Volume 1, No. 2
Effects of Hurricane Georges on Seagrass Disease in Florida Bay: Were there any?
by B.A. Blakesley, J.H. Landsberg, and M.O.Hall
Preliminary results from an assessment of seagrass disease in Florida Bay following Hurricane Georges were presented at a meeting in Miami (November 20, 1998) at which post-hurricane data collected in the bay by physical and biological scientists were discussed. Researchers from the Florida Department of Environmental Protections Florida Marine Research Institute (FMRI) have been studying seagrass disease in the bay since 1995 and are continuing this research with funding from Everglades National Park. This fall we were able to take advantage of an opportunity to study post-hurricane effects on seagrass disease.
Seagrasses are one of the most important components of the Florida Bay ecosystem. Many seagrass beds in the Bay are composed primarily of a single species of seagrass. This species, turtlegrass (Thalassia testudinum), forms the basis of a complex ecosystem that includes hundreds of species of flora and fauna. The seagrass communities in Florida Bay serve as permanent habitat or nursery grounds for more than 100 species of fish and 30 species of crustacea. They provide feeding and nesting grounds for sea turtles, manatees, dolphins, and birds. In addition, they provide attachment sites for organisms that live on the seagrass itself. The health of these seagrass beds is therefore vital to the stability of the Florida Bay ecosystem.
In the late 1980s, many Thalassia beds in the north-central and western regions of Florida Bay began to die rapidly. At the time of these die-offs, numerous interacting stressors were considered to play a role in the seagrass mortality. These stressors ranged from high salinity and temperature, elevated sediment sulfide levels, eutrophication, the proliferation of the seagrass pathogen Labyrinthula, and overdevelopment of seagrass communities because of lack of hurricanes. Seagrass die-off has now spread towards the southern and eastern portions of the bay and does not resemble the initial rapid die-off events of the late 1980s. Instead, this continuing die-off appears to be associated with a disease process and we suspect that several interacting stressors are now involved including lower light conditions. Low light conditions caused by resuspended sediments and persistent phytoplankton blooms may stress Thalassia making it more susceptible to disease. These low light conditions were not present during the rapid Thalassia die-offs of the late 1980s, but have characterized the environmental conditions during our study (1995-present).
The presence of the disease, caused by a marine slime mold called Labyrinthula, is evident when long brown streaks or lesions appear on the leaves of the turtlegrass. These lesions have been shown experimentally to be caused by the slime mold and to block photosynthesis when they spread throughout the leaf. We have been documenting the distribution and severity of these lesions and the slime mold infection in seagrass in 10 basins in Florida Bay two times a year for the last four years. Other FMRI researchers have also been intensively studying the distribution and abundance of all of the species of seagrasses in the Bay. Together, we have documented both a greater loss of Thalassia and higher incidences of Labyrinthula and its lesions in the western basins than in the other basins.
Hurricane Georges presented an opportunity to test the hypotheses that major storms may play a role in keeping the Bays Thalassia beds healthy. The meeting in Miami in November provided a forum for scientists studying the Bay to share their preliminary post-hurricane results. An information exchange such as this will help researchers determine what effects a hurricane the size of Georges may have upon Florida Bay good, bad, or both, or no effect at all.
At the November meeting, we reported that Hurricane Georges, which blew through the Keys and Florida Bay on September 25-26, is thought to have affected seagrass disease in the Bay, but perhaps not in a way that was originally anticipated. In order to determine what effects the storm may have had on seagrass disease, the data from previous years were compared with those collected 10 days after the storm passed.
Previous results from our disease study have shown that salinity plays an important role in controlling infection of Thalassia by Labyrinthula. Both laboratory and field data showed that the slime mold prefers higher salinities. In fact, at salinities below 15 ppt, Labyrinthula does not infect Thalassia in the laboratory; and in Florida Bay, Labyrinthula has never been found in this seagrass at such low salinities. We were initially interested in determining if the accumulated rainfall and runoff caused by the hurricane had significantly changed salinities in the Bay, but physical scientists at the November meeting showed that the hurricane had not produced sufficient rainfall to reduce salinities in the Bay enough to affect the slime mold.
Another interesting avenue of investigation came from observations made by researchers while collecting data immediately after the storm. We and our colleagues at the University of North Carolina noted that whole Thalassia shoots were not torn from the bottom by the storm and that, in fact, the only seagrass species which may have been affected in this way were the shallow-rooted species such as Halophila. However, observers out in the Bay did see large floating rafts of Thalassia leaves, and while examining shoots for disease in the lab, we observed that they had fewer of the outer (older) leaves than they normally do. These observations suggested that the storm had actually removed the outer leaves from the Thalassia shoots. These outer leaves are usually the most heavily infected, so the post-hurricane data would be expected to show a decrease in infection.
The general trend over the last seven seasons has been one of steadily increasing infection in the western basins. When we compared the post-hurricane data with those from the previous seasons, we found that in general, infection in the western basins had decreased, while infection had increased in the eastern basins, where infection had previously been relatively low. This scenario was further confused by observations made by Park Service workers stationed in Key Largo that the waters of Florida Bay had dramatically receded during the storm, dropping three feet in the eastern Bay. It would be expected that such a flow of water would remove the outer, older leaves (those usually most infected), thus lowering the observed percentage of infection in the partially denuded shoots examined after the storm.
- A summary of results from the post-hurricane assessment presented at the Miami meeting included these findings:
- Labyrinthula infection in Thalassia declined in the western basins after Hurricane Georges.
- Seasonal leaf loss was greater in the fall of 1998 than in the fall of 1997.
- The percentage of leaves with lesions declined in all basins except one (Madeira).
- In general, there were more attached dead leaves in the eastern basins in the fall of 1998 than in the fall of 1997.
Further analyses of the data suggest what may have actually happened:
(1) Labyrinthula infection in Thalassia in the western basins may have actually decreased over the past year because of improved water quality. Improved water quality may reduce stress in Thalassia, thus decreasing susceptibility to infection. In the western basins, clearer water and increasing abundance of other seagrass species that have lower light requirements than Thalassia were first noted in the spring of 1998. Such low light-adapted species (e.g. Halodule) respond quickly to improved light conditions and are able to colonize areas where Thalassia has disappeared. These trends continued in the fall and may have had a greater effect upon the seagrass communities in the western basins than relatively small Hurricane Georges did, especially since no dramatic perturbations in water levels were observed there during the hurricane.
(2) Labyrinthula infection in Thalassia in the eastern basins may have actually increased even more than scientists were able to detect the storm may have masked higher levels of infection. Physical scientists at the meeting presented measurements of higher salinities than usual (mid 30s to mid 40s ppt) in the eastern basins, both before and to a lesser degree, after the storm. Increased infection would be expected with an increase in salinities. Our previous work both in the field and in the laboratory has shown that the incidence of infection by Labyrinthula was higher at mid to high salinities. In addition, all evidence pointed to the fact that the storm caused the loss of many of the older, outer leaves, which are the ones that usually become more infected. We therefore hypothesize that because some outer leaves were washed away by the receding waters observed in the eastern Bay, even higher incidences and severity of infection would probably have been found if the storm had not passed through the area.
Analyses of the post-hurricane data will continue. In the meantime, we look forward to the spring sampling season in hopes that the data collected then will help us more fully understand the effects of a relatively small hurricane such as Georges. If salinities had not been higher this season, would the removal of the older, outer, more infected leaves have actually served to flush the Bay of the disease? Will even a hurricane the size of Georges help reduce infection in the western basins? Or will such a hurricane move infection (through the floating rafts of seagrass) to other parts of the Bay? We hope to have more complete answers to these questions, and to others not yet posed, in the spring. Stay tuned!!
For additional information write to:
Florida Department of Environmental Protection
Florida Marine Research Institute
100 8th Ave. S.E.
St. Petersburg, FL. 33701