Timor Sea Reef Connections

But work before this cruise has been limited and mostly focused on a few key sites. The Schmidt Ocean Institute expedition helped identify potentially unique and sensitive habitats, and improved 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, co-led the expedition, which ran from April 10 to May 4, 2015. The cruise had 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.

Far Waters

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 were 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 explored on this expedition. To gain insight about how these deeper reefs have fared over the years relative to shallower corals, they compared new observations against information about mesophotic corals collected as far back as 1999.

Making Connections

At Scott Reef the team studied 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 helped 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 was especially interested in processing deeper data. They also worked to add finer-scale detail for areas where modeling has been done.

Researchers lowered sensors to measure average currents, the intensity of turbulent mixing, temperature, salinity, and a range of biological and chemical parameters. They also deployed moorings in the central deep channel and around the reefs with similar sensor arrays—some for short-term recovery and two that were left for long-term data collection. Using a remotely operated vehicle (ROV), they surveyed 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 gathered data across a full tidal cycle to calculate fluxes of nutrients and heat, and determine which processes are indeed most important. They conducted similar monitoring within the reef lagoon, along with collecting water samples from Falkor’s small boats for nutrient analyses. All told, the work provided the broadest view ever of circulation and exchange within the full reef-lagoon system.

Middle Ground

Once work was completed at Scott Reef, Falkor moved to nearby deeper shoals for similar work. Nine shoals have been previously mapped and the team planned on studying more. However, only short visits were made to the nearby Echuca and Vulcan submerged shoals, gathering limited data due to the poor weather and sea state. Goals included further refining understanding of key patterns, and also to looking for differences.

While the Scott shoal has a shallow rim and breaking surf, the shoals that was 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 also explored 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 moved 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 was available, so work here required 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 the team was on the look-out for 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.

Using Connections

The work was done in order 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