The tidal influence on the Big Pine Key saltwater/freshwater interface was analyzed using time-lapse electrical resistivity imaging and shallow well measurements. The transition zone at the saltwater/freshwater interface was measured over part of a tidal cycle along three profiles. The resistivity was converted to salinity by deriving a formation factor for the Miami Oolite. A SEAWAT model was created to attempt to recreate the field measurements and test previously established hydrogeologic parameters. The results imply that the tide only affects the groundwater within 20 to 30 m of the coast. The effect is small and caused by flooding from the high tide. The low relief of the island means this effect is very sensitive to small changes in the magnitude. The SEAWAT model proved to be insufficient in modeling this effect. The study suggests that the extent of flooding is the largest influence on the salinity of the groundwater.

Coastal forest retreat in the Florida Keys during the 20th century has been attributed to a combination of sea level rise and hurricane storm surge impacts, but the interactions between these two disturbances leading to forest decline are not well understood. The goal of my research was to assess their effects over a period spanning more than two decades, and to examine the relationships between these press and pulse disturbances and freshwater availability in pine rockland, hardwood hammock, and supratidal scrub communities. Impacts and recovery from two storm surges, Hurricanes Georges (1998) and Wilma (2005), were assessed with satellite-derived vegetation indices and multiple change detection techniques. Impacts were greater at lower elevations, and in hardwood hammock, spectral signatures indicative of plant stress and productivity returned to pre-disturbance levels within a few years. In pine rockland, impacts were predominately related to Hurricane Wilma, however, a similar return to pre-disturbance conditions was absent, suggesting that trajectories of disturbance recovery differed between the two communities. Long-term monitoring of forest composition, structure, and groundwater salinity showed that compositional shifts in the low shrub stratum were associated with salinization of the freshwater resource attributable to sea level rise. Throughout the course of twelve months of climate and groundwater monitoring (2011-2012), groundwater salinity generally decreased in response to large precipitation events. Modeling of geophysical data indicated that groundwater salinity was an important predictor of community type. Isotopic analysis of d18O in plant stem water and foliar d13C was used to determine temporal and spatial patterns in water use and plant stress in two community dominants, slash pine, Pinus elliottii var. densa, and buttonwood, Conocarpus erectus. Both species relied heavily on groundwater, and plant stress was related to increasing groundwater salinity. The results of this work suggest that the interaction of press and pulse disturbances drive changes in community composition by causing mortality of salt-sensitive species and altering the freshwater resource.