Hurricane Georges Leaves Florida Bay Flustered but Not Flushed

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 suggest that the ecological health of Florida Bay depends, in part, 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 – wind speeds did not exceed tropical storm strength over most of the bay. However, some scientists are rethinking the role hurricanes have played in the evolution and maintenance of Florida Bay in Georges’ aftermath.

Hypothesized effects of hurricanes

The broad, shallow morphology of Florida Bay (1-m average depth) exerts a strong influence on its ecology. Tides are quickly and efficiently dissipated in the lacework of banks and shoals that cris-cross 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 plankton, which clouds the water and blocks light from reaching the seagrass beds. Lack of tidal flushing allows the biologically produced sediment to accumulate, and this reduces flushing further in a vicious cycle.

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 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 and basins. Too long without a hurricane would bring on a general ecological decline in the bay.

Georges was the first hurricane to affect Florida Bay in over 30 years (Table 1). Even though the eye of Georges passed directly over Key West on 25 September 1998 (Figure 1), wind speeds over most of the bay did not exceed the 63 knot threshold that distinguishes a hurricane from a tropical storm. Hurricane Andrew was too compact and passed too far north to have much effect on the bay. It may be that the 30-year hiatus between hurricanes has contributed to the seagrass die off 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?

Georges’ timing could not have been better for scientists seeking to answer this question. 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 1998, 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 compared to past storms

Over Florida Bay, Georges was far less powerful than Inez, Donna and Betsy, hurricanes that preceded it in the 1960s (Table 1). Maximum sustained wind speed peaked at around 45 knots at Long Key and Molasses Reef, in the middle and upper Keys, making Georges only half as powerful as Donna, Betsy and the infamous 1935 hurricane, which devastated much of the Florida Keys. Less than 2 inches of rain fell 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 1998. Runoff generated by this storm, nine days earlier, was still flowing into northeast Florida Bay at the time of Georges’ passage.

Storm surge and changes in water quality

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 the east, directly out toward the Gulf of Mexico, and shifted to southeast as the storm passed. This caused water levels to rise by about 1 meter on the Atlantic Ocean side of the upper Keys and 2 meters along the middle and lower Keys. Water levels within the bay generally fell by 0.2 to 1.0 meter, but the response varied greatly between locations depending on how the wind-driven currents interacted with the network of banks and shoals. Water levels actually increased by about 0.7 meter along the north shore of the bay (Figure 2). This suggests that water blown away from the Keys remained trapped behind the broad shallow banks in the central and western regions of the bay.

The wind and the large difference in water level across the Keys, combined to drive high rates of flow into Florida Bay. Current meters at Sombrero Key and in channels within Florida Bay measured velocities in the range of 1 meter per second. This is about five times greater than normal tidal currents. It appears that water that entered the bay through the Keys either remained in the southern portion of the bay flowed north along its 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 that fell nine days before Georges, on 16 September. Higher concentrations of ammonium were seen in the bay after Georges. Ammonium, which is part of the nitrogen cycle, was probably released to the water column from the sediment by wave action during the storm. Significantly, Georges failed to flush this nutrient-enriched water out of the bay (Figure 3). It appears that this water was trapped by the broad, shallow banks in the central and western portions of the bay.

Sediment erosion and redistribution

Georges contributed to the on-going process of sediment accumulation on beaches and mangrove islands, but the bay bottom remains largely unchanged. Accretion occurred on islands and along the mangrove coast where vegetation trapped sediment-laden water thrown up by Georges and sediments settled out. 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.

Ecological impacts

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) was beginning to shed older leaves from the summer growth. At locations visited after the storm, Georges had stripped the older leaves off the plants. Scientists speculate that the leaves, floating on the surface, were swept from the bay by the wind even as most of the water, and sediment and dissolved nutrients, remained trapped in the bay. Vast mounds of seagrass collected on beaches around Naples after the storm, much of it from beds within and to the west of Florida Bay.

Seagrass beds were also culled of other species of seagrass that are less firmly rooted in the sediments and it appears that Georges removed algal epiphytes from the remaining plants. This thinning and “pruning” of the seagrass beds may result in more vigorous growth 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 by removing the older leaves, which are more prone to disease, Georges may have reduced the incidence and spread of the disease.

The aftermath: reassessing the role of hurricanes

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. Even though Georges did not completely flush the bay of dissolved nutrients; it did have a cleansing effect by removing older leaves from the seagrass beds and other organic matter that was not well anchored. Organic material that could float to the surface was swept from the bay by wind. 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 scientists studying sediments that the more frequent winter storms are mainly responsible for the slow migration and evolution of the mud banks within the bay. 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.

References

Ball, M.M., E.A. Shinn, and K.W. Stockman. 1967. The geologic effects of

Hurricane Donna in south Florida. Journal of Geology 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: Journal of Geology

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.

U. S. Navy and Department of Commerce, 1994, Global tropical/ Extratropical

cyclone Climatic Atlas, Version 1.0, CD-ROM Jointly Produced by Fleet

Numerical Meteorology and Oceanography Detachment and the National Climatic

Data Center, March 1994.

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.

Acknowledgements:

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: Nineteen tropical storms 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 in the vicinity of 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. Data are from the U.S. Navy – Department of Commerce (1994).

Date	Name	Status	Wind Speed (knots)
October 1904	unnamed	T	55
June 1906	unnamed	H	70
October 1906	unnamed	H	110
October 1909	unnamed	H	85
November 1916	unnamed	H	70
September 1929	unnamed	H	90
August 1932	unnamed	T	55
September 1935	unnamed	H	130
November 1935	unnamed	H	65
August 1936	unnamed	T	50
October 1948	unnamed	H	115
September 1960	Donna	H	130
September 1965	Betsy	H	110
October 1966	Inez	H	75
June 1968	Brenda	T	25
September 1970	Felice	TD	30
September 1972	Dawn	TD	30
October 1987	Floyd	TD	30
November 1994	Gordon	T	45

Figure 1: Although Hurricane Georges passed directly over Key West, Florida, sustained wind speeds over most of Florida Bay did not exceed tropical storm strength. Winds over Florida Bay blew generally from the east and southeast.

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 nutrients 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.

Other web resources on Hurricane Georges:

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

- Hurricane Research Division 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