculated for each cay. These include attributes for interpreting historical cay changes and those relevant to developing metrics to quantify recent, and potentially future, cay changes. We briefly summarise how key cay geomorphologic attributes vary geographically across the Great Barrier Reef, with particular emphasis on the forty-five priority cays identified with collaborators at The Great Barrier Reef Marine Park Authority (The Reef Authority) and the Department of Environment, Science and Innovation (DESI).
This project also developed a framework to meaningfully classify priority cays based on their geomorphology and environmental drivers and thus provide a valuable tool to assist in the prioritization of management and monitoring activities. Two new, data-driven metrics were developed to classify and compare cays based on their geomorphic change potential and energy exposure. These are presented visually in the form of a one-page profile for each priority cay. A range of other key information is provided in these profiles, including imagery of the cay and reef platform, summaries of key morphological traits and management values, and a graphic showing model outputs of incident wave energy over each reef platform.
The classification scheme developed acknowledges the inherently dynamic nature of most coral cays and, by integrating the influence of key cay attributes and major drivers of shoreline change, it offers a meaningful metric for understanding and comparing cay behaviour using the best available data. However, we recognise that, like all such metrics, when applied at scale it necessarily involves simplifications. Improvements in the resolution, frequency and accuracy of input data layers would improve the classification and decisions supported by it. Efforts to improve these data should be a priority, especially for high value cays.
Finally, ways to efficiently map cay geomorphology using new technologies, and thus address some of the data deficiencies mentioned above, were developed. This project developed and tested replicable methods and workflows for monitoring cay change using drones. We recommend practical methods for data collection and processing to achieve vertical accuracies necessary for informing the detection and monitoring of change on cays. Guidelines for effectively collecting and georeferencing drone datasets are provided. Importantly, we have integrated our proposed workflows with GeoNadir, an efficient online platform, to manage, store, process and distribute drone imagery as findable, accessible, interoperable and reusable (FAIR) datasets in the form of orthomosaics, digital surface and terrain models.
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