SPECTRAL CHARACTERISTICS OF SELECTED HERMATYPIC CORALS 
FROM GULF OF KACHCHH, INDIA 
Nandini Ray Chaudhury 
EHD/ABHG/EPSA, Space Applications Centre, ISRO, Ahmedabad — 380015 — nandinire@sac.isro.gov.in 
Commission VIII, WG VIII/6 
KEY WORDS: Ecosystem, Hyper spectral, Multispectral, Image, Identification 
ABSTRACT: 
Hermatypic, scleractinian corals are the most important benthic substrates in a coral reef ecosystem. The existing, high (spatial) 
resolution, broad-band, multi-spectral, space-borne sensors have limited capability to spatially detect and spectrally discriminate coral 
substrates. In situ hyperspectral signatures of eight coral targets were collected with the help of Analytical Spectral Devices FieldSpec 
spectroradiometer from Paga and Laku Point reefs of Gulf of Kachchh, India to study the spectral behaviour of corals. The eight coral 
targets consisted of seven live corals representing four distinct colony morphologies and one bleached coral target. The coral spectra 
were studied over a continuous range of 350 to 1350 nm. The corals strongly reflected in the NIR and MIR regions with regional central 
maximas located at 820 and 1070 nm respectively. In the visible region the live coral spectra conformed to “brown mode” of coral 
reflectance with triple-peaked pattern at 575, 600 and 650 nm. All coral spectra are characterized with two distinct absorption features: 
chlorophyll absorption at 675 nm and water absorption at 975 nm. The live and the bleached corals get distinguished in the visible region 
over 400 to 600 nm region. Water column over the targets modifies the spectral shape and magnitude. First and second-order derivatives 
help in identifying spectral windows to distinguish live and bleached corals. 
1. INTRODUCTION 
Worldwide, hermatypic or reef-building corals occupy a keystone 
position in the overall functioning of reef ecosystem. Live coral 
cover of a reef habitat is one of the most widely accepted 
parameters of coral reef health (Green et al. 2000). Space-borne, 
optical remote sensing has sensor limitations in automated 
extraction of live coral cover parameter from space data. Digital 
delineation of this parameter has so far mostly used the expert 
knowledge of local reef systems in terms of specific 
geomorphological features and associated benthic classes 
(Hochberg and Atkinson, 2003). The existing, medium to high 
resolution, broadband, multi-spectral sensors have limited 
capabilities to provide synoptic data on live coral cover at 
community level. Poor penetration of sunlight and low signal to 
noise ratio of backscattered reflectance from deeper reef habitats 
are considered to be the major limiting factors to the performance 
of these sensors (Green et al. 2000). Therefore, live coral cover 
assessment through space-based imaging has been successfully 
confined to emergent, shallow, reef flat environments preferably 
at low tides. Relative abundance of reef biota and litho-substrates 
at different bathymetric depths with varying water column (i.e. 
depth, quality, etc.) add high degree of natural variability to coral 
reefs as optically complex, shallow water, remote sensing targets. 
This poses significant challenge in remote assessment of reef- 
sale biodiversity in terms of spatial detection and spectral 
characterization of individual corals at colony scale. 
As an underwater remote sensing target, coral reefs appear as 
Aes of diverse “substrates” or “bottom types" when viewed 
Tom the space (Hochberg et al. 2003). Spectral nature of the 
space-borne data has been recognized as the basic link between 
coral reef substrates and remotely sensed images (Hochberg and 
Atkinson, 2003). Substrate detection and quantification efforts 
have applied a deterministic, in situ data based, “reef-up” 
approach which often use proximal remote sensing of corals and 
other reef benthos. This approach calls for spectral cataloguing to 
identify distinct spectral features of a substrate. In situ reef-up 
approach has demonstrated the potential applications of field 
spectroscopy and hyperspectral analyses to characterize the 
reflectance properties of individual reef substrates which usually 
comprise an image (Goodman and Ustin, 2002). In situ 
hyperspectral signatures of corals and other reef benthos obtained 
with the help of portable field spectroradiometers have mostly 
been used for spectral discrimination of reef substrates (Hochberg 
and Atkinson, 2000, Hochberg et al. 2003, Hochberg and 
Atkinson, 2003, Kutser et al. 2003) and for health of corals 
(Holden and LeDrew, 1998; 1999; Clark et al. 2000). In situ reef 
spectra have been also used as input to simulate top-of- 
atmosphere spectral reflectance using radiative transfer models 
(Lubin et al. 2001). 
In situ spectral reflectance of corals per se has been recognized as 
a fundamental parameter in coral reef remote sensing (Hochberg 
et al. 2004). Spectral characteristics of coral organisms at colony 
or at community level have been correlated primarily with coral 
pigments, fluorescence and colony morphology (Hedley and 
Mumby, 2002; Hochberg et al., 2004; Joyce and Phinn, 2002 and 
2003). 
Broad-band, multi-spectral signatures obtained from image-pixels 
of high (spatial) resolution Indian Remote Sensing (IRS) satellite