Coral Reefs and Hurricanes
            
A hurricane is a violent environmental disturbance, 
tightly constrained in space and time. 
Its "footprint" of extreme impact may be only a few 
score miles across, so that its track can be represented 
by a line drawn on a map of the Caribbean. 
To a human observer, it seems like a very unique, very extreme event, 
unpredictable in its ocurrence and movement, highly localised, 
and contrasting sharply with a background of the more benign conditions 
usually found in tropical zones down south. 
(On a longer time-scale, however, hurricanes are common and ubiquitous; 
a map of Caribbean tropical storms and hurricanes for the last thousand years 
is black with their tracks). 
Thus, despite their small size, brief duration, and intensity,
any point within the hurricane belt is subject to their influence. 
The temporal structure of that influence depends upon the time scale 
of other processes influenced by storms. Thus, in relation to ecological 
processes of reef growth or sedimentation, on a time scale of hundreds or 
millions of years, hurricanes are able to be regarded as continuous force. 
On such a scale, it may be possible to distinguish different intensities 
of that force due to differences in hurricane frequency in space and time. 
On shorter time scales, the occurrence of hurricanes is irregular. Their 
influence on processes measured on a time scale of the same order as the 
interval between hurricanes, such as the generation time of living organisms, 
is better understood in terms of the time elapsed since the previous storm.

This preamble will help in understanding the impact of 
Hurricane Gilbert on Jamaican coral reefs. 
After Hurricane Charlie (1951), Jamaica enjoyed thirty years 
free from the impact of large hurricane-generated waves. 
All kinds of corrals flourished. Those which occupied space 
by rapid growth were especially successful. 
In extensive thickets, we find large quantities of elkhorn, staghorn, 
brain corals, sea fans, and sea feathers.
Rapid occupation of space was achieved by a slender branching morphology; 
strong enough to resist routine wave energies, 
but fragile under extreme conditions. 
In August 1980, the eastern, northeastern, and northern shores of 
Jamaica were attacked by Hurricane Alan, which wrought catastrophic 
damage on these fragile coral reefs. 
Fragile branching corals were smashed, some massive corals were 
toppled or shattered, the waves ripped up softer organisms like 
sea-fans and sponges, and all were bombarded with fragments, 
scoured by resuspended sand and struck by powerful waves. 
The recovery of reefs to their former luxuriance had not occurred 
by September 1988, when Hurricane Gilbert struck. Thus, although the 
physical impact of waves on the north coast was comparable to that 
of Hurricane Daniel, 
the damage to reef organisms was not as spectacular, because the time 
elapsed since the previous hurricane was so short, which was fortunate. 

But at Discovery Bay, 
the reef condition is now approximately what it was after Hurricane Allen. 
Massive corals that remained erect after Hurricane Allen 
mostly withstood Hurricane Gilbert. 
In reef channels, survival was less well: 35% in 1980, 56% in 1988, 
47% in 1951. 
Acropora cervicornis, which had begun to recover at some locations, 
was completely smashed again. Under Hurricane Gilbert, 
the corals were re-mobilized, scrubbed clean and re-distributed. 
So were the shallower stretches of brain coral rubble; cemented frameworks 
in deeper water remained intact: although scoured by sediment. Remobilization 
of the rubble substratum had serious consequences for corals 
(and other organisms) that had settled on it since 1980: 
opportunistic species such as Porites astreoides, 
P. porites, Agaricia agaricites and Madracis mirabilis. 
Many sea-fans, sea-whips and sponges were overthrown or broken, 
and piles of rotting corpses accumulated in channels 
and chutes on the deeper fore-reef.

Resuspended sediment, macerated tissues, terrestrial runoff 
including material from destroyed housing, and crushed wood 
from damaged boats greatly reduced underwater visibility (and thus 
light penetration) after the storm, and it took a couple of weeks 
to return to normal visibility. High organic 
loading in deposited sediments was evident for days or weeks in the 
blackening due to sulphate reduction under the anaerobic conditions 
brought on by decomposition.

Quantities of sediment were removed from the reef terraces. 
Some was dumped onshore as rubble ramparts or floods of sand, 
but most was carried downslope; not directly, but North North-east, 
aligned with the major waves. At 20-25 m, therefore, reef lobes to 
the east of channels suffered encroachment by sand. Sand clearly 
flowed down the fore-reef slope and some of it will have passed 
through the intermittent chutes, off the terrace to the island 
slope below. On the shallow terrace (3-15 m), small sand channels 
tributary to the major chutes, have been swept clean. Hardgrounds 
have been re-exposed.