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ASCA-SEAmester IV: Another exciting cruise along the ASCA line

By Jethan d’Hotman, Jordan van Stavel and Grant van der Heever, SAEON Egagasini Node
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As this was the fourth instalment of the SEAmester cruise (and the third time SEAmester partnered up with the Agulhas System Climate Array (ASCA), preparation for the cruise was like cutting butter with a hot knife.

For the ASCA team that took part in the mooring retrieval cruise the year before, the cruise was like much needed therapy.

En route to ASCA

Given the ASCA transect is approximately two days steam from Cape Town, this gave all on board a chance to settle in and combat their seasickness. Among the bird, mammal and weather observations, there was also time to conduct ad hoc biological sampling en route to the ASCA transect.

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The SA Agulhas II during a cold front on the ASCA-SEAmester IV voyage (image courtesy of Thando Mazomba)

SAEON Egagasini technician, Grant van der Heever, leaped at this opportunity. Equipped with the Ski-Monkey III camera system and the benthic dredge, the aim was to spatially map benthic habitats and associated ecological assemblages off the south coast of South Africa.

Despite the thick sediment plumes in inshore areas owing to bad weather and river outflows, three Ski-Monkey stations and two dredge stations were successfully conducted.

Operations at ASCA

Once at the ASCA transect, the scientific team began settling into the routine of the hectic work schedule for the stations. Every station consisted of five main operations – Plankton Drift net, Vertical Bongo, CTD*, Oblique Bongo and Neuston net.

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Image depicting the community composition at a soft-sandy station situated between Still Bay and Mossel Bay at a depth of 67 m. Interesting observations include polychaete tubes, velutinids (shell-less gastropods), starfish (sand sea stars and brittle stars) and an unidentified pipefish (Syngnathidae) (image courtesy of Katherine Hutchinson)

What happened to the dolphin and seal intended to be used for a practical demonstration for the SEAmester students?                                                                                                                                                                                                                                                                                                                                                                      

To add to the hectic programme, twice-daily SEAmester stations were conducted for student training purposes. Despite all the operations on deck and team members running around frantically, operations ran smoothly with almost no issues to report for the first half of the cruise.

Almost exactly at the halfway point, mother nature decided to add some excitement to our experience. The South African Weather Service (SAWS) had predicted three cold fronts that would intersect us over the coming days; these fronts would bring 9-m swells and wind gusts of 60 knots.

Given the unfavourable conditions and the loss of a dolphin and seal (intended to be used for a practical dissection demonstration for the SEAmester students), the team decided to stop all operations and wait for better, safer working conditions. Once the cold fronts had somewhat abated, work continued and 16 of the 20 stations were completed.

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Group photo of the ASCA science team for SEAmester IV 2019, taken on the SA Agulhas II helideck (image courtesy of Alexander Oelofse)

On the return leg, it was planned to use data from the Hull-mounted Acoustic Doppler Current Profiler (S-ADCP) to assist with the deployment of two satellite-tracked surface drifters (as part of the Global Drifter Programme). However, early on in the cruise it was discovered the S-ADCP was collecting erroneous data. Given this, maps of sea surface temperature and geostrophic velocities were used to assess the current state of the Agulhas Current and the two surface drifters were deployed on the inshore edge of the Agulhas Current (as suggested in d’Hotman et al., 2019 – https://repository.library.noaa.gov/view/noaa/20929/noaa_20929_DS1.pdf).

As is the custom for SEAmester cruises, a proportion of the science team was made up of students with little to no seagoing experience. Despite this they surpassed our expectations.

SEAmester provides a fantastic opportunity for all to learn and expand their seagoing experience. We look forward to any future collaborations with the SEAmester programme.

For more information, please see the full cruise report here.

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Recovery of the CTD after a three-hour-long CTD cast (image courtesy of the Tools of the Trade Team)

The sun setting over the Eastern Cape, a daily moment of appreciation (image courtesy of the Tools of the Trade Team)

* A CTD is an oceanography instrument used to measure the conductivity, temperature, and pressure of seawater. Wikipedia

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