Karst estuaries are governed by interactions between inland hydrological conditions and sea level Damian M. Menning a , Jonathan G. Wynn b , James R. Garey a,⇑ a Department of Cell Biology, Microbiology, and Molecular Biology, University of South Florida, 4202 E. Fowler Ave., ISA 2015, Tampa, FL 33620, United States b School of Geosciences, University of South Florida, 4202 E. Fowler Ave., NES 107, Tampa, FL 33620, United States article info Article history: Received 25 March 2015 Received in revised form 27 April 2015 Accepted 11 May 2015 Available online 19 May 2015 This manuscript was handled by Peter K. Kitanidis, Editor-in-Chief, with the assistance of Barbara Mahler, Associate Editor Keywords: Aquifer Geochemistry Mixing zone Salt water intrusion summary Karst 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 result- ing 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 signif- icant 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 sea- sonally, 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 dis- charged directly to the estuary and bypassed the spring. The number of spring reversals (salt water intru- sion 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 season- ally at the individual sites. Our data shows that this karst estuary system is governed by the interactions between inland hydrological conditions and sea level. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Submarine groundwater discharge (SGD) through subterranean estuaries has been recognized as a major source of nutrients to coastal ecosystems (Burnett et al., 2006; Moore, 2010; Smith and Cave, 2012; Szymczycha et al., 2012). This influx of nutrients may have dramatic impacts on the ecology of the surrounding estuaries (Johannes, 1980; Kotwicki et al., 2014) and can be seen by the resulting geochemical gradients (Burnett et al., 2003; Kim et al., 2005). Increased aquifer usage and/or increased sea levels may result in salt water intrusion to coastal aquifers by shifting the mixing zone underneath estuaries inland, and turning them from estuarine to salt marsh habitats. This may have dramatic effects on the biological communities which currently serve as nurseries for numerous invertebrate and fish species (Beck et al., 2001). Estuaries are semi-enclosed bodies of water which have a free connection to the open sea and within which sea water is measur- ably diluted with freshwater derived from land drainage (Pritchard, 1967). This definition generally applies to freshwater inputs from surface rivers and streams which may be secondary to subterranean drainage in karst regions. Moore (1999) defined the subterranean estuary as a coastal aquifer where terrestrial http://dx.doi.org/10.1016/j.jhydrol.2015.05.021 0022-1694/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +1 (813) 974 7103. E-mail address: garey@usf.edu (J.R. Garey). Journal of Hydrology 527 (2015) 718–733 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol