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Seafloor Characterization of the U.S. Caribbean |
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Day 10: 03/30/2006:
The first transect of the day followed a 650 m bathymetric contour 11 km south of La Parguera. The section of contour and depth were chosen so that the transect would go over several ridges and a large protrusion in the valley of two ridges. In addition to multibeam bathymetric data, multibeam backscatter data were used for transect positioning. The backscatter data added another dimension to the selection process, because it differentiates areas which reflect sound differently.
The ROV glided over soft loose sediment along the entire transect, except for where the protrusion was mapped by the multibeam data. The protrusion ended up being two large bare rocks (each 5 X 5m). They seemed out of place amongst the flat expanses of sediment. The ROV team had high expectations for the large protrusion, but when we finally reached the rocks they did not have the amount of life around them that was expected. The ROV inspected the bare rocks for 15 minutes. Several sponges and crinoids were seen, but that was it until a lonely fish was sighted in the distance just as the ROV was leaving.
The second transect of the day was positioned to target “El Hoyo” [the hole], a redhind spawning aggregation site, and an adjacent steep descent. El Hoyo is a 1.5 km basin-like feature at the top of the shelf slope. It is distinguished by a depressed center, 30 m deep, and an elevated ridge along its perimeter. The majority of the area within El Hoyo is colonized hard bottom with some loose sediment from the breakdown of corals. The hard bottom is covered by a great diversity of organisms including: gorgonians, hard corals, sponges, and fleshy algae. Any loose sediment is covered by a thin film of brownish cyanobacteria. At one point the ROV’s thrusters produced a small eddy which peeled away a small flap of cyanobacteria and stirred the loose sediment underneath.
The ROV began in the center of El Hoyo, passed some internal raised hard bottom features and then went over the perimeter ridge. A variety of fish swam in front of the camera, including angelfish, barracuda, damselfish, filefish, grouper, grunts, jacks, mackerel, morays, parrotfish, snappers, squirrelfish, tilefish and wrasses. It was a cornucopia of fish! It was a real treat to finally be able to identify the fish, after encountering so many unknown taxa in deeper waters. Two relatively large barracuda were very curious and kept swimming in front of the camera.
The ROV descended from El Hoyo’s ridge in an almost vertical trajectory. The trek downwards began at 20 m below the surface and ended at 250 m in less then 80 horizontal meters. The top third of the descent had many sponges and coral, but around 80 m they began to thin and were ultimately replaced by bare rock. At the bottom of the steep wall the ROV found the soft sediment the ROV team has become very familiar with and the transect was terminated.
Colonized hard bottom habitat in El Hoyo.
A green moray (gymnothorax funebris) poking its head out from between coral colonies.
By night the science team has been working beyond the shelf edge to map the features, ridges, drop-offs and topography of the seafloor off Parguera, Puerta RiFebruary 15, 2007ys, we have multibeamed 6.2 km^2. The shelf edge occurs approximately 10 km parallel from the shoreline. Depths drop rapidly along the shelf edge, sloping from about 50 feet water depth to 1,800 feet in little over 1.3 km. See the attached image for details of coverage in Puerto Rico
(Multibeam of Parguera, Puerto Rico).