DYNAMICS OF WATER LEVELS IN EPHEMERAL PONDS OF THE MUNSON SANDHILLS, LOWER COASTAL PLAIN OF NORTHWEST FLORIDA

Small, isolated wetlands known as “ephemeral” or “vernal” ponds are characterized by seasonal filling and drying cycles. Because these ponds are usually devoid of fish they provide a safe environment for the larval and adult species of amphibians and insects. Approximately one-half of all frog species and one-third of all salamander species rely on temporary wetland environments for development.

Several hundred ephemeral ponds, located in the Munson Sandhills south of Tallahassee, Florida, are part of a longleaf pine ecosystem that has been the site of numerous amphibian biodiversity studies. Many species are diminishing. Several factors may be involved, including loss of habitat due to human impacts, disease and the impacts of prolonged periods of drought on breeding sites.

This study involves determining the relationship between the hydrocycles in the ponds and variations in regional groundwater levels. The ponds are associated with a 5-10 meter layer of clean sands that contain thin lenses of clay. The sands rest upon limestone units that comprise the regional aquifer system. The relationship between water levels in ponds and the groundwater level was determined on both a regional and local scale. Water levels (or the ground surface at the bottom of dry ponds) determined using 2010 lidar surveys was compared to a 2006 map of the groundwater surface. Approximately 85 percent of the base of the ponds are within 2 meters of the groundwater surface. Local comparison of water levels in an ephemeral pond and nearby sink, exhibit a high correlation in water level variation, with the water levels in the pond being 0.2 meters higher that water levels in the sink. The water chemistry between the pond and sink is different, with water from the sink source having a higher pH and significantly higher TDS. A working model for the dynamics of water levels in the ephemeral ponds is that ponds fill under two conditions – (1) the regional groundwater surface is very near to the elevation of the base of the pond and (2) high seasonal rainfall provides additional water to the surface sand aquifer, which can produce a local, low TDS “cap” that rests upon the main groundwater surface. Climate change, with increasing periods of low rainfall and increased consumptive use of groundwater are potential treats to the long term sustainability of the ponds.