How resilient are coral reefs to tsunamis?

By Rebecca McLeod 24/11/2009

The September 29th tsunami waves left widespread destruction across the Pacific nations of Samoa, American Samoa and Tonga, evidenced by devastated villages, and multiple fatalities and injuries. But what about the natural environment? Was there paralleled damage under the sea, on the coral reefs that fringe these islands? Not only is this question of interest from an ecological perspective, but also from a social perspective: communities rely heavily upon these reefs for food, tourism and social wellbeing. As these communities begin to rebuild, and eventually encourage tourists to come back, the state of the coral reefs could be a factor in a tourism market that relies heavily on water-based activities.

The potential for damage to coral reefs following major storm surges and tsunamis is two-fold: the initial force of the waves hitting the reef, and the subsequent delivery of huge amounts of sediment and debris onto the reef as the wave retreats and during rainfall in the following weeks.

Using field tests, scientists at the ARC Centre for Excellence in Coral Reef Studies have demonstrated that the shape of the coral influences how vulnerable it is to damage by large waves. For example, table-topped corals (those with a broad top and attached to the reef by a stalk) are more prone to being overturned by a large wave than those that are mounded or branched. Location matters too, with corals on the front and crest of the reef where the waves break being more prone to damage by large waves than those situated on the top and back sides of the reef.

Arguably the greatest tsunami-associated threat to corals is posed by sediment and debris that is carried from the land as the waves retreat. Sediment can smother and even bury corals, leading to suffocation of the coral polyps. Extensive mechanical damage and scouring can result from debris such as building materials, machinery and cars getting pounded onto the reef.

Initial surveys of coral reefs along the west coast of Thailand following the Boxing Day tsunami in 2004 recorded mass destruction of corals. However, despite these initial concerns, repeat surveys in the following months found that the damage, although severe in small patches, was generally not as bad as first feared. This anomaly was due to much of the coral initially recorded as dead having in fact recovered. On the Island of Banda Aceh at the northern tip of Sumatara, scientists were in a unique position of being able to make before and after comparisons of reef health, as they had surveyed the reefs prior to the tsunami. The research team, led by Dr Andrew Baird at the ARC Centre for Excellence in Coral Reef Studies reported a remarkable resilience of coral communities to the tsunami waves. The damage observed was considerably less than that caused by destructive fishing practices (e.g. dynamite fishing) and coral harvesting, and was also considered to be significantly less than that resulting from hurricanes, due to differences in the distribution of wave energy in the water column.

I am not aware of any similar surveys being carried out in areas affected by the most recent tsunami (if you know of any please tell me!), but these experiences from the 2004 tsunami seem to suggest that the long-term impacts of the tsunami waves on coral reefs may not be all that bad. It seems plausible that those reefs in good health prior to the tsunami may experience a faster recovery than those already suffering from bleaching, grazing crown of thorns starfish, or destructive fishing practises.