Agatti Island of Lakshadweep Atolls: Coral Reef Bleaching

by Rohan Nath

Introduction 

Coral reefs support approximately 0.5 million species worldwide and are considered to be the  most diverse marine habitat. The annual growth of corals is based on environmental  conditions and are severely slow with a growth of only a few mm to 5 cm per year. Natural  and anthropogenic disturbances can influence the changes in the ambient environment and  affect the growth rate of coral reefs. Scleractinian corals get bleached and subsequently die  when there is a loss of endosymbiotic dinoflagellates due to elevated sea surface temperatures  (SSTs). This loss of algal endosymbionts under stress conditions is known as coral bleaching.  Coral bleaching can also be caused due to increased light intensity and salinity increased  copper concentration, as well as exposure to cyanide. There have been 29 widespread  bleaching cases in India since 1989. SST fluctuations have led to large scale bleaching and  mortality of corals in the Lakshadweep Islands, the Andaman and Nicobar, the Gulf of  Kachchh and the Gulf of Munnar. The Agatti Island (Fig. 1) have suffered from local coral  bleaching during 2010 as well as bleaching of coral-associated organisms like giant clams  and sea anemones. 

Fig. 1. An overview of the Agatti Island. Image Source: Google Earth.

Coral Bleaching 

The mortality of coral and associated organisms is prevalent throughout the island (Table 1). Table 1.  

Location Organism Mortality Rate
Northside of Agatti IslandCorals 64%-69%
Sea anemones 81%-83%
Giant clams  70%-85%
Southside of Agatti IslandCorals 75%-85%
Giant clams  83%-91%
Sea anemones  91%-93%

The mortality rate of corals, giant clams and sea anemones is higher on the south side of the  island compared to the north side of the island (Fig. 2). Variation in the depth during low tide  and high tide periods forms a possible reason for the mortality difference between the north  side and south side of the Island. The exposure of the coral reefs of the south side of the  Island to the sunlight during low tide is partial or full while the other sides are not. Low tide  exposure is a more valid reason for the difference in mortality than the elevated sea surface  temperature. The mortality of coral increases in May and June due to bleaching when the  temperature increases resulting in an extended exposure time during low tide and direct  contact with sunlight (Fig. 3).

Fig. 2. Percentage of bleaching in both North and South side of the Island.  Image Source: Vinoth, R., Gopi, M., Kumar, T. T. A., Thangaradjou, T., &  Balasubramanian, T. (2012). Coral reef bleaching at Agatti Island of Lakshadweep  atolls, India. Journal of Ocean University of China, 11(1), 105-110.
Fig. 3. (A, B) – bleached coral; C – bleached giant clam; D – bleached sea anemone. Image Source: Vinoth, R., Gopi, M., Kumar, T. T. A., Thangaradjou, T., &  Balasubramanian, T. (2012). Coral reef bleaching at Agatti Island of Lakshadweep  atolls, India. Journal of Ocean University of China, 11(1), 105-110.

There have been several bleaching events in the past due to elevated sea surface temperature  (Table 2). 

Table 2. 

Location and Year Bleaching  (%)Sea surface temperature of the  corresponding period (°C)Reference
Andaman, 2010 36-39 31-33 Krishnan et al.,  2011
Gulf of Munnar, 2008 10.5 31-33.5 Edward et al.,  2008
Gulf of Munnar, 2007 12.9
Gulf of Munnar, 2006 15.6
Gulf of Munnar, 2005 14.6
Palk Bay, 2002 50-60 32 Kumaraguru et  al., 2003
Gulf of Kachchh, 1998 11
Gulf of Munnar, 1998 82 3 above the seasonal average Arthur, 2000
Lakshadweep, 1998 89

The sea surface temperature of the Lakshadweep reef areas elevated at a rate of 0.21 °C per  decade from 28.50 °C in 1985 to 28.92 °C in 2005. The annual average maximum of sea  surface temperature remained the same, but the annual average minimum sea surface  temperature increased at a rate of 0.30 °C per decade from 27.2 °C to 27.8 °C. The El Nino  increased the sea surface temperature from 1997-1998 to 31 °C. 

Sedimentation and turbidity are also two important factors contributing to corals mortality.  However, the corals of Agatti Island currently do not face such a threat. The bottom  sediments are stabilized by the dense seagrass beds in the lagoon areas. 

Conclusion 

There is an average of 73% bleached corals, with bleaching-related mortality of sea  anemones (87%) and giant clams (83%) in the Agatti reefs. The bleached corals need time to  slowly recover in time and replaced by new coral colonies over the old ones. Past researches  proved that corals can recover after the removal of adverse conditions. Coral reefs are  important as a habitat and ecosystem for migratory fishes and need urgent attention. 

Reference 

1. Vinoth, R., Gopi, M., Kumar, T. T. A., Thangaradjou, T., & Balasubramanian, T.  (2012). Coral reef bleaching at Agatti Island of Lakshadweep atolls, India. Journal of  Ocean University of China, 11(1), 105-110.

Published by LakesOfIndia

Lakes of India is an E.F.I initiative aimed at sensitizing the larger public on freshwater habitats across the country. A blog platform where one can read about lakes across India. You can become a guest blogger to write about a lake in your hometown and initiate an action to protect that lake.

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