The transition of a freshwater karst aquifer to an anoxic marine system

Abstract

Jewfish Sink is located in the shallow seagrass flats of the Gulf of Mexico in west central Florida. Jewfish Sink was a submarine spring until the drought of 1961–1962 when it ceased flowing. Today, the sink is an anaerobic marine basin and provides the opportunity to study the implications of saltwater intrusion in coastal karstic areas. The biogeochemistry of Jewfish Sink was studied from summer 2001 through spring 2004. A distinct feature of the sink is the uniform cold temperature (16–17°C) of the deeper anoxic water that does not match groundwater found nearshore or onshore (22–24°C). There are four zones within the sink: oxic zone, transition zone, upper anoxic zone, and anoxic bottom water. The anoxic bottom water does not mix with water from above but may be linked to deep Gulf shelf water through ancient aquifer conduits. The other three zones vary seasonally in oxygen, salinity, and temperature because of limited mixing in the winter due to cooding and sinking of surface water. The walls of the anoxic zones have characteristic microbial mats that are found in other sulfidic karstic features in the area. Bacterial activity appears to be carbon, limited in the anoxic zones where sulfate reduction appears to be the major metabolic process. The reduction of sulfate to sulfide appears to be driven by irregular influexes of organic matter including macroalgae, horseshoe crabs, and stingrays that become entrapped within the sink. Bacterial activity in the oxic zones appears to be phosphate limited. Although the system is partially isolated from the overlying marine ecosystem, organic input from above drives the bacterial anaerobic ecosystem, resulting in a sulfide pump. In this model, sulfide percolates up through the karst and removes oxygen from the overlying sediment, which has likely caused changes in the shallow benthic ecosystem. Jewfish Sink appears to be part of an extensive anoxic subterranean estuary that extends under parts of at least three coastal counties in Florida and serve as a model for the effects of rising sea levels or aquifer mining.