Florida Keys Hurricane Impacts

Schmahl, G. gschmahl at ocean.nos.noaa.gov
Tue Oct 13 12:20:08 EDT 1998

Staff of the Florida Keys National Marine Sanctuary have made a quick, qualitative survey of some coral reefs of the lower Florida Keys following hurricane Georges.  To date, the following reefs have been surveyed:  Looe Key, Maryland Shoal, Western Sambos, Eastern Dry Rocks, Rock Key and Sand Key.  All of these reefs exhibited hurricane related impacts.  In general the center of the storm injury appeared to be at Western Sambos reef, with lesser amounts observed in either direction.  The power of the storm in this region was impressive.  Large amounts of sediment and rubble have been moved around.  In some cases 2 feet or more of sediment has been removed from the grooves between the coral spurs and deposited on the reef flat and back reef areas.  Many navigational markers were knocked down by the storm.  Marker 24, a cluster of five steel I-beams at Looe Key is still embedded in the substrate, but bent over at a 90 degree angle on the sea floor.  Many of the other reef markers are also down including ones at Western Sambo, Maryland Shaol, Coffins Patch and Newfound Harbor.  One survey site marker for the EPA coral monitoring program, which is a 1 inch square stainless steel stake drilled and epoxied into the reef substrate, was observed to be bent over and cracked at the base.  

Hurricane impacts were observed down to 70 feet (Looe Key) and 60 feet (Western Sambo). However, in general, the impoacts at these depths were relatively minimal. At these locations the tops of many barrel sponges (Xestospongia muta) were sheared off.  There was also evidence of movement of sediment and rubble, and a few coral heads were toppled.  One significant observation is that all reefs, including the deeper ones, have been essentially "scoured" of most benthic macroalgae (with the exception of Halimeda).  Prior to the storm, it had been observed that there was an unusually abundant amount of benthic algae, primarily Dictyota, on all reefs.  This algae is gone now, giving the reef a "clean" appearance that I have not observed in many years.

The fore reef zone of these spur and groove reefs also, in general, look pretty good.  As already mentioned, a large amount of sediment and rubble has been removed from the grooves.  I observed a number of coral heads, some quite large (2 meters high), that were broken off and lying in the grooves.  There seemed to be a quite substantial amount of this at the west end of Looe Key, where I also observed a portion of a reef spur that had cracked off and incompletely separated from the reef.  Again, most of the benthic macroalgae on the fore reef is now gone.  Acropora palmata on the fore reef was impacted (major branches boken off, etc.) but for the most part survived.   

The major imacts to the surveyed reefs occurred in the shallow reef zones.  All reefs surveyed sustained significant injury to Acropora palmata populations which were located in the shallow reef flat and back reef environments.  In some areas, such as Western Sambo and Looe Key, the elkhorn coral has been eliminated from portions of the reef flat and back reef areas.  I was surprised by the lack of broken branches and other rubble material in these areas - it has been simply swept away.  However, there are some surviving bases at both reefs that can act as a population source for regeneration, as well as some broken pieces that could reattach. Losses of palmata at Western Sambo in this zone is probbably greater than 50%.  At Looe Key the only palmata fragments I could find on the reef flat was at the western end of the reef. The extensive palmata zone at Rock Key was heavily impacted, but there appeared to be many more colony bases intact and broken pieces around than at Western Sambo.  A large amount of newly exposed rubble has been redeposited on the reef flat.  This newly exposed material, which is yellowish white, has been rapidly colonized by a fine, filamentous green algae, which in some cases is quite heavy, giving some areas a greenish hue when observed from the surface.

These are only "quick look" observations.  Further surveys are ongoing.  The FKNMS is fortunate in that there are a number of quantitative monitoring stations on these and other reefs that have been established in recent years.  These stations will be re-surveyed to give a more detailed account of hurricane impacts.

G.P. Schmahl, Lower Keys Regional Manager
Florida Keys National Marine Sanctuary
216 Ann St., Key West, FL   33040
(305) 292-0311

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