Hurricanes Are Just A Fact Of Life For Corals

Author: Alex Mustard

As many readers may already know, I jumped in at the deep end as a Divequest Leader when, on the last day of our highly successful coral spawning trip to Grand Cayman, we got caught up with Hurricane Ivan. Human beings are a remarkably adaptable species, and despite 240mph winds, 3 nights in a hurricane shelter and emergency evacuation by private jet, my mind was soon far more concerned for Cayman’s reef! I’ve recently been back to Cayman, and I was quite taken aback to find that as far as the dive sites are concerned you would hardly know that there had been a storm.

I was quite surprised by this and I set out to find out more about the relationship between hurricanes and coral reefs. It is perhaps contrary to our intuition but hurricanes are far from a disaster story for corals and are just part of the natural cycle of the reef. J. Willard Marriott once said, “the stronger the wind, the stronger the trees”, and his words hold true underwater. Caribbean corals have evolved in an environment prone to hurricanes and their survival strategies are geared around them. Category 5 hurricanes are usually a once in a lifetime event for humans. However with an average of 4 hurricanes a year in the Caribbean, a long-lived coral colony (that can easily outlive us) can expect several big storms during its life and clearly needs adaptations to cope.

Biologists often compare the impact of hurricanes on reefs to that of forest fires on land: they clear out space and prevent a single species monopolising the environment and thus promote biodiversity. Short-term destruction gives rise to long-term enhancement. Key to this is that hurricanes do not impact all species in the same way. Fast growing species (sponges, seafans, branching corals) are ripped off the reef more easily by the storm waves than slow growing robust species (bolder corals). Hurricane George in Florida is a typical example: it knocked over three quarters of branching corals on the worst effected reefs, but only about 5% of the bolder corals. Hurricanes are a natural process and are crucial in keeping the balance between the species in the ecosystem. Without these storms the fast-growing branching species would overgrow and kill off the slow growing bolder corals.

That said, being plucked from the reef is not the end of the story for branching corals like Elkhorn and Staghorn either. As long as the broken colony ends up in a suitable position it can regenerate: after Hurricane Greta in Belize about 40% of the torn away Elkhorn corals reattached and continued to grow. The shallows provide the best conditions for coral growth, but at the risk of the most storm damage. Clearly, corals that are adapted to live in these areas, such as Elkhorn, must be geared towards a ‘boom and bust’ lifecycle and not surprisingly they have evolved adaptations such as regenerative power and a fast growth rate.

Another major effect of hurricanes is that they scour algae (such as Dictyota) from the reef and clear out real estate for new corals to colonise. This ties in with what I heard from my friends in the days following the hurricane who have commented on how clean the reef looks. Furthermore the removal of algae often reveals lots of small coral colonies that were already there and gives them the chance to grow. In Florida, following Hurricane George, scientists actually measured an immediate increase in colony abundance because they were suddenly able to see all these previously hidden corals. The removal of sediment from the reef area (gravity usually ensures transport is downhill over the wall) is often even more effective in creating new space for colonisation. Even though many of Cayman’s wall sites are cut with sand-filled gullies, these sediment rivers usually only flow during storm events. A once in a century storm event, like Ivan, often accounts for more sediment transport than the slow wave by wave creep of sand over all the intervening years.

Anyway, back to the corals. All this new rock is ideal for fresh colonisation by new coral recruits, which can struggle to get a successful foothold on an algae covered reef. Recruitment events following hurricanes are often particularly extensive. In the Pacific nation of Tuvalu (which curiously makes its money by owning Internet domain of .tv) Typhoon Bebe scraped the reef flat bare, but within 10 years this ideal surface was re-colonised with Staghorn corals each of which had grown to more than 2 feet across. After Hurricanes Hugo and Andrew in Florida, finger coral rapidly settled in large numbers on to the free space created in the shallows. Hopefully the timing of Hurricane Ivan was particularly fortuitous in this respect. Ivan struck 10 days after we had witnessed a mass coral spawning event involving at least 10 species and with luck, the resulting coral larvae will have already settled into their new home.

The massive waves are not the only problem for the reef. Hurricanes typically pour huge volumes of freshwater, sediment and nutrients into the ocean, that can kill and smother the coral, block out the sunlight and trigger planktonic algae blooms. Scientists believe that it was sediments rather than the waves that killed the most corals off Honduras following Hurricane Mitch. Mangroves and seagrass filled lagoons are the best protection for reefs against these threats, which soak up a lot of the excess water and sediment releasing it more slowly back to the ocean. Again Grand Cayman is lucky in this respect: the small land area combined with the sponge-like qualities of the Central Wetland and extensive Sounds should help safeguard the reefs.

Another risk is that hurricanes can trigger outbreaks in coral diseases. I’m sure you’ve noticed that you tend to get ill when you are stressed or tired from work. Well it’s the same for corals. The additional environmental pressures associated with a hurricane can make corals much more prone to disease. One year after Hurricane Mitch coral diseases were much more common than usual in the Bay Islands. This outbreak was probably triggered by a combination of stress from the hurricane and abnormally high sea temperatures before the storm.

For many of the reefs inhabitants, Ivan will have had little impact. Most fish will be fine, assuming that they weren’t swept from the reef, and opportunistic species often multiply because food can be more abundant after a storm. Following Hurricane Isabel in 2003, populations of some species of fish and well as sea birds have increased in the US. Lobsters tend to do less well in rough weather, and can easily be killed, crushed when their shelters collapse or damaged by moving rocks. But many will migrate into deeper water as the storm starts to build. Our Divequest group dived on the North Wall of Cayman as the swell built up the day before Ivan and I saw plenty of lobsters deeper down the wall. Most of the larger creatures will also be fine, although I expect the stingrays in the North Sound were keen for tourists armed with squid to return to the islands!

Grand Cayman’s reefs will have certainly felt the force of Ivan, and like on the land certain areas may have been damaged extensively. But like the Nation itself, that is dedicated not only to recover but to be an improvement on the pre-Ivan version, the hurricane will probably prove a benefit to Cayman’s reefs in the long term.

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