By Kevin O’Brien
Last month, scientists from the PIFSC Coral Reef Ecosystem Division (CRED), along with scientists from the School of Ocean and Earth Science and Technology at the University of Hawai‘i at Mānoa (UH), began work on a collaborative project funded by the National Science Foundation to provide bathymetry (or depth) data for the waters surrounding the island of Mo‘orea in French Polynesia. The Moorea Coral Reef Long Term Ecological Research (LTER) Project is one of many LTER sites, both marine and terrestrial, that are operated around the world to provide insight into ecological processes through long-term monitoring.
The Moorea Coral Reef LTER site was established in 2004, and the website for this project describes the research effort there as “an interdisciplinary, landscape-scale program whose goal is to advance understanding of key mechanisms that modulate ecosystem processes and community structure of coral reefs through integrated research, education and outreach.” Fulfilling that goal requires a better understanding of the movement of water around this island’s forereef as well as into, through, and out of its lagoons. Modeling wave dynamics and currents requires high-resolution bathymetry data collected from around the entire island. This need is where CRED’s mobile multibeam mapping assets came into the picture.
CRED’s multibeam survey launch, the R/V AHI (Acoustic Habitat Investigator), is a 25-ft aluminum SAFE Boat with an enclosed cabin and an inboard diesel engine. The AHI was custom built for the purpose of mapping benthic habitats in shallow waters, and it is equipped with a 240-kHz Reson 8101ER multibeam transducer mounted to the bottom of its hull. This transducer is paired with a suite of onboard sonar and computer equipment, making AHI a totally self-contained platform for the collection of multibeam data with the capability of accurately mapping the benthos to a depth of up to 250 m and sometimes to deeper depths. As its name implies, the multibeam system aboard the AHI transmits acoustic energy toward the seafloor via 101 separate beams spaced 1.5° apart, and, as it surveys, depth data is collected at 101 spots on the seafloor spread out over a swath that covers about 7 times the depth of the water column.
The goal of the CRED team this summer was to successfully map all accessible shallow-water reef environments surrounding the island of Mo‘orea at depths between 10 and 150 m or deeper if conditions allowed. This mapping effort, in conjunction with collection of multibeam data at deeper depths by the UH R/V Kilo Moana, will give LTER researchers the data they need to refine and test their wave and circulation models and apply these data to the broader array of LTER projects that take place on Mo‘orea. Open ocean waves and currents, and the zooplankton and other material that they transport, influence the reefs and lagoons around Mo‘orea, but their effects are poorly understood. The LTER scientists’ hydrodynamic modeling requires high-resolution bathymetry data to accurately account for effects of depth and seafloor roughness on waves and currents and to better link open ocean processes with the processes that occur in coastal reefs. Additionally, the availability of high-resolution bathymetry maps likely will be useful for several other research projects at Mo‘orea and Tahiti.
To transport the AHI and all its associated expedition equipment from Hawai‘i to French Polynesia, the vessel was loaded onto the deck of the Kilo Moana on July 3 in Honolulu. The Kilo Moana, as part of a UH cruise to support LTER research in Mo‘orea, was in French Polynesian waters until Aug. 10. The field team for the PIFSC mission included 5 CRED scientists—Frances Lichowski, Jeremy Taylor, Rhonda Suka, Kevin O’Brien, and chief scientist John Rooney, Ph.D.—and Joyce Miller of UH.
Upon its arrival in Tahiti on July 17, the Kilo Moana pulled into Papeete Harbor, where it was met by the CRED team. After the ship cleared customs and immigration, the AHI was offloaded from the deck into the water with the ship’s crane, and the short (15 nmi) transit from Papeete to Mo‘orea was made in the AHI that afternoon by Rooney and O’Brien. Researchers aboard the Kilo Moana then proceeded to conduct offshore multibeam and water-sampling operations around Mo‘orea and Tahiti while the CRED team made their home base at the Richard B. Gump Research Station, a facility of the University of California, Berkeley, located on the shore of Cook’s Bay, Mo‘orea.
Between July 18 and Aug. 4, the CRED team successfully mapped approximately 95% of its targeted area on Mo‘orea: the reef passes and lagoon areas as well as both shallow and deep forereef areas, including a dramatic near-vertical wall that plunges from a depth of 70 m down to depths below 300 m most of the way around this island. In many places, the multibeam equipment on the AHI was able to accurately map deeper than 150 m, with some surveys reaching depths of nearly 400 m.
On a typical day of surveying during this mission, the CRED team split into 2 work parties. Two team members were selected to operate the AHI, requiring a refreshing morning swim to retrieve the boat from its mooring at the Gump Research Station. Each day, a new survey area was chosen, and the boat transited to the survey area to conduct multibeam surveys, returning to Gump Research Station in the evening. On some days, operations required a complete circumnavigation of the island. While the AHI was on the water, the other work party remained onshore to process and clean the data collected the previous day as well as to provide shore-side support in the event it was needed.
The mission went remarkably smoothly, with only one mechanical setback that required swapping of an onboard diesel generator because of a blown oil seal. In the final days of this mission, from Aug. 4–7, the team finished surveying all targeted areas on Mo‘orea and turned its attention to mapping a few select areas off the northwestern shore of Tahiti, areas that are also of interest to LTER researchers.