Passage of hurricane Georges over the Florida Keys in September 1998 left the waters of Florida Bay agitated and murky, but Georges failed to cleanse the bay of accumulated nutrients and sediments. Some scientists have linked the recent 30-year lull in hurricane activity to recent episodes of seagrass die-off and persistent plankton blooms, which began in the late 1980s. They argue that the ecological health of Florida Bay depends on regular cleansing by hurricanes. Detailed observations by scientists immediately following Georges’ passage suggest that this hurricane did not affect the bay as anticipated by this theory. It may be that Georges simply was not strong enough. However, some scientists are rethinking the role hurricanes have played in the evolution and maintenance of Florida Bay in the aftermath of this storm.

The ecology of Florida Bay is affected by its extremely broad, shallow morphology. Tides are quickly and efficiently dissipated in the lacework of banks and shoals that fill the bay. This means that regular flushing by tides is much less in Florida Bay than in most other coastal marine ecosystems. Tidal flushing aids in the cycling of nutrients and moderates changes in temperature and salinity. Reduced flushing in Florida Bay contributes to the accumulation of organic matter and associated nutrients. This contributes to the growth of algae that cause stress on seagrasses and cloud the pristine waters. Lack of tidal flushing allows the biogenically produced sediment to accumulate, further reducing the flushing by tides.

Some ecologists see salvation in the power of hurricanes to move water and sediment. Evidence of the destruction wrought by past hurricanes litters the landscape of south Florida. Laid beside the weak tidal flushing in Florida Bay, it is easy to see how the how these intense storms might serve a constructive purpose in the ecosystem of Florida Bay. The theory is that the health of the ecosystem requires the occasional hurricane to flush out accumulated nutrients and keep sediments from building up in the shallow tidal channels. Too long without a hurricane would bring on a general ecological decline in the bay.

Georges was the first hurricane to pass over Florida Bay in over 30 years. Hurricane Andrew was too compact and passed too far north to have much effect on the bay. It may be that this long hiatus between hurricanes has contributed to the seagrass dieoff and plankton blooms that began to plague the bay around 1990. If this is so, can we expect to see an improvement in Florida Bay because of Georges?

The timing could not have been better. At the time that Georges hit, a number of research projects were underway to look at all aspects of the ecosystem. Scientists were able to revisit the bay soon after the storm and measure changes from conditions that existed before the storm. In November 1989, these scientists met to compare what they had seen and assess Georges’ effect. Their discussions focused on three questions. Did Georges replace a large volume of Florida Bay water with ocean water or rainfall? Did erosion and redistribution of sediment change the morphology of the bay? Did Georges have ecological effects that will persist over a number of years? This report provides a summary of what is known so far.

Georges was a far less powerful storm over Florida Bay than Inez, Donna and Betsy, hurricanes that preceded it in the 1960s. On 25 September 1998, hurricane Georges passed directly over Key West, about 80 miles southwest of the middle of Florida Bay (Figure 1). Maximum wind speed in the vicinity of the bay peaked at around 45 knots, making Georges only half as powerful as Donna, Betsy and the 1935 hurricane, which devastated much of the Florida Keys. Less than 2 inches of rain feel over the inner bay during Georges. By comparison, the rain gage at Joe Bay, in northern Florida Bay, recorded over 10 inches of rainfall on 16 September. Runoff generated by this storm over south Florida was still flowing into northeast Florida Bay at the time of Georges’ passage.

Records of water levels and currents in and around Florida Bay show a marked response during Georges’ passage (Figure 2). Winds blew over the bay initially from east, directly out toward the Gulf of Mexico, and shifted to southeast as the storm passed. This caused water levels to increase by about 1 meter on the windward (southwest) side of the Keys, outside of the bay. Water levels within the bay generally decreased by 0.2 to 1.0 meters, but the response varied greatly between locations depending on how the wind-driven currents interacted with the system of banks and shoals. Water levels actually increased by about 0.7 meters along the north shore (Figure 2). This occurred probably because water blown away from the Keys remained trapped behind the broad shallow bank in the central and western regions of the bay.

The wind and a large difference in water level across the Keys, combined to drive high rates of flow into Florida Bay. Current meters measured velocities in the range of 1 meter per second, which is about fives greater than normal tidal currents. It appears that this large inflow of water either remained in the south region of the bay or flowed north along the western boundary.

Water quality in Florida Bay after Georges was not notably different from conditions normally seen over the past nine years of systematic monitoring. Elevated salinity levels, present at the end of August, had declined when the bay was resampled after the storm. This was probably the result of the large amount of rainfall on 16 September. Higher concentrations of ammonium were seen in the bay after Georges. Ammonium may have been released to the water column by resuspension of sediments and irrigation of porewater related to wave action during the storm. Significantly, this ammonium-enriched water remained largely trapped in Florida Bay (Figure 3) where broad, shallow banks prevented flushing by wind-driven currents.

Georges contributed to the on-going process of sediment accretion on beaches and mangrove islands, but the bay bottom remains largely unchanged. Accretion occurred where sediment-laden water thrown up by Georges could get trapped and drop its load of sediment. The mud banks, basins and channels that make up the bottom of the bay were not affected. Some localized erosion occurred on banks exposed to particularly long wind fetches, and thus high wave energy, during the storm. These observations are similar to the lack of effect seen after Donna and Betsy, which were much more powerful hurricanes (Stockman and Shinn 1967, Pray 1966, Perkins and Enos 1968).

Georges’ direct impact on vegetation was limited to defoliation in both the mangroves and the seagrass beds. Sediment and beach wrack were reworked along the north shore of the bay. New sedimentation may have improved some areas of critical crocodile nesting habitat.

Georges hit Florida Bay at a time when Turtle grass (Thalassia) is beginning to shed older leaves from the summer growth. Georges stripped these off the plants and blew them out of the bay. The beds were also culled of other species of seagrass that are less firmly rooted in the sediments and it appears that Georges removed epiphytes from the remaining plants. Vast mounds of seagrass collected on beaches to the north after the storm, much of it from beds within and to the west of Florida Bay. This thinning and pruning of the seagrass beds may give the seagrasses a boost in productivity in the months that follow.

In the western part of the Florida Bay, researchers noted a decline in the severity of disease infection in the Turtle grass beds following the storm. The incidence of infection by Labyrinthula, a fungus, is related to the salinity, and that complicates the interpretation of these observations. However, it is possible that removal of the older leaves, which are more prone to disease, may have reduced the incidence and spread of disease in these beds.

Results so far suggest that Georges was not able to flush a large proportion of water out of Florida Bay and replace it with ocean water, and there was little, if any, change in the banks and channels. It appears that Georges has had a beneficial effect on the seagrass beds, but scientists will have to wait to see whether this will have a lasting effect.

Some scientists are now reassessing the roles of hurricanes and winter storms in light of these results. In many ways, the effects of hurricane Georges on Florida Bay resembled those of a strong winter storm rather than what many had anticipated from a hurricane. Observations following Georges confirm conclusions reached earlier by Wanless and Tagett (1989) that the more frequent winter storms are mainly responsible for the slow migration and evolution of the mud banks within the bay. Even though Georges did not completely flush the bay of nutrients, it did have a cleansing effect by removing older leaves from the seagrass beds and other organic matter that was not well anchored. It is still possible that hurricanes can have drastic and persistent effects on the ecosystem of Florida Bay. However in the aftermath of Georges it appears that this may occur only for the most powerful and least common hurricanes.

Ball, M.M., E.A. Shinn, and K.W. Stockman. 1967. The geologic effects of Hurricane Donna in south Florida. J. Geol. 75: 583-597.

Perkins, R. D. and Enos, Paul 1968, Hurricane Betsy in the Florida-Bahama area-geologic effects and a comparison with Hurricane Donna: Jour. of Geol. 76:710-717.

Pray, L. C., 1966, Hurricane Betsy (1965) and nearshore carbonate sediment of the Florida Keys: Geological Society of America Annual Meeting, San Francisco, 1966, Program p. 168-169.

Stockman, K.W., R.N. Ginsburg, and E.A. Shinn. 1967. The production of lime mud by algae in south Florida. J. Sediment Petrol. 37(2): 633-648.

Wanless, H.R. and M.G. Tagett, 1989. Origin, growth and evolution of carbonate mudbanks in Florida Bay. Bulletin of Marine Science 44:490-514.

The Georges workshop was held on 20 November 1998. The following researchers contributed their observations and ideas at this workshop: Joe Boyer, Mike Durako, Bob Halley, Jan Landsberg, Tom Lee, Chris Madden, Dewitt Smith, Ned Smith, and Paul Willis. This report summarizes these and related discussions. The workshop was organized by the Program Management Committee for Florida Bay and Adjacent Marine Systems with the support of The Nature Conservancy. More information can be found on the worldwide web at http://www.aoml.noaa.gov/flbay/.

Table 1: Twenty tropical storms have passed within 40 km of Florida Bay (25.1N 80.7W) between 1894 and 1994. Third column refers to status of the storm during its passage over Florida Bay (H-hurricane, T-tropical storm, TD-tropical depression). Fourth column is estimated wind speed over the bay. Only eleven of the storms were hurricanes at the time they impacted Florida Bay.

Figure 1: Estimated winds from Hurricane Georges as it passed over Key West. Winds over Florida Bay blew generally from the southwest for much of the storm. Measure wind speeds were much less than shown here.

Figure 2: Water levels in Florida Bay responded dramatically to the wind. Broad banks in the central and western part of the bay prevented water from being blown completely out. Instead, water blown out from behind the Keys piled up along the north shore and in the mangrove swamps there.

Figure 3: Georges stirred up sediments and raised concentrations of ammonium in the water slightly. An area of higher ammonium concentrations was observed outside of the bay after the storm. This water may have come from Florida Bay. However, the much higher concentrations of ammonium remaining in the central and northern portions of the bay show that Georges did not completely flush nutrients from the bay.

http://tropical.atmos.colostate.edu/forecasts/index.html
- long-range prediction of hurricane seasons

http://www.nhc.noaa.gov/1998GEORGESadv.html
- archive of forecasts, advisories and discussions

http://www.nhc.noaa.gov/ftp/pub/forecasts/discussion/MIATWSAT
- summary of the 1998 hurricane season, including info on Georges

http://www.aoml.noaa.gov/hrd/Storm_pages/georges/top.html
- hrd wind analyses produced for operational use

http://storm.rsmas.miami.edu/~jgottsch/georges.html
- Key West radar images of rainfall during Georges

http://storm.rsmas.miami.edu/~jgottsch/georgesamx.html
- Miami radar images of rainfall during Georges

http://www.fiu.edu/~boyerj/data.html
- SERP post-Georges data

http://coastal.er.usgs.gov/flbay/html/SEP98/ref.html
- archive of daily reflectance images

http://coastal.er.usgs.gov/flbay/html/SEP98/sst.html
- archive of daily sea surface temperature images

http://www.aoml.noaa.gov/ocd/sferpm/salmaps.html
- Florida Bay Hydrographic survey maps - Salinity

http://www.aoml.noaa.gov/ocd/sferpm/chlmaps.html
- Florida Bay Hydrographic survey maps - Chlorophyll

http://www.aoml.noaa.gov/ocd/sferpm/tranmaps.html
- Florida Bay Hydrographic survey maps - Transmittance

http://sflwww.er.usgs.gov/projects/circulation/data/sal_maps/index.html
- USGS Salinity Maps