The best laid plans

By Suzanne Egan, University of Tasmania

Due to the nature of the support offered by the CSIRO Marine National Facility to the CAPSTAN Program, our story takes place on a transit leg of the RV Investigator.  This means that it was already sailing from Hobart to Fremantle for its next major voyage. For this reason, our Chief Scientist, Dr Leah Moore, had to design a 48-hour sampling program that lay along the route through that part of the Southern Ocean.

Google map of the eastern edge of the Great Australian Bight showing the bathymetry of the region.  9 yellow stars indicate the study sites in 3 canyons next to each other along the slope off Portland that were proposed for the 2019 CAPSTAN voyage.
1) The original plan with three transects along canyons off the Portland Coast

Fortunately, the southern Australian coast offers many unique and varied locations for such an investigation, so Dr. Moore chose to look a little more closely at an area off the south-western coast of Victoria, near Portland. This region is unique due to the low terrestrial sediment input that occurs, so material moving in the submarine canyons that cut across the continental slope is dominated by pelagic sediment (of oceanic origin), some in the form of fluidised flow deposits (cold water carbonate turbidites). The planktonic fossils in these sediments can also be used to help recreate past climatic conditions.

The other interesting phenomenon that occurs in this region from a biological point of view is the Bonney Upwelling, which is the largest and most predictable along the Great Australian Bight. The upwelling of cold nutrient-rich deep ocean water is episodically active along the coast from Portland, Victoria through to Robe, South Australia. The continental shelf narrows to approximately 20 km here and the upwelling water is funnelled up the underwater canyons onto the shelf, where it fuels phytoplankton (microscopic plants) and plankton (microscopic animals) blooms, which in turn feed whales, seals and birdlife.

Map of the great Australian Bight showing the sea surface temperature between 20 degrees C in the northern most part of the Bight (yellow) and 12 degrees C (purple) to the south.  The area focused on during the 2019 CAPSTAN marine science training voyage is outlined by a red box near Portland.
2) Sea surface temperature map for the Great Australian Bight on 26th April 2019 shows warm water adjacent to the coast in the study area (red box), meaning cold bottom water is not present. Satellite data was obtained from Copernicus Marine Environment Monitoring Service and provided by V. Tamsitt.

The plan going into the voyage was to map two canyons (maybe three if there was enough time), collect sediment cores, bathymetry, oceanographic and biological data within the Bonney Upwelling area (Figure 1).

And, of course, the best laid plans… so once on board with access to higher resolution geophysics data Dr. Moore decided to modify our station locations. The easternmost canyon was not going to be suitable to place the cores at the depths originally planned as it was much steeper than expected. The adjacent canyon was an interesting Y-shape and much broader enhancing the opportunity to place cores, with an increased chance of finding the turbidites we were looking for. In addition, consultation of satellite, meteorological, and Argo float data indicated that the Bonney Upwelling was not active, so an alternative plan was required and quickly (Figure 2). 

Bathymetric map of the canyons off Portland showing the ship's track (black line), sample stations within the y shaped canyon, and waypoints to guide the ship's track during the 2019 CAPSTAN voyage.
3) Final voyage track showing waypoints (so the ship would follow the canyon) and station locations.

To maximise the time on station and to try to get as many cores collected as possible (its safer to collect them in daylight) the biological and oceanographic data were collected in the early hours before dawn or in the late afternoon to evening. Overnight we mapped the sea floor at high resolution to assist with the AusSeaBed Project (http://marine.ga.gov.au/). The final voyage map reflects the waypoints for ship navigation and station locations along the canyon (Figure 3).

As a teaching voyage it was an amazing opportunity to see how decisions were made and why (this explanation came after we left the stations due to time constraints) and how to adjust on the fly to changing conditions and circumstances while still ensuring project objectives and data quality were not compromised. As a result, all the equipment we deployed on this scientific voyage generated high quality data and a complete suite of samples, which rarely happens at sea. It was also great for our Chief Scientist to then sit down and talk us through all the decisions she had made and why, and convey that to us.  The best laid plans…

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