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Science Features - Discoveries of the Deep—The Surprising Undersea World at Pulley Ridge

In the deep waters of the Gulf of Mexico, approximately 250 km west of Cape Sable, Florida, and 70 km west of the Dry Tortugas, are a series of drowned barrier islands known as Pulley Ridge. The ridge was found in 1950, but it wasn't until recent years that scientists discovered something extraordinary.

Picture of Octocoral.

The southern portion of the ridge is a thriving coral reef, a pristine habitat teeming with life and color. Here, more than 60 species of fish swim in predominantly clear, warm water. An abundance of algae sprinkles the seascape in vibrant reds and greens. Brilliant blue-purple corals stretch across the sea floor like giant plates. Octocorals, with tiny featherlike tendrils and colors that vary per colony, reach out with sometimes light and delicate and sometimes bright and knobby arms. And in the soft light that filters down from the distant surface, luxuriant fields of the leafy algae Anadyomene menziesii rise from the sea floor like patches of lettuce at dusk.

The reef was discovered in 1999, as scientists and graduate students from the USGS Center for Coastal & Watershed Studies and the University of South Florida (USF) boarded the research vessel Bellows and set sail for the Pulley ridge area, where a bathymetric map of the ocean floor showed a mysterious bump.

Picture of Collecting Bucket Sample.
Collecting Bucket

When they reached the ridge, they attached a bucket to a chain and tossed it overboard. It sank to 60 m. They let the bucket drag for a moment before hauling it back up to the deck. The bucket returned with a magnificent bouquet of green leafy algae and bright purple coral. They were stunned. No one on board had ever heard of such growth so far beneath the ocean's surface.

Five years of expeditions with more bucket samples and a few newer technologies--including a remotely operated vehicle (ROV), a one-person submarine, and the USGS SeaBOSS (Sea Bottom Observation and Sampling System)--revealed the stunning and unusual underwater paradise.

Picture of One-Person Submarine.
One-person submarine

Through consultation with colleagues across the Nation, scientists have determined that the Pulley Ridge reef, at depths of 60 to 80 m, is the deepest photosynthetic coral reef found on the U.S. continental shelf.

Compared to the majority of coral reefs, which have been dying over the past 20 to 30 years, the health of the reef is also surprising. The photosynthetic corals at Pulley Ridge are thriving on only 1 to 2 percent of the available surface light--roughly 5 percent of the light typically available to shallow-water reefs. Despite the low-light conditions, the corals at Pulley Ridge are up to 60 percent live in some localities, a considerable achievement compared to many in the Florida Keys, where coral reefs popular for snorkeling are only about 5 percent live.

Picture of Spotfin Hogfish.
Spotfin Hogfish

The reef is also unusual because of the variety of life it supports. Alongside the coral cover usually found in shallow water are deep-water features such as grouper holes, tilefish mounds, and coralline algal nodules. French angelfish and bicolor damselfish share a world with spotfin hogfish and bank butterflyfish, representing an atypical mixture of both deep- and shallow-water species. The extent of algal cover and abundance of herbivores indicates productivity on the sea floor is unusually, if not uniquely, high at this depth in the Gulf of Mexico and Caribbean. Also unique to the Gulf of Mexico is the discovery of the jellylike alga, Verdigellas peltata, which has been found in the Bahamas but had not been known to inhabit the Gulf.

Picture of Verdigellas Peltata.
Verdigellas Peltata

There are many factors that help account for this unique habitat. The corals have clearly adapted to the low-light conditions--instead of growing vertically, as is typical, many colonies grow flat, maximizing their ability to absorb the precious sunlight. The barrier islands, submerged by rising sea levels 13,000 years ago, provide elevated topography and lithified substrate for the hard bottom community. Clear and warm water is brought in courtesy of the Loop Current, whose western edge flows through the region, and the ridge benefits from the thermocline, a water mass known to provide nutrients during upwelling to shallow reefs in Florida.

This unique find has created quite a stir, catching the interest of the general public as well as a wide range of researchers, from marine biologists to specialists in rock sedimentation. In its studies at Pulley Ridge, the USGS has had the opportunity to partner with a variety of organizations including USF, National Geographic, the National Oceanic and Atmospheric Administration (NOAA), and the Florida Institute of Oceanography.

Later this year, scientists will again set sail to explore the depths at Pulley Ridge. For this trip, USGS scientists will be joining colleagues from the Florida Department of Environmental Protection, the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University, Corpus Christi, NOAA, the Florida Institute of Oceanography, Mote Marine Laboratory, and the USF College of Marine Science. With so many rare and unique phenomena, Pulley Ridge poses as many questions as it does possible answers, not the least of which is why the coral is so healthy as compared to its struggling counter parts in shallower areas. The area is sure to be a research treasure for many expeditions to come.

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