Endangered Cold-water Coral Reefs

Gregor P. Eberli, Mark Grasmueck, Thiago Correa

One of the unique characteristics of the Deepwater Horizon oil spill in the Gulf of Mexico is the creation of oil plumes in the water column and the risk they produce for planktonic and benthic (sea floor) life. For the first time, not only coastal habitats, but also deep-water ecosystems are at risk. Thousands of barrels of oil a day are gushing out at great depth (1502 m/ 4928 ft) but not all the oil reaches the sea surface. The stratified water of the Gulf captures or slows the ascent of the oil. In addition, the dispersants being pumped into the escaping oil near the source produces small droplets that float for a considerable time in the water column and probably never reach the surface. Several plumes have been reported; the latest of these plumes to be discovered on May 27, 2010 is 22 miles long, 6 miles wide and several hundred feet thick.

Oil plumes within the deep waters pose a particular danger to life in within the water column, which is the habitat of planktonic microorganism and deep-water fish such as blue fin tuna. Deep water communities in the Gulf of Mexico and the Straits of Florida include coldwater corals that live in water depths of 600 – 1500 m in waters as cold as 3° Celsius. These cold-water coral communities are in great danger.

Cold-water corals, unlike their shallow-water counterparts do not live in symbiosis with the unicellular algae called zooxanthellae, but are animals that feed on organic matter floating through the water column. It is known that most of the food consumed by the cold water corals is produced in the surface waters and then sinks down to the corals. The enormous oil plumes in the water column are situated between this source of food and the deep-water corals.

oil plumes

As the organic material sinks through the water column it will pass through the oil plumes and get contaminated by the micron-sized oil droplets within the plumes. It is most likely that the delicate cold-water corals are not able to digest these oil-ladden food particles and will perish in large numbers.

deep water coral
Deep-water coral with a crab in 670 m water depth in the Straits of Florida. Photo courtesy John Reed, Harbor Branch Oceanographic Institute.

Is this really a problem? How many cold-water corals exist in the deep oceans? The fact is that there are more deep-water coral reefs in the Gulf of Mexico and the Straits of Florida than shallow-water reefs on the Florida shelf. Deep-sea coral ecosystems are common off the southeastern U.S. and include a variety of high-relief, hard-bottom habitats at numerous sites from the Blake Plateau off North Carolina, southward through the Straits of Florida, and in the eastern Gulf of Mexico. This distribution matches the path of the Loop Current that forms from the water masses in the Gulf of Mexico, enters the Straits of Florida to form the Florida Current and further north, together with the Antilles Current, forms the Gulf Stream. New data from the spill indicates that some of the plumes are caught in the Loop Current and may start to make their way into the Straits of Florida. The Straits of Florida is a prime cold water province where in many places the seafloor is covered with up to 70% of deep water coral mounds.

Three-dimensional renderings of coral mound fields in the Straits of Florida created at the University of Miami from the multibeam high-resolution bathymetry maps illustrating the abundance and variability of the mound morphology.  Current direction is indicated in red. Top left: Middle of the Straits of Florida. Top right: Coral ridges along the lower slope of the Miami Terrace. Bottom left: Mounds along the base of Great Bahama Bank. Bottom right: Coral mound ridges offshore Bimini.

Along the slope of the Miami Terrace, kilometer-long ridges cover the lower slope. In the middle of the Straits just offshore of Miami, the entire sea floor looks knobby because cold-water coral form mounds, each approximately 20 – 40 meters high, like an underwater chain of mountains. Along the slope of Great Bahama Bank the mounds form impressive edifices of over a hundred meters in height. In one 49 square kilometer area over 500 mounds exist. Similar mound fields are being studied further north along the slopes of Little Bahama Bank and onto the Blake Plateau.

The migrating oil plume has the potential to destroy or greatly diminish these communities by interrupting the food supply from the surface waters to the deep-water corals. There is no known technique to clean the water column from these oil plumes, and as a consequence the hidden oases of corals in the deep, cold waters of the Gulf of Mexico, the Straits of Florida and the Blake Plateau are in severe danger of being wiped out by this oil spill.

mound field
Mound Field at the Great Bahama Bank slope
(A) Bathymetric map of 847 cold-watercoral mounds based on multi-beam data imaging mounds of various size and footprint (C). They cover 3km2 of 47km2 sea floor. (B) Oblique view of the 83m-high Twin Peak mound (location stippled area).

The Mound Field in the Middle of the Straits of Florida

mound field
Oblique bathymetric view in the middle of the Straits of Florida based on multi-beam data overlain by side-scan sonar mosaics depicting a knobby sea surface formed by coalesced cold-water coral mounds (Inset, upper right shows the location in the Straits). The darker backscatter amplitudes in the side-scan sonar mosaics are standing cold-water coral colonies as confirmed with ground truth analysis by submersible video transects.  The lighter backscatter amplitudes in the flat area on the left and on the coral mounds are the muddy carbonates. The coral mounds align to form chevron-like patterned ridges that trend NW-SE and NE-SW in a prevailing north flowing current.

Coral Ridges on the Slope of the Miami Terrace

coral ridges
Oblique bathymetric view across the Miami Terrace (3x vertical exaggeration). The long ridges extending perpendicularly from the terrace break into the Straits of Florida are  formed by cold-water corals.  They cover most of the lower Terrace slope. The slope is sub-divided (black dashed line) into upper (~6 to 11°) and lower (~2°) sections, in which ridges vary in size and frequency. Inset box shows the location of the study area (red rectangle) within the northern Straits of Florida.

Coral Coverage on Ridges of the Slope
of the Miami Terrace

coral coverage
Figure 5: (A) Bathymetric map in plan view of the lower slope of the Miami Terrace with the dense coral patches in maroon that are preferentially on top of the ridges facing the dominant current direction. (B) Rose diagram shows that the current regime is predominant due south (mode of 170°) with an average velocity of 15 cm.s-1. (C) Rose diagrams showing the azimuth of the dense coral patches. The dense coral patches covers 20% of the entire area.