Florida’s Blue Holes

Mote Marine Laboratory scientists along with a team of collaborating institutions (Georgia Tech, Florida Atlantic University/Harbor Branch, Montana State University, and USGS) and a team of volunteer technical divers just completed their final week’s worth of research cruises to explore a unique, scarcely studied, underwater landscape on the West Florida Shelf — one whose impact on the Gulf has yet to be characterized but is likely very important — Florida’s blue holes.

Most of us have heard of sinkholes on land, but did you know that sinkholes also exist in the ocean floor? Underwater sinkholes, springs and caverns are “karst” (calcium carbonate rock) features that are scattered across Florida’s shelf floor; they vary in size, shape and depth, but most are ecological “hot spots,” oases in the relatively barren seafloor.

If you are lucky enough to dive down to one of these blue holes, you will see a diverse biological community of marine species swimming around you and spread out across the sea floor. Then as you swim away from the rim of the blue hole, the seafloor will evolve from a coral landscape with sponges, mollusks and other benthic (bottom) dwellers to a seagrass meadows or soft corals. As you get farther and farther away, the seagrass and soft corals will give way to a relatively barren sea floor.

These blue holes are clearly supporting communities of marine fauna and flora, so it’s important and downright fascinating to understand why — what these unique areas have to offer that sustain these populations.

Part of the reason blue holes have evaded regular scientific exploration is their location. The opening of a blue hole can be hundreds of feet down, where only very experienced technical divers can reach, and the openings are often too narrow for automated submersibles. In fact, most reports of blue holes have come from fishermen and sport divers, not from scientists or research cruises.

The team took on interdisciplinary exploration of two blue holes,  “Amberjack hole” named after one of the fishes found above these habitats frequently and “Green Banana” named after a pile of green bananas that were floating by one day (per a fisherman legend).

Some exploratory questions the researchers intend to answer from the data gathered are:

Are these submersed sinkholes connected to Florida’s ground water? Is there potential for groundwater intrusion into the Gulf or saltwater intrusion into Florida’s groundwater?
Is there any reason to suspect a particular bluehole is secreting nutrients and thus affecting primary production? (“Primary producers” are life forms that use sunlight energy to build nutritious, complex carbon molecules and other compounds consumed by different living things in the ocean’s food web.)
Do microenvironments harbor unique or new species of microbes? (Microbes include bacteria and other microscopic life.)
Is the site likely to become a protected area?

They used an array of scientific methods and instruments to answer these questions, including a “benthic lander,” a large, multipurpose instrument that was lowered by scientific divers to the base of the holes and measured pore water nutrients and fluxes of various chemicals across the sediment-water interface.

Chemical signatures, radon and radium isotopes, helped scientists investigate whether there is fresh water from the Florida Aquifer making its way out into the Gulf through these holes. If blue holes are connected to groundwater through pathways underneath the continental shelf, then these shelf features could be a point source of land-based nutrients to the Gulf and a newly discovered source of primary productivity; and conversely, there could be potential for seawater intrusion up into the ground water with storm surges and sea level rise.

A second component of the exploration included benthic surveys, during which divers swam around and recorded the sea life present at the rim of the blue hole and above the hole in the open pelagic zone. The abundance and diversity of some species, especially protected species like whale sharks, sea turtles and commercially important fishes, were documented and given to appropriate agencies. If there is enough abundance and biodiversity of life in these habitats, they may be considered for marine protection.

Author: Robert Beckner

Bob Beckner has been diving for 36 years and technical diving for 27.  Growing up in Florida led him to cave diving and wreck diving and working in several dive shops along the way.  He was fortunate enough to have had Terrence Tysall as a technical instructor and learn in a very “GUE” like fashion before GUE existed.  He has participated in several USSR Monitor expeditions with the Cambrian foundation, gone with Mote to the Gulf Blue Holes and worked with Karst Underwater Research for 13 years helping Explore and Document Florida’s Karst features. His favorite KUR projects are Weeki Wachee Springs, Peacock Springs, Werner Boyce State Park/Deep Salt Springs and any site that needs photo documentation. Number one bucket list dive type: mine diving.  He lives in Central Florida under the watchful and protective eye of Sheriff Grady Judd.

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