DOUBLE KEYHOLE: AN ACTIVE SPRING-FED KARST ESTUARY IN THE GULF OF MEXICO

Karst estuaries are ecosystems created by freshwater inputs that flow directly into the sea through karst conduits and/or matrices. We determined the hydrological, physical, geochemical and biological characteristics of a karst estuary resulting from the discharge of Double Keyhole Spring into the Gulf of Mexico. Sampling was carried out within the conduit, the discharge pond and two sites along the estuary toward the Gulf of Mexico. SCUBA divers deployed instrumentation within the spring conduit and collected discrete water samples over a two year period. The spring discharge is brackish due to mixing between Gulf of Mexico saltwater and freshwater from the aquifer. The composition of the spring discharge varies seasonally, showing increased marine influence during the wet season. In June 2012, Tropical Storm Debby resulted in measurable freshwater inputs from the upper portion of the Upper Floridan aquifer that discharged directly to the estuary and bypassed the spring. The number of spring reversals into Double Keyhole Spring increased as the dry season progressed, stopped immediately after Tropical Storm Debby, and then increased into the next dry season. Geochemical differences were found along the spring/estuary transect on each collection date and seasonally at individual sites. This karst estuary system appears to be governed by interactions between inland hydrological conditions and sea level. We examined the microbial communities within the system using Quantitative PCR as a measure of abundance and Length-Heterogeneity PCR to measure species richness and diversity. We found that the ultimate driver of biological change within this karst estuary was the volume of aquifer discharge from the spring. Physical gradients formed by aquifer discharge had a stronger influence than geochemical gradients on the microbial communities in the karst estuary. Microbial abundance varied inversely with aquifer discharge, while OTU richness varied directly with aquifer discharge. Microbial communities varied most between low and high aquifer discharge conditions observed before and after Tropical Storm Debby . Finally, as a management tool, hydrological condition could potentially serve as a surrogate for the state of the biological communities within the karst estuary/aquifer system.