About halfway between Northwestern Australia and Indonesia lie some of the planet’s most remote and healthy coral reefs, with biodiversity in places rivaling that of the much better known Great Barrier Reef.
Yet the physical connections between these reefs, the factors responsible for their health, and the conditions most likely to threaten them are not fully understood. In April, Falkor will steam to the region for a collaborative project aimed at exploring these connections. The work will include expanding on previous research at shallower reefs, as well as the first ever exploration of some deeper sites.
The reefs are part of a large group of submerged and emergent features on the western margin of what’s known as the Oceanic Shoals bioregion, in the Timor Sea. Located above large natural gas reserves, they have been of increasing interest in recent years both because of their proximity to the 2009 Montara oil spill, and because of plans to station a floating liquefied natural gas (FLNG) processing facility 100 kilometers to the east, which will likely be the world’s largest floating structure.
But work to date has been limited and mostly focused on a few key sites. The Schmidt Ocean Institute expedition will help identify potentially unique and sensitive habitats, and improve understanding of key ecological processes that shape the reef communities and contribute to their ability to bounce back from disturbances such as bleaching events caused by high water temperatures, and cyclones.
Greg Ivey and Ryan Lowe from the University of Western Australia, and Andrew Heyward from the Australian Institute of Marine Science, will co-lead the expedition, which runs from April 10 to May 4, 2015. The cruise will have a science party of 14, with additional participants from both of those institutions, as well as from Stanford University in the U.S, and Griffiths University in Australia.
The first research site begins about 400 kilometers north of Broome, the expedition port. For 20 years, Australian researchers have been studying one of the region’s most prominent features, Scott Reef, which has emergent North and South plateaus split by a 450-meter-deep channel (see image below). The South Reef includes a 300-square-kilometer protected lagoon with depths down to about 70 meters.
Besides scientists, the only regular visitors are Indonesian fishermen who sail there each year to gather sea cucumbers, shark fins and other species. Though such fishing does have significant impacts, the location is so remote, and human activity is so limited, that the reefs are considered some of the most pristine on the planet. Their diversity is just shy of what’s found in the Coral Triangle region to the north—bounded by Indonesia, the Philippines, and the Solomon Islands—which boasts the highest reef diversity in the world.
Corals living in the South Reef lagoon will be of special interest for the team because they are found in what’s known as the mesophotic zone. This is the low end of the depth range that can receive enough light to support significant growth of reef-building species that depend on photosynthesizing algae for their food. Some research suggests mesophotic reefs could be better protected than their shallower counterparts from damages due to warming seas and other threats, a hypothesis the team will be exploring on this expedition. To gain insight about how these deeper reefs have fared over the years relative to shallower corals, they will compare new observations against information about mesophotic corals collected as far back as 1999.
At Scott Reef the team will study ties between oceanographic circulation and the reefs, for instance how currents in the deep channel, and water circulation in and out of the lagoon, correlate with growth and diversity patterns on the lagoon floor. Data collected will help the team to improve preliminary circulation models that exist already for Scott Reef, and begin developing models for other sites they will visit. Very little modeling work has been done below 30 meters for any of the reefs, so the team will be especially interested in processing deeper data. They will also be working to add finer-scale detail for areas where modeling has been done.
Researchers will lower sensors to measure average currents, the intensity of turbulent mixing, temperature, salinity, and a range of biological and chemical parameters. They will also be deploying moorings in the central deep channel and around the reefs with similar sensor arrays—some for short-term recovery and two that will be left for long-term data collection. Using a remotely operated vehicle (ROV), they will survey various areas to confirm where coral growth is greatest, to allow correlation to physical parameters and to features identified on high-resolution sonar maps.
The deep channel between the reefs is likely the main path for water into the mesophotic reef habitats, in contrast to the wind and tidally-driven surface flows over the shallower habitats. Past research in the area has suggested that the large subsurface waves known as internal tides—formed below the surface when tidally driven seawater interacts with the sloping seafloor around the reefs—may play a major role in delivering nutrients to the reefs and governing temperature patterns.
Using the moorings and observations from Falkor’s sonar system, the team will gather data across a full tidal cycle to calculate fluxes of nutrients and heat, and determine which processes are indeed most important. They will conduct similar monitoring within the reef lagoon, along with collecting water samples from Falkor’s small boats for nutrient analyses. All told, the work will provide the broadest view ever of circulation and exchange within the full reef-lagoon system.
Once work is complete at Scott Reef, Falkor will move to nearby deeper shoals for similar work. Nine shoals have been previously mapped and the team plans to study at least two. The goal will be to further refine understanding of key patterns, and also to look for differences.
While the Scott shoal has a shallow rim and breaking surf, the shoals that will be targeted during this second phase of the expedition do not. They rise from depths of 100 to 200 meters, up to 15 or so meters below the surface. This can mean very different current patterns, and different parameters may prove the most important in determining coral diversity. The team will also be exploring a hypothesis based on previous research that water mixing at the edges of these shoals may bring up nutrients to support more abundant fish populations.
Unexplored Deep Reefs
Finally, the team will move to an even deeper—and unexplored—reef system with plateaus that rise from depths of about 500 meters up to 200 meters below the surface. Very little information about the two target areas is available, so work here will require extensive mapping using Falkor’s sonar.
These reefs are located below the reach of sunlight, meaning only deepwater species are found. Here, corals have to filter water, so there may be very different physical factors associated with their habitat patterns. Internal tides may also play a key role here as the mixing they cause can move algae and other food found in shallower waters, down to these depths, providing food to filter feeders such as deep-sea corals.
Once the work is complete, the team will be able to better understand the roles of water flow in maintaining individual reefs and shoals, and the regional connectivity between sites. Reefs don’t exist as isolated islands; they are connected to and dependent on the currents that bathe them with nutrients and deliver the offspring from other reefs that are vital to their continued health. And each reef in turn contributes its own progeny to the currents that reach other areas.
Biologically, both the emergent reefs and submerged shoals are like stepping stones across the Timor Sea, and better understanding the connections between them will offer substantial benefits. There might, for instance, be current patterns that suggest strong connections between widely separated reef shoals and reveal the places where drilling accidents would be most likely to deliver oil to reefs. This is important information for environmental assessments of proposed exploration and drilling activity, and also for predicting impacts if a spill were to occur. At the more basic level, better appreciation for connections between reefs and oceanographic circulation patterns should help researchers design more effective reef conservation and monitoring programs.
by Mark Schrope
Data & Publications
ROV ROPOS Dives can be found on YouTube.
The resulting shipboard dataset is being stored at the Rolling Deck to Repository and is now available.
Acoustic Backscatter, Swath Bathymetry, ROV Navigation, Falkor Navigation and Digital Imagery from ROV ROPOS is available through the Marine Geoscience Data System.
Digital Stills are also available here via the Australian National Data Service.
Imagery is also being ingested into an interactive map, at the North West Atlas.
Metadata of the ROV dives is available at the eAtlas.
Rayson, M., Bluteau, C., Ivey, G. and Jones, N. (2016). Observations of high-frequency internal waves and strong turbulent mixing in a channel flow between two coral atolls, Conference Proceedings, Proceedings 8th International Symposium on Stratified Flows, San Diego, California, USA.
Rayson, M., Bluteau, C., Ivey, G. and Jones, N. (2016). Observations of high-frequency internal waves and strong turbulent mixing in a channel flow between two coral atolls, Oral Presentation, Proceedings 8th International Symposium on Stratified Flows, San Diego, California, USA.