Thomas V. Armentano, Everglades National Park, South Florida Natural Resources Center, 40001 State Road 9336, Homestead, Florida 33034-6733.

Anecdotal reports of dead and dying mangroves on islands in Florida Bay began to appear in the late 1980s and continued in following years. Aerial overflights in 1992 confirmed that, particularly in island interiors, stands of dead mangroves were common in eastern and central Florida Bay. However little information was available on the extent of the mortality pattern, its rate of development and its causes. To address these uncertainties, several studies were begun involving related aspects of the mangrove mortality concern. The present work has concentrated on determining the current composition and structure of the vegetation on selected islands and inferring the recent past vegetation from aerial photographs and the presence of woody material and other field evidence of earlier plant communities. From these data, relationships to hurricanes and droughts will be developed that should help determine the causes for the time trends in mangrove mortality and recovery that appear to characterize the islands and northern coastal area of Florida Bay.

A literature review suggests that the patterns of mangrove mortality observed in Florida Bay may be driven largely by intrinsic forest stand processes and disturbance interventions largely independent of human activities. Several papers by Lugo and colleagues, indicate that a pattern, possibly cyclic, of mangrove forest development, senescence and recovery appears to characterize some mangrove islands in the Caribbean Sea. Droughts and hurricanes intervene, however, interrupting stand development. Salinas development in island interiors often leads to hypersalinity and consequent accelerated mortality of mangroves creating a pond or basin devoid of vascular vegetation. Hurricanes, in contrast, may destroy forests and shift or deposit large quantities of sediment, thereby rendering islands open to recolonization by mangroves (Craighead, several refs.). This concept has been accepted as an hypothesis to be evaluated at least for islands dominated by mangrove forest or scrub, without discounting the possibility that other factors are involved. Initial inspection of islands from the western, central and eastern portions of the Bay, for example, shows that elevation differences, significantly influence the potential for vegetation development. Thus certain keys may be too low (e.g., most of Club Key) or too high (e.g. Murray Key) to expect the simplified model described above to be valid. Salinity of Florida Bay waters might also affect vegetation patterns by influencing pore water salinity on the islands.

Detailed vegetation inventories have been completed on Clive and Pass Keys (western and eastern Florida Bay, respectively). So far about 30 keys have been inspected either on foot or by low altitude overflights . Low-level photographs of most of these islands were made in 1995 and a photographic series is available dating back to 1935. In general, on many of the islands which aerial inspection revealed as exhibiting various stages of mangrove mortality in 1992, seedlings and twig sprouting of partially dead mangroves can now be seen. Expansion or rapid growth of halophyte cover (e.g., Batis maritima and Salicornia virginica) also is common. The extent to which any of these patterns represents "recovery" and the initiation of a new sequence of vegetation development is unclear, particularly given that the current pattern is quite variable from island to island. Future work will concentrate on further quantitative inventory of present vegetation on selected islands and aerial photograph interpretation. Results will be combined with parallel studies by the Florida Department of Environmental Protection (Marine Research Institute) and National Biological Service in order to better understand underlying causative mechanisms.

P. Carlson, S. Brinton, Florida Marine Research Institute; T. Armentano, D. Smith, Everglades National Park; J. Absten, Florida International University.

Widespread mortality of mangroves occurred in Florida Bay during spring and early summer 1991 and again in spring 1992. Black mangroves (Avicennia germinans) growing on keys with shallow, central basins (salinas) appeared to be most severely affected, suggesting that hypersaline conditions in Florida Bay might have played a role in mangrove mortality.

Our investigation focused on the role of climate and porewater salinity in mangrove mortality. Because the mangrove mortality was most acute in spring 1991, we reviewed environmental data for the period from 1987 to the present looking for unique climatic or physical conditions concurrent with mangrove die­off. We assembled temperature, rainfall, and evaporation data from National Weather Service reporting stations in South Florida. Surface water salinity and water level data were obtained from Everglades National Park monitoring program stations at Whipray Basin and Buoy Key, respectively. Water level records for Key West and Vaca Key tide gauges were supplied by the National Ocean Survey.

To hindcast porewater salinities during the 1991 die­off episode, we sampled porewater on two islands every 4 to 8 weeks between May 1992 and August 1993. Dump Key was severely impacted by die­off; Clive Key was not. PVC well screen was used to collect depth­integrated porewater samples from 5 to 35 cm depth at three stations (1,10,and 20 m from shore) along two transects on each island. Chloride and sulfate concentrations were measured each time; sulfide concentrations were measured less frequently.

Temperature, rainfall, and evaporation data indicate that spring 1991 was similar to preceding and following years. Mean maximum temperatures ranged from ca. 24°C in winter to ca. 33°C in summer; no freezes occurred in winter 1990­1991. Biweekly rainfall measured at Flamingo was similar in 1990, 1991, and 1992, and wet and dry season rainfall totals for Flamingo and Tavernier also showed no significant departures from average values during this period. Surface water salinity records for Whipray Basin show hypersaline conditions for most of 1989, 1990, and 1991. However, maximum surface water salinity occurred in fall 1989, and mangrove die­off occurred in spring 1991 and spring 1992, a period when surface water salinity was declining.

Porewater salinity at Clive Key varied from 40 ppt in winter to 49 ppt in summer. At Dump Key, salinity varied from 40 ppt in late winter to 67 ppt in summer. Regression of porewater salinity against tidal inundation frequency resulted in estimates of porewater salinity greater than 90 ppt during periods of infrequent tidal flooding. Mangrove transpiration caused significant increases in porewater sulfate:chlorinity ratios during the spring season.

Frequent tidal inundation in winter 1990­1991 is the single most striking climatic event coinciding with mangrove mortality. In particular, inundation frequencies in December, January, and February were much higher in 1991 than in 1990. As a result of residually high surface water salinity and higher than normal inundation frequency, porewater salinity in low islands within Florida Bay might have exceeded lethal levels for Avicennia. The cause of the shift in winter tidal inundation frequency is not known, but other researchers have described a shift of ca. 10 cm in mean sea level with a quasi­decadal frequency. Small changes in sea level, cyclic or otherwise, might have a pronounced influence on the mangrove communities of low islands and the mainland shoreline of Florida Bay, especially when they coincide with periods of hypersalinity in Florida Bay surface water.

This project addresses question C.2. (Tasks ii and vii) of the Seagrass, Mangrove, and Hardbottom Habitats section of the Science Plan for Florida Bay: "What environmental factors explain the pattern of mangrove die­back within the Florida Bay ecosystem?" On­going studies of mangrove mortality at FMRI include expansion of quarterly porewater monitoring to additional islands and mainland sites, historical GIS analysis of mangrove communities, and analysis of longer­term water level records.

Last updated: 07/16/98
by: Monika Gurnée
gurnee@aoml.noaa.gov