Natural Resource Disaster Assessment (NRDA): Mesophotic Reef Investigations Post-DWH Event

Science Center Objects

A series of ROV cruises was proposed to survey selected reefs for fish and soft coral diversity.

PROJECT COMPLETED

Roughtongue Reef Map
Roughtongue Reef 

The Science Issue and Relevance: On April 20, 2010, the BP Macondo oil well, being drilled by the Deepwater Horizon rig, blew out, and was uncontrolled for 87 days. During this time, approximately 4.1 million barrels of crude were discharged into the Gulf of Mexico, along with 43,800 barrels of dispersant. This oil and dispersant mix overlaid the Pinnacles tract, a series of hardbottom mesophotic reefs along the 100 meter depth contour inshore of the well, for much of the blowout. These reefs supported a well-studied assemblage of soft corals and fishes. Between 1997 and 2003, USGS had conducted ROV explorations of the area, so baseline data existed. The Natural Resource Damage Assessment conducted to assess damages due to the oil spill needed to know if harm had occurred on the reefs, and if so, how much.

Methodology for Addressing the Issue: A series of ROV cruises was proposed to survey selected reefs for fish and soft coral diversity. Two cruises have been conducted; one in late 2010, following the spill, and one in 2011. A third cruise occurred in 2014. Data consists of video transects and still photos taken over the reefs, covering as much as possible the regions surveyed during the 1997-2003 studies. Reefs were selected to have a range of biotypes and expected exposure levels, including control reefs that did not have documented oil exposure, reefs that were 80 miles from the wellhead, and reefs that were 37 miles from the wellhead. Video cameras on the ROV’s were set to a low tilt, and the ROV was driven in a straight line at a slow speed for five minute transects. While not transecting, the ROV was run in an exploratory mode, taking close-ups of soft corals, collecting corals, and collecting core samples. Transects were scored for meeting visibility, height, speed and type, then acceptable segments were viewed by multiple observers and all fish identified and counted. Soft corals were identified and scored for damage. Counts, species lists, and diversity were compared to the 1997-2003 cruise data, and between the impacted and control sites. A third cruise is scheduled for June 2014, to revisit the same sites and assess status of the reefs, and marked corals.

Fish and coral inhabitants on Roughtongue Reef
Fish and coral inhabitants on Roughtongue Reef

Related Products: A final internal report to NOAA was completed in Fall 2015.

 

Coral on Roughtongue Reef
Coral on Roughtongue Reef