**6. Malvan region**

The diurnal trends of SST in the warmest quarter over a period from 2010 to 2020 provided information on the direction of changes in SST. The BT value for Malvan was observed 29.89° C (± 0.49° C) means 0.5° C above the thermal threshold value (**Figure 3**). Over a period from 2010 to 2020, it was observed that the SST value during the warmest quarter had crossed the BT in each year except two years: 2012 and 2019. Year: 2012 and 2019 have experienced significantly less SST as compared with other years. The maximum SST 31.21° C has been recorded in the year 2010. During the year 2020, the SST values crossed the BT values and persisted for three months with two intermittent breaks. This study also computed variations of daily SST anomaly. The absolute range of positive anomaly for the Malvan region varied between 0.01° C to 1.8° C, and a maximum of 1.8° C was recorded in 2010. The frequency and intensity of positive anomaly have increased continuously over the region. The range of DHW was found to be 0.2° C to 7.84° C, and the maximum was recorded in 2010, whereas the minimum was recorded in 2012. The "Alert Level-1" status was recorded in the year 2010 and 2016, while "Warning" status was recorded in the year 2015, 2017, and 2020. The DHW under "Alert Level-2" status was absent over the Malvan region. The year 2019 has shown the least coral bleaching thermal stress indices because of the long period cyclone '*Vayu*' was formed in the Arabian Sea, which triggered heavy to extreme rainfall.

Field observations data were collected over both locations during the period from 2010 to 2020. The recorded satellite-derived SST and field observations data indicated that corals in both regions experienced prolonged heat stress, which is the primary cause of back-to-back coral bleaching events. Field survey at Gulf of Kachchh region revealed bleaching of *Coscinaraea, Favia, Goniastrea, Goniopora, Leptastrea, Porites, and Turbinaria* (**Figure 4**)*.* Field surveys at the Malvan region were revealed partial and whole colony bleaching of *Porites* spp*., Favites spp., Turbinaria mesenterina, Pseudosiderastrea tayami, Cyphastrea serailia, Plesiastrea versipora, Goniopora* spp., *Siderastrea savignyana* (**Figure 5**). Gulf of Kachchh region recorded ~4% and ~ 10% bleaching during 2016 and 2019 temperature peaks. During field survey at Malvan region in 2014, the bleaching prevalence was observed 14.58%; in the year 2015, the bleaching prevalence was observed 54.20%; in 2016, the bleaching prevalence was observed 46.76%; in 2017, the bleaching prevalence was observed 20.22%; in 2018, the bleaching prevalence was observed 5.07%, and in 2019, the bleaching prevalence was observed 8.37% (**Figure 6**) [25, 30]. The effects of increasing thermal stress on corals were correlated with field observations data. We found that the DHW derived from SST and field observations were positively correlated with a correlation coefficient of 0.71 (**Figure 7**). The significant correlations indicate the SST peaks during the warmest quarter were the predominant cause of mass coral bleaching and mortality.

SST-driven impacts trigger cascading effects at the ecosystem level by reducing coral species heterogeneity, weakening the reef carbonate framework, loss of reef functionality, and negatively impacting the reef-associated biodiversity [9, 24]. This

#### **Figure 4.**

*Coral bleaching observed at Laku Point reef, Gulf of Kachchh region during 2016 and 2019. (a-b) Healthy and bleached* Turbinaria sp.; *(c) bleached* Gonipora sp.*; (d) partially bleached* Favites sp.*; (e) partially bleached*  Porites sp.*; (f) bleached sea anemone* Heteractis sp.; *(g) bleached colonies of* Dipsastraea sp*.; (h) bleached colonies of* Favites sp*. and* Porites sp*.; (I & j) bleached colonies* Dipsastraea sp*.*

*Climate Change Induced Thermal Stress Caused Recurrent Coral Bleaching over Gulf… DOI: http://dx.doi.org/10.5772/intechopen.96806*

#### **Figure 5.**

*Coral bleaching observed at Malvan region from 2014 to 2019. (a-b) Bleached* Favites sp*.; (c) bleached massive*  Porites sp*.; (c & d) bleached and dead coral colonies covered by turf algae and sediments.*

**Figure 6.** *Coral bleaching prevalence at Malvan Marine Sanctuary, Central West coast of India.*

heat shock-mediated bleaching mortality of corals has emerged as the greatest threat to the existence of reefs globally as these habitats fail to recover [10, 16, 19, 35]. The present study highlights that the satellite-derived SST data may be used as a convenient tool for thermal stress-driven coral bleaching events, which will improve reef management practices in the thermal stress-impacted coral reef environment. Climate change poses a threat to the persistence of the coral reefs in tropical seas. The mass coral bleaching estimated during 2010 and 2016 was correlated with a multivariate ENSO index. The year 2010 and 2015–2016 were a strong ENSO years.

#### **Figure 7.**

*Correlation of duration of thermal stress (i.e., DHW) and bleaching percentage during the period from 2014 to 2019.*

The tropical Indian Ocean is warming rapidly compared to rest of the tropical oceans [36], and warming of the Arabian Sea has increased significantly since the 1990s [37, 38]. In recent years, rapid warming event caused severe negative ecosystem impact in the Indian Ocean. For instances, 2015–2016 ENSO caused a significant decline in oil sardines fishery in South-West India [39], and phytoplankton community shift in the North-eastern Arabian Sea [40], as well as coral bleaching and mortality in Lakshadweep archipelago [41]. Understanding how the Indian Ocean suffered severe coral bleaching and mortality in 2015 following a 7.5 maximum degree heating weeks (DHWs), which caused a 60% coral cover decrease from 30% cover in 2012 to 12% in April 2016 [11]. Therefore, ENSO induced heat stress driven coral bleaching, coral mortality and ecosystem level impact of recurrent mass bleaching events require global scale quantification of the magnitude, intensity, and duration of thermal stress for each reef location for formulation of improved and timely management policies.
