Karst estuaries are governed by interactions between inland hydrological conditions and sea level

- We define and describe a particular kind of karst estuary.
- Aquifer discharge correlates to tidal height and aquifer level.
- Sea level and inland hydrological characteristics determine the mixing zone.
- Geochemical gradients through the estuary are determined by aquifer discharge volume.

SummaryKarst estuaries represent unique systems created by freshwater inputs that flow directly into the sea through karst conduits and/or matrices. In order to determine the characteristics of a karst estuary resulting from the brackish discharge of Double Keyhole Spring into the Gulf of Mexico, we monitored short-term tidal fluctuations, long-term rainfall patterns, aquifer levels, spring discharge, and multiple geochemical parameters along a transect from the spring through the estuary. We monitored four sites along a spring/estuary transect and a nearby inland freshwater spring to represent the freshwater portion of the Upper Floridan Aquifer. Datasondes were deployed in Double Keyhole Spring to measure discharge volume, tidal fluctuations, and physical water parameters for two years. Water samples were collected quarterly from both springs and the surrounding surface sites over the same time period. An isotopic/trace element mass balance tracer method was used to determine the hydrogeological components of the spring discharge from three possible sources: (1) freshwater from the upper portion of the Upper Floridan aquifer, (2) freshwater from the lower portion of the Upper Floridan aquifer, and (3) saltwater from the Gulf of Mexico. Within the water column of the submarine spring conduit, there were no significant differences of the sampled parameters over short sampling distances (<400 m) and periods (<1 h). Spring discharge directly correlated to aquifer level and negatively correlated to tidal level. The brackish nature of the spring discharge is due to simple mixing between Gulf of Mexico saltwater and freshwater from the lower portion of the Upper Floridan aquifer. The composition of the spring discharge varied 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 (salt water intrusion events) into Double Keyhole Spring increased as the dry season progressed, stopped immediately after Tropical Storm Debby, and then gradually increased into the next dry season. Statistically significant geochemical differences were found along the spring/estuary transect on each collection date and seasonally at the individual sites. Our data shows that this karst estuary system is governed by the interactions between inland hydrological conditions and sea level.