Abstract
The worldwide waning health of coral reefs implies an increasing need for monitoring them at colony scale over large areas. Relaying fieldwork considerably, the remote sensing approach can address this need in offering spectral information relevant for coral health detection with 0.5 m spatial accuracy. We investigated the potential of spectral diversity indices to achieve the discrimination of coral-dominated assemblages and health states from novel satellite imagery (WorldView-2, WV2). Both Equitability’s (E) and Pielou’s (P) operators were used to quantify the evenness of the corrected visible spectral bands (two times 26 combinations of five bands) corresponding to remotely sensed colonies. Three scleractinian corals (Porites lobata, P. rus and Acropora pulchra) that are primarily involved in Moorea’s reef building (French Polynesia) were examined in respect to their health state (healthy or unhealthy, referring to both bleached and dead coral, hereinafter). Using four classifiers, we showed that the Support Vector Machine (SVM) greatly discerned among the six coral classes based upon the five WV2 spectral bands (93%), thus surpassing the classification issued from the three traditionally used bands (80%). Coupling the WV2 dataset with Egreen-red, Eyellow-red or E“coastal”-blue-green allowed the SVM performance to attain 96%. On the other hand, adding the E“coastal”-blue to the WV2-dataset contributed to a substantially increase of the classification accuracy derived from the Random Forest classifier, stepping from 64% to 77%. Significant contributions of spectral diversity indices to surveying coral health were further discussed in the light of spectral properties of coral-related pigments. These findings may play a major role for the extensive monitoring of coral health states at a fine scale, and for the management and restoration of damaged coral reefs.
Highlights
Coral ecosystems worldwide were estimated to provide US$ 375 billion worth of ecological services, such as disturbance regulation, food production or cultural goods [1]
Since only severely bleached coral can be identified by hyperspectral derivative spectroscopy [14,16] with high accuracy, we develop hereinafter an innovative classification methodology based on diversity indices [18] applied to coral spectral signatures so that distinction between healthy and unhealthy states, more subtle than bleached states, can be completed
Increasing the classification accuracy derived from the three visible bands QB2 sensor by 13%, the spaceborne WV2 sensor equaled spectroscopy-induced capabilities of airborne remote sensing [13,14,15,16], despite a coarser spectral resolution
Summary
Coral ecosystems worldwide were estimated to provide US$ 375 billion worth of ecological services, such as disturbance regulation, food production or cultural goods [1]. These marine biodiversity hotspots are increasingly undermined by a wealth of stressors, ranging from the global sea level and temperature rising [2] to ocean acidification impairing the calcification rate [3] through increased frequency of tropical cyclone events [4]. Coral-related benthic habitat classification was underpinned both by the enhancement of the spatial resolution of multispectral spaceborne sensors leading to the 2.4 m
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