Assessment and monitoring of coral reef using hyperspectral and multispectral data: application to ecological indicator implementation for reef conservation and management
Résumé
Background Coral reefs are facing increasing environmental pressures along with anthropogenic stressors and disturbances, leading to a worldwide degradation of their status. There is an urgent need for developing adequate tools to assess reef state and resilience at large scale. This can greatly help to propose adequate management strategies for coral reef degradation mitigation and promoting reef resilience. Methods Using a spatial approach based on high resolution multispectral and hyperspectral image analyses, the present study aims at drawing, for the first time, a baseline of current coral reef structure and status in a highly heterogeneous shallow reef flat in Reunion Island (South-West Indian Ocean). We first developed an automatic image processing methodology which could be repeated over time to extract accurate spatial quantitative metrics on benthic coral reefs. Geometric and atmospheric corrections were performed prior to image processing. Benthic cover estimates were derived from airborne hyperspectral image processing using water column correction and unmixing models implemented with the four main coral reef benthic components: corals, algae, seagrasses and sand. Coral reef health status was evaluated through both coral cover estimates and a new index called Hyperspectral Coral to Algae Index (HCAI), defined as the ratio of living coral cover to the sum of living coral and algal covers. Ground truth data acquired simultaneously were used to validate processing accuracy. A significant positive correlation was obtained between coral cover recorded in situ and estimated from image analysis. We then illustrated, through a diachronic analysis between 2009 and 2015, the value of hyperspectral and multispectral imagery in quantifying spatial-temporal changes of benthic components for metrics extracted using the developed method. Results Our results reveal an overall decrease of the HCAI index and percent cover of all the dominant coral classes along the reef (e.g., −28.24% for the coral class), while habitat classes dominated by algae strongly increased during the period. The comparison of HCAI with the “live coral cover” extracted from Global Coral Reef Monitoring Network (GCRMN) stations showed similar trends although these two approaches differ fundamentally in many aspects regarding spatial and ecological scales (resolution, parameters measured, precision of the information obtained). Local and high- frequency measurements, as conducted by GCRMN, can be supplemented with broader spatial measurements of habitat dynamics and fragmentation through low-frequency spatial monitoring. This Spatial approach is also more suitable for describing natural patterns emerging from a range of gradients. Conclusion Remote sensing analyses based on hyperspectral spatial approach, as developed in this particular study, opens new perspectives for coral reef monitoring, which have profound implications for coral reef management and conservation in the light of rapid environmental changes.