Deep Thoughts blog: Exploring blue holes

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May 8, 2019: Preparing for our deep dive into Florida’s blue holes


Photo: Past dive into a Florida blue hole. Credit: Conor Goulding/Mote Marine Laboratory.
A new round of dives will explore these mysterious environments more deeply.

In just a few days, Mote Marine Laboratory scientists will set out on a 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 meadow. As you get farther and farther away, the seagrass 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. That is one of question that sparked the interest of a small group of scientists at Mote Marine Laboratory, Georgia Institute of Technology, Florida Atlantic University, and the U.S. Geological Survey.

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.

However, that’s all about to change as Mote gears up for this interdisciplinary exploration of one blue hole, named “Amberjack hole” after one of the fishes found above these habitats frequently.

Some exploratory questions the researchers intend to answer 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 will use an array of scientific methods and instruments to answer these questions, including a “benthic lander,” a large, multipurpose instrument that will be lowered by scientific divers to the base of Amberjack hole and will measure pore water nutrients and fluxes of various chemicals across the sediment-water interface.

Chemical signatures, radon and radium isotopes, will help scientists investigate whether there is fresh water from the Florida Aquifer making its way out into the Gulf through Amberjack hole. 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 will include benthic surveys, during which divers will swim around and record 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, will be 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.

More blog posts will follow, outlining this exciting research and the scientists aboard the RV Clark as the exploration gets under way.

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