Geo-Spatial Technology
In this article remote sensing application is used for mapping of physical feature of coastal ecosystem. Coral reefs are mapped using satellite images. Coral reefs are described as live coral, dead coral and coral in bleaching condition.
Need of Coral Mapping
Coral reefs are the foundation and primary structure of highly productive and diverse marine ecosystem. This is the reason, Coral reefs are called the “rainforests of the sea,” Corals grow in shallow water close to the shore and thrive within a limited range of temperature, salinity and turbidity.
These ecologically and economically important marine habitats have been significantly degraded and destroyed by human and natural disturbance such as global warming or rise in sea surface temperature. When the environmental conditions that the coral requires are altered, the stress placed on the coral often causes bleaching.
Decline in the density of zooxanthellae leads to coral bleaching. Large-scale bleaching is predominantly triggered by elevated sea-surface temperatures. Several mass-bleaching events have been recorded on coral reefs around the world.
Mapping and Monitoring Coral Bleaching analysis using Satellite Images
Field sampling is a common approach to monitor coral reef environments, but this approach is also limited scope. It requires large amounts of transect data to monitor the extensive reef environment.
Whereas remote sensing offers an effective, complementary approach that can overcome the limitations of field sampling, particularly in the case of monitoring of reefs in poorly accessed areas.
The loss in zooxanthellae from corals during bleaching events results in an optical signal that is strong enough to detect through remote sensing technology. However some coral bleaching studies have suggested that
it is difficult for satellite sensors to detect coral bleaching, mainly because of low spatial resolution (~ 30 m) of satellite images relative to the scale of reef heterogeneity but bleached corals can be easily detected by sensors with high (<2 m) spatial resolution. The bleached coral has a high reflectance in blue-green spectral bands.
Satellite Image Selection for Coral Reef Mapping
Many high resolution satellite data such as Quickbird and worldview data could be used to investigate the coral assemblages affected by bleaching. In this explanation Quickbird was chosen because of its high spatial resolution (multispectral 2.4 m and panchromatic 0.6 m).
It would contribute to better mapping of coral reefs. In this given example the changes in coral coverage were identified from panchromatic-fused multispectral (PAN+MSS) images from Quickbird satellite just before and after the bleaching event in 2007.
Comparison of images before and after the bleaching event allows more accurate assessment of the detection limits for bleached corals in a pixel because an increase in reflectance will be recorded where bleaching has taken place.
The Quickbird images used in the study were acquired on 27 June and 25 September, 2007 with full 16-bit radiometric resolution and spatial resolution of 2.4 m (MSS) and 0.6 m (PAN) to quantify coral bleaching.
Satellite Image processing for Coral Reef Mapping
In image processing
- Image was radiometrically calibrated to retrieve at-sensor radiance, then atmospherically corrected
- In addition to its corrections for atmospheric absorption and scattering
- Water column correction was performed using the depth-invariant index to enhance the information on bottom type.
- Images were classified as coral, algae, seagrass, pavement, sand and rubble using a maximum likelihood decision rule after training samples had been selected for each of the benthic cover types.
- Morphological operators combined with visual have been used to improve the visibility and detectability of different benthic features.
- For identification of coral cover, the coral location, color, texture and association were used for better interpretation.
Interpretation from Classified Image
- From this classified output, the coral cover areas were masked out and overlaid over the original image to further reclassify the coral cover visually as live coral, partially bleached coral (a portion bleached, with some live coral tissue), fully bleached coral (100 % bleached with no algal colonization) and dead coral (coral tissue areas partially overgrown by an algal turf layer).
- The image interpretation analysis indicates that 61.79 % of corals were partially bleached, 11.86 % fully bleached and 4.2 % dead. Therefore, changes in the distribution of coral cover could be tracked across the inner reef area by analyzing high resolution satellite data like Quickbird imagery.