TERRAIN CONDUCTIVITY SPATIAL VARIABILITY IN A MANGROVE UNDER TWO DIFFERENT HYDROLOGIC CONDITIONS, INDIAN RIVER LAGOON, FLORIDA

Recent studies indicate that hydrological processes control many aspects of ecosystem structure and function in mangroves.  Previous research at this study site has shown that porewater salinities and chemical composition are controlled by evaporative enrichment of mixed precipitation and Indian River Lagoon water.  Differences in groundwater conductivity at discrete locations correspond with changes in mangrove species composition.  In this study, we use EM-31 and EM-34 terrain conductivity as a proxy for groundwater conductivity to create a continuous site-wide spatial of conductivity and to examine the spatial variability of those data.  Transects taken along an access road provide a one-dimensional glimpse at the significant changes in conductivity across the transition from the mangrove to woody upland environments.  Readings were taken throughout the interior of the study site to create a two-dimensional, map view of conductivity variations within the different mangrove community types identified.  Spatial variability in the vertical and horizontal directions was quantified through the use of null-hypothesis statistics.  The shallow subsurface, represented by EM-31 readings, shows a greater degree of variability than the deeper subsurface, represented by EM-34 readings.  Changes in terrain conductivity correlate with transitions between mangrove community types, suggesting that the plants play a role in the conductivity of the groundwater on a small, spatial scale.  Spatial readings were taken during two different phases of mangrove management.  During the first phase, surface water inundated the mangrove infrequently, only during extreme tidal events.  In the second phase, Indian River Lagoon water was pumped into the mangrove and kept at a constant level of ~ 0.4 m.  A comparison of data taken from the two phases shows very few differences in either the EM-31 or EM-34 data.