Hurricanes: Friend or foe to coral reefs?
June 1 is the start of hurricane season in the western Atlantic, and for many in the southeast United States and Gulf of Mexico it means stocking up on emergency supplies and regular visits to the National Hurricane Center website. The public is well aware of the damage a storm can cause on land, but did you ever stop to wonder how hurricanes affect life beneath the water?
Here in the Florida Keys, storms have the ability to both help and hinder the coral reef environment.
The high winds of a storm can cause powerful waves. Storm surge and waves can topple entire coral heads, or shift sand which can scour or smother coral colonies. Delicate branching corals – like staghorn and elkhorn – are among the most vulnerable to breakage and may be reduced to rubble during a severe storm. Yet sometimes, the breaking of coral into pieces may actually help a coral colony reproduce through a process called fragmentation. If conditions are favorable and coral pieces come to rest in an area where they can reattach to the seafloor, fragmentation can result in colonies flourishing in a new location. In other instances, corals that reproduce through broadcast spawning may get a little extra help from late summer storms by aiding in the dispersal of their larvae to new reef locations.
During summer months, ocean surface temperatures increase just like air temperatures. If sea surface temperatures rise beyond the coral’s preferred temperature range, the coral animal can become stressed and bleached, or possibly die. However, as a storm passes over the ocean it absorbs some of the heat from the water, cooling the sea surface and giving corals a reprieve from the heat. The winds of a storm can further reduce water temperature by causing upwelling, which brings deeper, cooler water to the surface. Even the clouds of a storm can help reduce heat by blocking the sunlight, and dropping water temperatures to a more comfortable level for coral animals.
The 2005 Hurricane Season
2005 was the hottest year on record in the Northern Hemisphere since the start of reliable records in 1880. As a result of this extensive thermal stress, the corals of the Caribbean experienced widespread bleaching and death, the worst in recorded history.
That summer, corals of the Florida Keys bleached like their Caribbean counterparts, but were spared widespread mortality due to the passing of several strong hurricanes which cooled Keys waters in their wakes. Hurricanes Dennis, Katrina, Rita and Wilma were part of a record-setting hurricane season, and their effects on the sea surface temperature of the Florida Keys can be seen in the graph to the right.