Characterization of Mesophotic Coral Reefs Using the Seabed AUVRoy A. Armstrong1 and Hanumant Singh2

1Bio-optical Oceanography Laboratory, Deapartment of Marine Sciences, University of Puerto Rico, Mayagüez, PR

2 Woods Hole Oceanographic Institution, Woods Hole, MA

Why Study Mesophotic Coral Reefs?

• In shallow waters (< 20 m) of the Caribbean Region, a coral decline of 80% over the last 30 years has been documented.• Deeper reefs (> 30 m) are largely unknown. • They appear to be healthier than shallow water reefs.• Habitats of commercially important fish species.• Source of coral larvae for recruitment and potential recovery of the shallower reef areas.• There could be several times as much reef habitat deep as there is shallow.

Figure 1. The Seabed AUV. The two hull design allows for a vehicle that is passively stable in pitch and roll. The multiple thrusters allow independent control of depth and heading which make it ideal for working as a hovering platform for near bottom imaging work in rugged terrain.

The Seabed AUV

The Seabed Autonomous Underwater Vehicle (Figure 1) is a highly successful platform that was built in 2001 for the express purpose of providing high resolution color imagery underwater and has since been used for a variety of scientific applications including coral reef ecology, benthic habitat studies, archaeological sites in deep water, and chemical, geological and geophysical mapping.

Why Study Deep Coral Reefs With AUVs?

• Effective airborne or satellite remote sensing of coral reefs is limited to shallow, optically clear water.• The deep hermatypic corals (30 to 100 m) lie beyond the range of safe diving operations.• The Seabed AUV is an ideal platform for large scale benthic imaging.• AUVs can be configured to carry a wide variety of imaging sensors and other instruments such as CTDs, side scan sonars, multi-beam and pencil beam sonars, chemical sensors, optical sensors and radiometers, and others.

The Role of AUVs

Since the insular shelf and slope areas between 30 to 100 m in the U.S. Caribbean have an area of approximately 3,300 Km2 (or 43% of the total area between 0-100 m, Figure 2), it is impractical to rely solely on diving or even ROV surveys to adequately map and characterize these potential deep reef habitats.

Figure 2. Bathymetry of the Puerto Rico – US Virgin Island geological platform to 100m depth.

As a leading instrument in this field, the Seabed AUV has been successfully deployed for the imaging and mapping of the deep coral reef zones in the US Virgin Islands and Puerto Rico (Armstrong et al., 2002); Singh et al., 2004; Armstrong et al., 2006; Armstrong 2007). The main sensor on the Seabed AUV is a high resolution digital camera, which was used to collect a set of approximately 6000 images at the Hind Bank Marine Conservation District (MCD), south of St. Thomas, United States Virgin Islands.

Introduction

Mapping and monitoring deeper coral reefs is necessary for selecting and protecting unique areas of high living coral cover, diversity, and structural complexity that could aid in the recovery of the shallower fish and benthic communities. The high quality digital imagery provided by the Seabed autonomous underwater vehicle (AUV) is being used to characterize and quantify the coral reef habitat present to depths of 100 m, which for most areas of the world, remain largely unknown. Since deeper reefs appear to be largely unaffected by hurricane disturbances, human impacts, and disease, they could serve as refugia for a large number of species during times of environmental stress. A long-term benefit of this research is to provide important scientific results that can be used by state and federal agencies to develop efficient management strategies to protect the mesophotic coral reef habitats of the US Caribbean.

Underwater imagery is typically characterized by low contrast and low color fidelity. The nonlinear attenuation of the visible spectrum in seawater causes parts of the visible spectrum to be preferentially attenuated. Thus most underwater images tend to be saturated in the blue-green region. The 12 bits of dynamic range provided by the camera that is part of Seabed allow us to compensate and color balance the imagery to obtain “true” color (Figure 3) and then allows us to mosaic these images into a composite view (Figure 4).

Figure 3. Original image (left) and the corresponding color compensated and normalized image (right). This image was taken on the Hind Bank MCD at a depth of 36 m using the Seabed AUV.

Figure 4. Photomosaic of the deep reef at the MCD showing corals, sponges, algae, and sand.

References

Armstrong, R.A., H. Singh, and F. Gilbes. 2002. Benthic Survey of Insular Slope Coral Reefs Using the SeaBED AUV. Backscatter 13(3): 22-25.Singh, H., R.A. Armstrong, R. Eustice, C. Roman, O. Pizarro, and J.Torres. 2004. Imaging Coral I: Imaging Coral Habitats with the SeaBED AUV. Subsurface Sensing Technologies and Applications 5(1):25-42.Armstrong, R.A., H. Singh, J. Torres, R. Nemeth, A. Can, C. Roman, R. Eustice, L. Riggs, and G. Garcia-Moliner. 2006. Characterizing the deep insular shelf coral reef habitat of the Hind Bank Marine Conservation District (US Virgin Islands) using the Seabed Autonomous Underwater Vehicle. Continental Shelf Research 26, 194-205.Armstrong, R.A. 2007. Deep zooxanthellate coral reefs of the Puerto Rico - U.S. Virgin Islands insular platform. Coral Reefs, 26(4):945.