Detection of coral spawn slicks at The Great Barrier Reef with multispectral imagery and synthetic aperture radar : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Manawatū, New Zealand
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Date
2024
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Massey University
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Abstract
Understanding the fate and dispersal of coral spawn slicks is crucial for conserving coral reef ecosystems, which are under extreme pressure from climate change and habitat loss. Corals reproduce through broadcast spawning, which involves the synchronous mass release of eggs and gamete, which may manifest as coral spawn slicks on the sea surface. This study is the first attempt to integrate satellite-based multispectral and Synthetic Aperture Radar (SAR) data for detecting coral spawn slicks. In addition, other features that may be confused with coral spawn slicks, such as algal blooms (biogenic slicks) and natural sea surface slicks, are also investigated. Numerous slick-like features were detected in PlanetScope imagery between 2016 and 2022 on the Great Barrier Reef. Their spectral properties were analysed with the help of ancillary information to determine coral spawn slick validity. Corresponding Sentinel-1A SAR data were then examined to see if these validated slicks were visible in this alternate data type. Coral spawn slicks were confidently detected on two occasions. The remaining slicks are likely macroalgae mats and cyanobacteria blooms, such as Trichodesmium and Sargassum. The coral spawn slicks displayed pink hues and were small and localised compared to other biogenic slicks, which often spanned vast areas. The red / green band ratio (RGR) was effective for discriminating coral spawn slicks but was limited by the spatial resolution of PlanetScope. Slicks of all types were often aligned within natural sea surface slicks (natural films), highlighting that coral spawn slicks may be undetectable in SAR images when this phenomenon occurs. The presence of look-alike features challenged the identification of coral spawn slicks in SAR, yet biogenic slicks could be matched on several occasions – though not consistently. There was evidence that cyanobacteria blooms appeared as areas of low backscatter, whereas macroalgae mats displayed positive backscatter compared to the surrounding ocean. This study showcases how multispectral imagery and SAR technologies may complement each other, with multispectral imagery providing detailed information on spectral properties and SAR giving insights into the surface structure.
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