Spectral Discrimination of Live and Bleached Corals: A Case Study on <em>Turbinaria peltata</em> (Esper, 1794) Using Field Spectroscopy

Scleractinian corals represent the foundation species of reef ecosystems. Bleaching is a physiological, cellular response to environmental stresses wherein marine invertebrates including corals expel their endosymbiont, unicellular microalgae or zooxanthellae from their host tissues. Field spectroscopy helps to characterize the health of corals in terms of reflectance spectra or spectral signatures, i.e. reflected light as a function of wavelength. This chapter reports a case study on spectral discrimination of in situ hyperspectral signatures of live, apparently healthy and bleached corals collected from a single colony of Turbinaria peltata (Esper, 1794) sampled from Laku Point reef in Gujarat coast of India. Derivative analyses on the in situ reflectance data identify five narrow windows in the visible light region (green and red light regions) to spectrally discriminate live and bleached coral polyps of the T. peltata species. This study highlights the potential of field spectroscopy in characterizing coral health in situ through non-invasive sampling

Coral bleaching due to increased sea surface temperature in Gulf of Kachchh Region, India, during June 2016

327-332The 2015-2016 E1 Niño Southern Oscillation event was one of the extreme climate events which elevated the sea surface temperature (SST) of tropical oceans, which in turn increased the level of thermal stress on corals. Coral bleaching event is mainly caused due to high positive SST anomaly, i.e., when SST exceeds its normal summer maxima. Corals in the Gulf of Kachchh region of Gujarat earlier experienced coral bleaching events during 1988, 2010 and 2014. For this study, SST was derived from NOAA OISST data set which is available daily at 0.25° global grids from 1982 to present. The climatologically warmest month for the Gulf of Kachchh region is June when the maximum monthly mean temperature is 29.31°C, as observed from NOAA OISST. The present study focuses on monitoring daily SST anomalies during summer 2016 for the Gulf of Kachchh reefs and field observations on early responses of coral bleaching from Laku Point reef, a site known for high coral diversity. It was found that in summer 2016, SST rose to 30.62 °C and recorded a maximum positive anomaly of 1.31°C in the month of June. A total of 72 days out of 122-day monitoring period showed positive SST anomaly, including 28 days of continuous positive thermal stress in June 2016.To validate coral bleaching forecast at the end of the regional warmest quarter, a field visit was carried out at Laku Point reef near Poshitra village in the southern coast of the Gulf of Kachchh. A total of 13 coral species and a sea anemone were found bleached in various proportions during the field sampling after two months of prolonged thermal stress. The field data showed an average of 3.9% bleaching of corals at colony scale. The maximum proportion of colony scale bleaching was observed in Porites lutea species

Benthic coverage and bottom topography of coral reef environment over Pirotan reef, Gulf of Kachchh region, India

Remote sensing measurements in coral reef environments commonly confront the problem of overlying atmosphere and modification of spectral signal due to water column over the bottom substrates. In order to correct these problems, hyperspectral observations offer an advantage over multispectral observations. Airborne hyperspectral remote sensing data from Airborne Visible Infrared Imaging Spectrometer- Next Generation (AVIRIS-NG) sensor was acquired during low tidal condition on 14 February 2016 at Pirotan reef, Gulf of Kachchh region, India. The objective of this study is to map benthic coverage and bottom topography over Pirotan reef. The methodology involved atmospheric correction, simultaneous retrieval of water parameters, bathymetry, water column correction and mapping. Atmospheric correction was performed by removing path radiance and aerosol contribution and dividing by atmospheric transmittance and incoming solar irradiance to obtain remote sensing reflectance. Model derived error minimization technique was used for simultaneous retrieval of water parameters and bathymetry. Derived water parameters were used to account for water column attenuation and retrieve concomitant true bottom signature

Assessment of coral reef thermal stress over India based on remotely sensed sea surface temperature

Sea Surface Temperature is a critical physical attribute of coastal marine habitats. Remote sensing from satellite is the most widely used approach for monitoring the stress on coral reef ecosystems on large scale. Regional coral bleaching monitoring framework has been applied in five major Indian coral reef regions to investigate the threats. Degradation of coral reefs is a major environmental problem worldwide. ENSO (El Niño Southern Oscillation) event is one of the extreme climate change event, which elevate Sea Surface Temperatures (SSTs) of tropical oceans. This warming of SST increases the level of thermal stress on coral reefs. Coral reefs are the most sensitive ecosystem of all coastal ecosystems due to temperature change and exhibit bleaching when SST exceeds their normal summer maxima and remain high for more than 28 days. SST data from NOAA OISST v2 over period from 1982 to 2018 (37 years) were used for this study. Coral bleaching indices have been observed over five major Indian coral reef regions. Bleaching Threshold (BT), Positive SST Anomaly (PA) and Degree Heating Week (DHW) are commonly used indices for calculating thermal stress on the coral reefs computed from satellite-derived SST data. The BT value for each regions based on the long-term SST data analysis and found different. These thermal stress analyses over India were found close with the recorded mass coral bleaching events during 1998, 2010 and 2016. In addition, this study included calculating the thermal stress over India during Mass Coral Bleaching years