The effects of hurricanes on coral reefs

Anjani Ganase, marine biologist, looks at how hurricanes affect coral reef ecosystems

Coral reef ecosystems are shaped by their surroundings. The amount of light they receive, the temperature of the water column, even the movement of the water (currents) all govern whether a coral species can survive and reproduce. As corals are only mobile during their larval phase, the spot where they choose to settle and grow becomes a very important choice; not too hot or cold, just enough light and shelter. As a result, we find shifts in coral reef types as environments change. However, despite a coral’s ability to adapt to long-term environmental conditions, similar to us on land, coral reefs can also be devastated by large disturbances that bring destruction or death. One example is hurricanes.

Hurricane force winds uproot trees and damage infrastructure on land. These same winds drive intense wave surges along coastlines, causing violent and irregular water movement that can be felt at great depths. Water movement is a part of a coral’s life, especially those living in the shallows, where the shape and size of the coral might be determined by the rhythmic movement of the water. However, the powerful waves generated by the hurricane can break and overturn even the hardest coral structure. Damage at greater depths is two-fold, directly breaking corals because of surging water; and secondly, from debris falling from the shallows.

Apart from the physical stress that the surging waters exert on the reef benthos, the turmoil of water movement from hurricanes also tends to mix the upper portions of the ocean’s water column, homogenising water conditions to the depths. Usually, seawater becomes naturally colder and saltier with depth; and organisms pick and choose the depth that best satisfies their needs. Drastic changes in temperature, salinities, oxygen concentrations and other conditions can result in death of small fish and invertebrates, such as lobsters that are unable to tolerate the extremely low salinities as a result of the excess rainfall. Larger mobile creatures, including bigger fish, sharks and marine mammals can escape the hurricane by moving to other areas. One study on black tip sharks observed that they moved away from the reef when hurricanes approached and returned following the storm, and suggested that the sharks were able to detect changes in the surrounding pressures driven by the storm (Heupel et al. 2003).

BEFORE: A healthy coral reef on the Great Barrier Reef before being hit by a category 5 cyclone (Cyclone Ita). Photo by XL Catlin Seaview Survey

AFTER: The same coral reef after being hit by a category 5 cyclone (Cyclone Ita). Photo by XL Catlin Seaview Survey

The effects of hurricanes may still be felt days after the storm has moved on. Waters can continue to surge days after a hurricane. In addition, the runoff from the land can carry sediment down to the coastal ecosystems, burying corals and muddying the water column blocking out the sunlight. The large amount of fresh water run off from the rivers also continues to reduce salinity. The recovery of the corals and associated marine life, greatly depends on subsequent disturbances, whether it is another hurricane or man-made, as well as whether the long-term environmental conditions continue to promote coral recruitment and growth.

On longer time scales, the frequency of hurricanes experienced in an area also governs the resultant community composition of coral reefs. Some locations in the Caribbean are hit more frequently by hurricanes, such as the Bahamas, which is impacted by hurricanes almost annually. In contrast, Trinidad and Tobago, the southernmost islands in the Caribbean, have a history of very low hurricane disturbance. The time between hurricane impacts on coral reefs in the Bahamas is much shorter compared to that in Trinidad and Tobago. This means that coral reefs in the Bahamas are likely to be very different to Tobago’s. Bahamian reefs are therefore likely to be composed of species that have faster growth rate, and therefore able to recover quickly between storms.

Studies have shown that full recovery of coral reefs (regrowth of corals) following a hurricane can be as quick as five years if no other storm hits. However, it has also been shown that overall Caribbean reefs have become impaired in their ability to recover from hurricanes; they recover at a slower rate (> 8 years) if at all, to their pre-disturbed states (Gardner 2005). Sometimes the recovery is too slow to allow sufficient regrowth of coral skeleton before another hurricane passes. This recovery impairment of corals can be related to additional stressors that are experienced by the corals. Interestingly, the impacts of hurricanes on coral reefs were more intense during the 1980s, but this does not mean that hurricanes have become less damaging to coral reefs (in fact, we know that hurricanes have become more intense over the last forty years); rather coral reefs are being more damaged by other disturbances at a greater rate. These other disturbances are typically man-made, which alter the long-term living conditions of the reef environment making it less suitable for corals. 

On top of the standard management of coral reefs that is important to sustain long-term coral health, there is need for a recovery management plan for reefs following significant disturbance events. If a coral reef has been badly damaged following a hurricane, areas of severe coral loss should be temporarily closed to fisheries and recreational use in the following years. This will reduce the number and frequency of stressors acting on the reef and help the reef to recover. Furthermore, any nearby land-based activity that may affect the reef should also be limited and monitored closely until the reef has recovered to a state where it can once again act as a buffer to other disturbances.

References:

Gardner, T. A., Cote, I. M., Gill, J. A., Grant, A., & Watkinson, A. R. (2005). Hurricanes and Caribbean coral reefs: impacts, recovery patterns, and role in long‐term decline. Ecology, 86(1), 174-184.

Heupel, M. R., Simpfendorfer, C. A., & Hueter, R. E. (2003). Running before the storm: blacktip sharks respond to falling barometric pressure associated with Tropical Storm Gabrielle. Journal of fish biology, 63(5), 1357-1363.