INVESTIGATING THE RELATIONSHIP BETWEEN GROUNDWATER SALINITY AND VEGETATION ON A BARRIER ISLAND AT CAYO COSTA, FLORIDA

Barrier island groundwater and natural vegetation communities are vulnerable to saltwater intrusion by storm surges. Cayo Costa, a barrier island in Southwest Florida, is exposed to tropical storms and hurricanes which affect the distribution of freshwater within the unconfined aquifer. The water table elevation and salinity of shallow groundwater (depth < 12 ft) were examined during the wet and dry seasons along a transect that extended through five natural plant communities to create a baseline relationship between groundwater salinity and vegetation. Elevation data was obtained with a GNSS rover, and shallow groundwater was sampled by driving push piezometer points into sediments at depths ranging from 3-12 ft. The water table elevation was measured with a Solinst water level meter and sampled with a Solinst pump and YSI handheld probe (pH, temperature, conductivity). The water table elevation, salinity values, tidal data, and vegetation data were analyzed using statistical regression to correlate water quality, water level, and vegetation type. An in-situ vegetation survey was conducted and natural communities confirmed using iNaturalist and publicly available data. Remote sensing images from NASA’s Landsat and ESA’s Sentinel satellite missions were examined over the past 37 years to identify past changes in natural communities to use as a proxy for changes in groundwater salinity. Transect results show a clear trend of saline to freshwater (38.73 - 0.26 ppt) from the beach dunes to the maritime hammock with variations between the wet and dry seasons. Archived satellite images show the erosion of freshwater-adapted natural communities on the south end of the island and the formation of interdune areas in the central region. Identifying the relationship between groundwater salinity and vegetation helps to reconstruct the past and predict future freshwater availability on barrier islands for humans and vegetation, which is critical for coastal management and resiliency planning for Cayo Costa and other barrier island environments in Southwest Florida.