Relationships Between Vegetation and Ground Conductivity in a Mangrove Near Indian River Lagoon, Florida.

Geophysical techniques can provide rapid and non-invasive methods of characterizing the shallow structure of mangroves. In this study, electromagnetic surveys with the EM-31 and EM-34 instruments from Geonics, Inc. were conducted to assess the spatial variability of ground conductivity, a proxy for groundwater salinity, in a mangrove on North Hutchinson Island, Florida, a carbonate barrier island.  Relationships between ground conductivities and pore-water salinities have been measured in prior surveys.  We present here apparent conductivities measured along five profiles that traverse the field site; data were inverted to obtain ground conductivity for the vadose and saturated zones.  At this site, average water-table elevations were 0.2 m below ground level.  The mangrove roots systems extend to .6 m to 1 m below the ground surface.  Sampled pore-water conductivities range from near freshwater to hypersaline. The average vadose-zone ground conductivities derived from inversion of the EM-31 data are 1400 mS/m, but range from 75 mS/m to 12,000 mS/m.  The average (EM31) saturated -zone ground conductivities are 1900 mS/m, and range more narrowly from 820 mS/m to 2400 mS/m.   There is a larger degree of variability observed in the vadose zone than the saturated zone, but the saturated zone generally has higher conductivity values associated with it.  Notably, the density of mangrove vegetation shows a strong correlation with ground conductivity in both zones.  All data show vegetated areas have more variability than salt pan areas.