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 Island | Corals | 64%-69% |
| Sea anemones | 81%-83% | |
| Giant clams | 70%-85% | |
| Southside of Agatti Island | Corals | 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).
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.
India Ocean Project 