Coral Ecosystem Connectivity 2013: From Pulley Ridge to the Florida Keys

Expedition Purpose

Why Are Scientists Exploring Deep Coral Fore Reefs?

A key purpose of NOAA’s Office of Ocean Exploration and Research is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species.

Coral reefs provide habitats for some of the most diverse biological communities on Earth. Most people have seen photographs and video images of shallow-water coral reefs, and many have visited these reefs in person. Shallow-water corals often have microscopic algae called zooxanthellae (pronounced “zoh-zan-THEL-ee”) living inside their soft tissues. Because they are capable of photosynthesis, these algae provide an important source of nutrition for many coral species and may also be involved with the corals’ growth. In deeper waters (below 150 m – 200 m), light levels are not adequate to support photosynthesis; but ocean explorers have discovered extensive mounds of living coral in depths from 400 m to 700 m. These deep-water corals do not contain zooxanthellae, and do not build the same types of reef that are produced by shallow-water corals; but the diversity of species in deep-water coral ecosystems may be comparable to that of coral reefs in shallow waters (for more information, activities, and lessons about coral reefs, visit the National Ocean Service Coral Reef Discovery Kit at

Recently, ocean explorers have discovered a third type of coral ecosystem: light-dependent deep reefs living in what coral ecologists call the mesophotic zone (or “twilight zone”) in depths of 30 m to over 150 m, depending upon water clarity. Shallow-water coral reefs have been intensively studied by scientists using self-contained underwater breathing (SCUBA) equipment, while deep coral systems are being investigated with human-occupied submersibles and remotely operated underwater vehicles (ROVs). Mesophotic coral ecosystems are beyond the safe range of conventional SCUBA equipment, yet are too shallow and close to shore to justify the use of expensive submersibles and ROVs; but advances in undersea technologies over the past decade have begun to make investigating these ecosystems possible. While only a few studies of mesophotic zone reefs have been done using these new capabilities, data from these studies suggest these ecosystems include coral, sponge, and algal species that provide important refuges and nursery habitats for corals and fishes found on shallower reefs.

Scientists believe it is urgent to understand the connections between mesophotic and shallow-water coral ecosystems, because shallow coral ecosystems around the world are threatened by climate change, fishing, pollution, invasive species, and other human activities such as dredging and anchoring. These threats not only jeopardize coral reefs, but also endanger many benefits provided by these reefs such as supporting recreation and tourism industries, protecting shorelines from erosion and storm damage, supplying foods that are important to many coastal communities, and providing promising sources of powerful new antibiotic, anti-cancer and anti-inflammatory drugs.

Mesophotic coral ecosystems are subject to many of the same threats faced by shallower coral ecosystems, but the extent of these threats to mesophotic ecosystems is unknown and needs to be evaluated. Since mesophotic corals are adapted to live in low-light conditions and require sunlight for survival, anything that limits light penetration (such as dredging or sediment runoff from the land) can be very harmful. Mesophotic coral ecosystems may also include species that are only found within this depth range or geographical location. These species are known as endemic species, and are especially vulnerable to disturbances from human activities and may face extinction if they are overexploited.

Pulley Ridge is a mesophotic coral ecosystem off the southwest coast of Florida in 60-80 meters depth, and is the deepest light-dependent coral reef that has been discovered off the United States. Pulley Ridge was originally discovered in 1950, and was found again in 1999 by scientists from the U.S. Geological Survey (USGS) and graduate students from the University of South Florida. Since then, a series of expeditions have revealed that coral ecosystems at Pulley Ridge are considerably healthier compared to many in the Florida Keys, and are unusual because of the variety of life they support. Scientists hypothesize that Pulley Ridge may play an important role in replenishing key fish species, such as grouper and snapper, and other organisms in downstream reefs of the Florida Keys and Dry Tortugas. Since most of Florida’s reefs have severely declined over the past 30 years, this potential role means it is important to protect, and manage, Pulley Ridge as a possible source of larvae that can help sustain Florida’s reef ecosystems and the tourism economy that depends on them.

In 2011, NOAA’s National Centers for Coastal Ocean Science’s Center for Sponsored Coastal Ocean Research began a five year project to investigate the role that reefs of Pulley Ridge and the northern Gulf of Mexico may play in replenishing key fish species and other organisms in the downstream reefs of the Florida Keys and Dry Tortugas. The overall goal of the project is to create a comprehensive understanding that can support robust resource planning and management in the eastern Gulf of Mexico. The Pulley Ridge Project is a collaboration of more than 30 scientists from eleven different universities pooling their expertise through NOAA’s Cooperative Institute for Marine and Atmospheric Studies (CIMAS) in coordination with the Cooperative Institute for Ocean Exploration Research and Technology (CIOERT).

During the project’s first year (2012), fieldwork began that included:

  • Installing moored instruments at Pulley Ridge and the Dry Tortugas to measure temperature, salinity, and currents;
  • Field-testing moored larval light traps to resolve any design issues and work out specific sampling procedures (for more about larval light traps, please see (PDF, 3.8 MB) );
  • Conducting transect surveys with an ROV to quantify benthic habitats and organisms, and to identify suitable sites for technical divers to collect specimens;
  • Collecting specimens with technical diving (for more about technical diving, please see the Expedition Purpose for the Cayman Islands Twilight Zone 2007 Expedition;
  • Sampling plankton using the Multiple Opening/Closing Nets and Environmental Sampling System (MOCNESS), which is a system of nets that can be opened and closed at different depths, and also carries a number of sensors for measuring environmental parameters as it is towed (such as conductivity, temperature, pressure, fluorescence, optical transmission, dissolved oxygen, and light levels). For more about MOCNESS, please see ;
  • Using a CTD to measure temperature, dissolved oxygen, salinity, fluorescence, turbidity, and depth (for more about CTDs, please see the “Where’s the Oxygen” lesson;
  • Deploying a drifter to measure surface currents; and
  • Beginning to archive information about Pulley Ridge in a database at Harbor Branch Oceanographic Institute.

The Coral Ecosystem Connectivity 2013 from Pulley Ridge to the Florida Keys Expedition will continue this work.