Florida Coastal Sinkhole Project

Introduction
    The coastal lowlands of west-central Florida are known for their shallow karstified limestone which contain numerous sinkholes, karst windows and cave systems.  The inland cave systems are fresh water but onshore systems within 5 km of the Gulf of Mexico are often influenced by saltwater.   Some of these systems vent freshwater into the Gulf as submarine springs.  Many of the karst windows are interconnected by extensive underwater cave systems, some of which have been mapped by cave divers.  There is also an even more extensive crevicular system (Hobbs HH 1994, Galassi DMP 2001) that interconnects most of the caves, springs, sinkholes, and karst windows within an aquifer system but cannot be explored directly. Similar submerged karst systems are found throughout Florida, the Bahamas and the Yucatan Peninsula (e.g. Pohlman JW, Iliffe TM Cifuentes LA 1997).

    In coastal karst environments, transitions between fresh and seawater systems may occur because of  sea level changes, or in more recent history, the overuse of freshwater aquifers for public water supply and agriculture.  Aquifers  discharge freshwater into shallow marine waters by diffuse seepage through the crevicular system or more rarely through larger submarine springs via cave conduits (Swarzenski and Reich 2000).  This discharge of fresh aquifer water into coastal  marine waters is widespread around the Florida peninsula.  

    When the flow of freshwater into coastal marine waters ceases due to sea level changes (Brinkmann and Reeder 1994) or overuse of the aquifer, the caves, sinkholes and crevicular system that underlies the shallow seawater would be expected to be inundated with seawater and in the absence of flow the seawater should  become anoxic.  

    Our hypothesis is that these anoxic marine systems are dominated by anaerobic respiration through sulfate reduction to create sulfide which drastically changes the biogeochemical processes in the underlying limestone system and  has a major effect on the biota living within the overlying marine sediment.  We propose that this condition is becoming more frequent along the Florida coast due to the overuse of the freshwater aquifer and that the  Jewfish Sink system (see below) can serve as a model for the biogeochemical processes caused by saltwater intrusion into former freshwater coastal aquifers.  More broadly,  this system can serve as a model for changes to be expected from rising sea levels.


Figure 1. Map of central western Florida showing the locations of the sinks and cave systems relevant to this project.

Description of the Field Area and Preliminary Observations
    The coastal region of Pasco County in Florida overlies miocene Karst limestone  with a number of known caves and sinkholes, many  that are part of the local aquifer.  Jewfish sink is located in the Gulf of Mexico approximately 1 km from shore near Aripeka in Pasco county and was once an active freshwater spring (Fig 1 and Fig 2) ).  In the early 1960s the flow stopped, presumably from overuse of the aquifer and Jewfish sink became a deep (65m) anoxic basin (Wetterhall 1965, Rosenau and Faulkner 1977). The conditions at Jewfish sink representing a transition from a freshwater to a marine system are similar to those proposed for a large sediment-filled sink in the  Florida Keys (Shinn et al. 1996).   My graduate student (Michael Garman) and I have measured dissolved oxygen, temperature, sulfide, nitrate, ammonia and conductance while exploring the sink using SCUBA on  a series of dives approximately every other month for the past 18 months. Our preliminary observations can be summarized as follows:  The water in the sinkhole appears to be stagnant except for the annual sinking of cold surface water during the winter.  By summer, the sink may reach an equilibrium in which sulfide production driven by organic carbon oxidation in the anoxic bottom water is in equilibrium with oxygenated surface water.  The sink becomes a diffusion system in which oxygen diffuses downward from the surface where the water is in equilibrium with the atmosphere, and sulfide diffuses upward from the bottom water where sulfate reduction occurs. Both oxygen and sulfide concentration are near zero at the chemocline.  Sulfide measurements in the water below the chemocline range from 1 to 15 mg/L, while nitrate and ammonia range from 1 to 6 mg/L.  The sulfide below the chemocline appears to support two communities of sulfur oxidizing bacteria as evidenced by two pH minima (Figure 3).  There is an upper community at the chemocline that likely uses oxygen as an electron acceptor and a second community below the chemocline that likely uses nitrate as and electron acceptor.  The density relationships between the two sulfur oxidizing bacterial communities and the concentrations of oxygen, sulfide and nitrate can be expressed as a series of diffusion equations with initial concentrations of sulfide and nitrate dependent on the flux of organic carbon in the sinkhole  There are extensive bacterial mats in the anoxic regions of Jewfish Sink that support both prokaryote and eukaryote microorganisms (Fig 4).


Figure 2.  Diagram of Jewfish Sink

    Preliminary sulfide measurements of sediment pore water in and around the sinkhole are in the same range as those measured in the water column below the chemocline in the sink.  This suggests that the sink is connected to an extensive underground crevicular system that was once part of the aquifer but is now marine and anoxic.  Sulfide appears to percolate from the crevicular system up through the limestone, causing the thin layer (2-10 cm) of medium grain sand that overlies the limestone to be largely anoxic. Direct observation reveals that only the top 1 or 2 mm of the sand is oxygenated, while the sand below is black and sulfidic.  We suspect that the enormous amount of sulfide coming from the crevicular system through the sediment scavenges most of the oxygen that diffuses down into the sediment and has a much more profound affect than ordinary sulfide flux generated from anaerobic bacteria living within the sediment itself.  This system appears to extend for at least several km along the near shore in this region  and may have a large impact on the biodiversity of the benthic invertebrates. For example, starfish and sea urchins are not found in the area surrounding Jewfish sink although they are commonly found elsewhere along the Gulf coast.
    We have also begun to explore a number of nearby onshore sinkholes (Fig 1).  Palm sink is  2 km from the Gulf and has freshwater in the upper 30m but Gulf water in the deeper regions (30-50m).  Vents through which the Gulf water enters the sinkholes are visible at the bottom of the sink and flow appears to be tidally influenced.  Bacterial mats similar to those found in the anoxic regions of Jewfish sink can be seen within  these vents. Ward’s sink is located 15 km southeast of Jewfish Sink and is nearly identical in size and depth (65m) but is entirely freshwater and lacks the sulfide and bacterial mats of the other sinks.   The Crystal Beach cave system is an active freshwater spring 35 km south of Jewfish Sink that vents freshwater into the Gulf 200 meters offshore and may be similar to the extinct spring system of which Jewfish Sink is a remnant.  The cave system is tidally affected and in some areas has a halocline with freshwater overlying gulf water. Macrofauna such as starfish and sea urchins are commonly observed near the spring.  Jewfish Sink and these other sinkholes and cave systems are a good model to study the effect of the transition of a freshwater spring system to an anoxic marine system: Ward’s sink represents a completely freshwater system with no marine influence.  Palm sink is in transition from a freshwater system to an anoxic marine system, while the Crystal Beach Spring is a tidally influenced active spring system similar to the condition of Jewfish Sink before it became an anoxic marine basin.


Figure 3. Sample of Physical Data Collected at Jewfish Sink
         
    We have collected samples from the water column in Jewfish sink from several depths above and below the chemocline as well as samples of  sediment and bacterial mats.  We have amplified 16S rRNA genes with eubacterial primers (Lane 1991) and cloned libraries of the 16S amplicons.  We currently have several hundred independent clones from the different samples and have begun to sequence them with the intent of identify the members of the bacterial communities present in the Jewfish system by phylogenetic analysis with known sequences from Genbank.  We are just beginning to repeat the process with primers specific for Archaea (Barnes et al 1994.).


Figure 4.  Wall of Jewfish Sink below the redox boundary.  The individual strands of mat are up to 15 cm in length.

    The sediment on horizontal ledges and fissures within and around the mouth of Jewfish spring support what appears to be a thriving community of nematodes and other meiofaunal animals.  Meiofauna are microscopic animals between 50 and 500 microns in length and have been used as indicators of biodiversity (Kennedy and Jacoby 1999).  The only macrofaunal invertebrates appear to be some encrusting sponges. Because of the high sulfide levels in these sediments, we suspect that the meiofauna are sulfide tolerant and it is likely that some of them have bacterial symbionts (ref).  Hand sorting and identification of meiofauna is tedious and requires expertise that few scientists possess so we are taking a molecular approach to meiofauna similar to the approach for bacteria.  To measure the biodiversity of the meiofaunal community, Melissa Adorno, an undergraduate in the lab has extracted DNA from the sediment and amplified metazoan 18S rRNA genes.  She has cloned libraries and prepared DNA from two hundred clones that awaits DNA sequencing.  We expect to be able to reconstruct the meiofaunal community structure from these sequences.

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