CHENNAI: Despite sea surface temperature (SST) hovering above the bleaching threshold limit of 30°C, the coral reefs in the Gulf of Mannar have shown remarkable resilience this summer, with no signs of mass bleaching.
R Murugan, Wildlife Warden of the Gulf of Mannar Marine National Park, told TNIE that while the US-based National Oceanic and Atmospheric Administration (NOAA) had issued a general alert on potential coral bleaching due to above normal SST, underwater monitoring of reefs paints a more reassuring picture.
“There is some bleaching, but nothing alarming like last year,” Murugan said. “This is largely due to manageable marine heat stress and occasional rain. Last year, the SST rose beyond 33°C, peaking close to 34°C. This year, it has stayed between 31°C and 32°C. As of now, there is no threat of mass bleaching and mortality.”
Last year, a comprehensive rapid assessment carried out in April-May showed that 17% of the coral cover had bleached due to an unprecedented marine heatwave. Both fast-growing coral genera like Acropora, Montipora and Pocillopora and slow-growing genera like Porites, Platygyra and Dipsastraea were affected.
Palk Bay, too, recorded bleaching of 14% of live corals. Although officials feared mass mortality, ocean temperature dipped by mid-May, allowing partial recovery, though 4% mortality was still recorded.
Coral bleaching has become a near-annual event in the Gulf of Mannar between April and June. The phenomenon occurs when water temperatures rise above tolerance levels, causing corals to expel the symbiotic algae, zooxanthellae, that provide them with both colour and nutrition. If temperature normalises in time, corals can recover, but prolonged exposure leads to mortality.
“The bleaching threshold in Gulf of Mannar is 30°C, and this level is typically reached every summer. So mild to moderate bleaching is common. However, the Gulf of Mannar corals are historically resilient and have bounced back well after stress events,” officials said. The last severe bleaching occurred in 2016, when coral cover dropped from 38.9% to 22.7%.