data usually show spectral mixing of corals and other reef
substrates like benthic algae, reef-sand, etc. Therefore, it becomes
necessary to understand the spectral behaviour of corals in situ to
comprehend space-borne sensor-level signal of corals from a reef.
In situ spectral behaviour of Indian corals per se, in visible, near-
and mid-infra-red (NIR and MIR) regions of the spectrum was so
far unknown. This study attempts to characterize seven, live
(belonging to seven genera and four distinctive colony
morphologies) and one bleached hermatypic corals from Gulf of
Kachchh, India with in situ hyperspectral data over a continuous
spectral range of 350 to 1350 nm.
2. STUDY AREA
Indian coast is endowed with spatially limited but strategically
located coral reef habitats which offer a myriad of marine
biodiversity combined with unique regional characteristics. In
India, major coral reefs occur in four distinct locations. Two of
them occur in gulf settings: Gulf of Kachchh in Arabian Sea and
Gulf of Mannar in Bay of Bengal while the other two are off-
shore island groups of Lakshadweep in Arabian Sea and
Andaman and Nicobar in Bay of Bengal. Indian coral reefs share
sixty genera of hermatypic, scleractinian corals out of the one
hundred and eleven genera reported in the world (Venkataraman,
2003). Thus, Indian corals share 54% of global coral diversity at
genera level.
The southern part of Gulf of Kachchh from Gujarat coast
represents the sturdiest scleractinian coral species of India.
Occurring in the northernmost limits of Indian reef regions
(22?20' to 22?40' north latitudes and 68?30' to 70?40' east
longitudes), these corals grow in a highly turbid and saline,
macro-tidal environment marked with semi-diurnal desiccations
due to fluctuating tidal exposures (Navalgund et al. 2010). Out of
the sixty genera of scleractinian corals reported from India, only
twenty genera are found in Gulf of Kachchh (Venkataraman,
2003) reef region which is declared and protected as a Marine
Sanctuary since 1983.
A: Location of Gulf of Kachchh in India
B: Location of Study Areas: 1 and 2 in
Gulf of Kachchh
1: Paga Reef
2: Laku Point
> Z
IRS P6 LISS-IIF CC
Dated: 14'^ February, 2009
ATEM Tu
Daterk 16° March, 2005
Figure 1. Location of the study sites
Paga reef and Laku Point (near Poshitra) were chosen as two
specific sites (Figure 1) for collection of in situ coral spectra from
Gulf of Kachchh. These sites had been reported to have relatively
high generic diversity of scleractinian corals within the region
(Patel, 1978). Moreover the yearly, equinoctial spring tides
(negative low tides) result in maximum exposure of inter-tidal and
sub-tidal areas of these reefs facilitating in situ coral Spectra
collection with virtually no water column. Paga is an off-shore
patch reef where diverse coral colonies occur mostly in the reef
slope, reef crest and outer reef flat areas while Laku point is à
narrow fringing reef where coral colonies grow in shallow, rock.
pools.
3. MATERIAL & METHODS
3.1 Multi-spectral Signatures of Reef Substrates from
Resourcesat-1 LISS-IV Data
Linear Imaging and Self Scanning sensor: LISS-IV, onboard
Resourcesat-1 (IRS-P6) and 2 (RS2) satellites have been the most
preferred imaging sensor for Indian coral reefs for its high spatial
resolution (5.8 m at nadir) complemented with three spectral
channels (located in Green, Red and NIR regions) and 10 bit level
of quantization. LISS-IV in multi-spectral mode has performed
considerably well to characterize reef geomorphology of the
smaller reefs of Central Indian Ocean (Navalgund et al. 2010).
However, at an orbital altitude of 817 km, detection capability of
this sensor gets spatially limited for coral colonies within a reef.
Three discrete, broad-band, spectral channels, positioned at 530 to
590 nm, 620 to 680 nm and 770 to 860 nm usually fall short to
spectrally resolve a “pure coral signature”. Cohabitation of macro
reef-benthos like corals and macro-algae along with underlying
litho-substrates (sand, mud, etc.) under varying depths of water
column make this task all the more difficult. Atmospheric
interferences also alter the strength of the back-scattered signal
through atmospheric absorption and scattering. The back
scattered signal from a reef for a single pixel can thus be a mixed
representation of the natural heterogeneity present in the
corresponding reef area. This problem has been demonstrated
with the help of Figures 2 and 3 as a case study using a subset of
an archived IRS- P6 LISSIVMX (multi-spectral) data acquired on
16" March, 2005 pertaining to Paga reef. The spectral behaviour
of selected reef substrates have been analysed with respect to Top
of the earth's Atmosphere (TOA) spectral radiance. No
atmospheric correction has been performed on this subset image
for this study.
Figure 3 shows multi-spectral signatures (in terms of mean
spectral radiance of randomly selected thirty pixels for each
group) of five distinct reef categories obtained from the subset of
LISS-IVMX image of Paga reef (Figure 2). This subset image
was digitally enhanced by applying standard deviation stretch for
better visual appreciation of the reef substrates. Four out of the
selected five pixel-groups which represent four different reel
substrates are clearly, visually distinguishable by their respective
tones. In a standard LISS-IVMX False Colour Composite (FCO:
white pixels representing pure, exposed sand get well
distinguished from rest of the reef pixel classes, as à substrate
giving the highest spectral response in all the three channels. For
Paga reef the submerged sand pixels on reef flat (free of any kind
of benthic substrates) appeared in light green tone as the sand is
mixed with silt and clay.
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