A novel spaceborne proxy for mapping coral cover.
Proceedings of the 12th International Coral Reef Symposium.
Spaceborne remote sensing has resolved critical issues for mapping coral reef structure that human- and ship-based surveys could not have overcome, namely the spatial continuity regardless of the water depth. With the emergence of very high spatial resolution sensors, the spatial capabilities of satellites have outperformed those of aircraft, providing spectral information at the dominant benthos scale but over large areas. With the launch of the WorldView-2 (WV2) sensor, coral reefs can now be surveyed using eight bands at ~2 m spatial resolution, somewhat bridging the gap with high spatial resolution, hyperspectral airborne sensors. The WV2 spectral capabilities were utilized for modeling an in situ gradient of Live Coral Cover (LCC). Georeferenced underwater photoquadrats were collected to discern among ten benthic classes, ranging from coralligeneous sand to live Synarea rus bommie or Acropora pulchra thicket, and to compute the LCC. From the benthic images of the five WV2 visible bands (purple, blue, green, yellow, red), 20 pairwise combinations were tested in the form of a Normalized Difference Ratio (NDR). Four spectral combinations were revealed with high correlations (>0.8) with in situ ground-truthing. Associating the common green band with the innovative purple band, the NDR green-purple showed a strong linear relationship with the LCC (R2=0.96, p<0.001). The successful combination was mapped over the entire scene, reliably highlighting live corals while being less sensitive to dead corals and sediment. Discussed to be a proxy for the amount of zooxanthellate-borne pigments such as perdinin, the green-purple NDR holds great promise to map, detect and predict change in coral reefs at the dominant benthos scale regionally, aiding their management and conservation.
James Cook University