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Assessment of Coral Loss, Post-Hurricane Georges, at Selected Florida Keys Reefs

A total of 272 coral colonies were counted in pre-Hurricane Georges video. Post-hurricane, a total of 236 stony corals were counted for a net loss of 36 colonies.

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by John Dotten¹ and Dave Eaken¹

¹Florida Fish and Wildlife Conservation Commission-Fish and Wildlife Research Institute

Original presentation of this information was made at the International Conference on Scientific Aspects of Coral Reef Assessment Monitoring and Restoration in Ft. Lauderdale, Florida, April 14–16, 1999.

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Storm History

Hurricane Georges, formed in the far eastern Atlantic from a tropical wave early on September 15, 1998, became a tropical storm on the morning of September 16. By late afternoon on September 17, satellite imagery indicated that Georges had developed an eye and reached hurricane strength. It has been estimated that Georges reached peak intensity with winds of 155 mph (Category 4 hurricane) on September 20. Georges’ first of many island landfalls occurred at Antigua, then at the U.S. Virgin Islands, Puerto Rico, Hispaniola, and Cuba. Georges moved into the Florida Straits early on September 25 and re-intensified, making landfall near Key West, Florida, with maximum sustained winds of 105 mph and a peak storm surge of 10.82 feet recorded on Bahia Honda Key. Georges made its final landfall near Biloxi, Mississippi, early on September 28. Downgraded to a tropical depression by mid-morning on September 29 while located near Mobile, Alabama. Georges soon lost its tropical organization and dissipated over the continental United States.

Introduction

The Environmental Protection Agency's (EPA) Coral Reef Monitoring Project (CRMP) is a collaborative effort between the University of Georgia, the University of Charleston, South Carolina, and the Florida Department of Environmental Protection Marine Research Institute. The primary goal of the project is to measure the status and trends of four types of coral habitats within the Florida Keys National Marine Sanctuary (FKNMS). There are 40 permanent reef sites from Key Largo to Key West that have been sampled annually since 1996 to determine richness and percent cover of stony coral species. In 1998, annual sampling of these sites was conducted from May through August. Hurricane Georges passed through the Florida Keys on September 25. Selected sites were re-sampled between October 1998 and February 1999 using underwater video transects to document changes caused by the storm event.

Methods

Using a Sony CCD-VX3 with full automatic settings ,video data were collected along transects 0.50 m wide and 21–24 m long during the regular 1998 field effort and, subsequently, at selected post-hurricane sites . A convergent laser system enabled the videographer to maintain camera distance above the benthos at 40 cm. Illumination was provided by two 50-watt external lights. Using two 8 mm editing machines run simultaneously, pre and post-hurricane footage of the selected 13 transects (six shallow reef and seven deep reef) were compared to determine numerical loss (or gain) of coral colonies.

Results

Shallow Reefs
The six shallow reef sites were hardest hit by Hurricane Georges: all showed decline in coral colonies (see bar graph below). Based on pre-storm values, Looe Key Reef Shallow #4 exhibited the greatest loss (40.0%) of coral colonies. The least impacted was at Looe Key Reef Shallow #2, which demonstrated a decline of only 8.3% of coral colonies. Mean coral loss was 24.6% for the six shallow reefs examined. Shallow reefs lost an average of 26.7% of the boulder corals and 63.9% of the branching corals. Conch Reef Shallow #4 lost a large pillar coral (Dendrogyra cylindrus) colony, although the actual decrease in branching corals was 25% (4 colonies to 3).

Deep Reefs
Results from analyses of the deep reefs showed a higher degree of variability. The mean coral colony loss for the seven transects was only 3.6%; however, the highest incidence of coral loss (44.4%) in this assessment was at Sand Key Reef Deep. Branching coral loss at deep sites was 100.0%. In particular, Sand Key Reef Deep #1 lost 100.0% of the nine finger coral colonies observed in the pre-hurricane video (see before and after image 1). (The Porites porites species complex includes one or more disputed species.) The number of coral colonies unexpectedly increased at three deep transects. The greatest increase of coral colonies (38.5%) was at Sombrero Reef Deep #3. There was also a 4.3% mean increase in boulder coral colonies at the deep reefs .

Before and After Image 1

Before and After Image 2

Before and After Image 3

Bar Graph of Coral Loss and Gain

Summary

A total of 272 coral colonies were counted in pre-Hurricane Georges video. Post-hurricane, a total of 236 stony corals were counted revealing a net loss of 36 colonies. Stony coral loss for all 13 transects was 13.24%. Of the total 272 corals, 246 were characterized as boulder corals (i.e. Montastraea annularis, Colpophyllia natans) and 26 colonies were categorized as branching corals (i.e. Porites porites, Acropora palmata). Sixteen boulder corals were removed (6.50% of total colonies lost). Movement of large (> 1.0m) boulder corals documented at Looe Key Reef Shallow #2 (see before and after 2) may represent a significant loss of coral cover. For all analyzed transects combined, 20 of 26 (76.92%) branching corals were lost. All remaining branching coral colonies showed evidence of storm damage.

How did Georges increase the number of hard corals?
Pre-hurricane video transects showed high macroalgae cover, primarily Dictyota spp. Hurricane Georges’ wave surge and scouring removed nearly all of the Dictyota spp. from the analyzed transects. Small coral colonies revealed in the post-hurricane video had been obscured in the pre-hurricane video (see before and after 3).

Write this down, there may be a quiz later!
A net loss of 36 coral colonies (13.24%) resulted from Hurricane Georges for the 13 transects analyzed. Branching coral loss was 76.92% (i.e. P. porites, A. palmata) and boulder coral (i.e. M. annularis, C. natans) loss was 6.61%. Macroalgae removed by Hurricane Georges revealed small corals in the post-hurricane video, which resulted in increases in colony numbers at three deep reef transects.

Future analyses of 1998–1999 Coral Reef Monitoring Project video data will document change in percent coral cover Keys-wide. Net change attributable to Hurricane Georges may be difficult to discern.

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