By Kelsey Archer Barnhill
On Sunday afternoon the conditions were just right and we were treated to another day of ROV operations. We returned to Cadamosto Seamount where we had a spectacular dive with the highest density of octocorals and sponges we have seen on the cruise so far! Sponge aggregations, coral gardens and vertical walls colonised with sponges were just some of the incredible images we saw. The dive began at 2000m and we finished our climb up the flank of the seamount at 1735m. It was a challenging dive technically, with the current direction facing the ROV and the steep rocky cliffs at the site. At the start of the dive we saw a fishing line in the water and spent the better part of an hour repositioning ourselves to avoid becoming tangled in it. After a ship manoeuvre, the ROV was able to safely descend and touch bottom where we immediately saw a rock covered with octocorals. Coral abundance and the size of the sponges we saw was impressive, and we also noticed quite a few Brisingiids. So far we have seen the coral Metallagorgia on every dive in Cabo Verde, but they were especially abundant here. We sampled the soft coral Anthomasthus, a corallium-like octocoral, elephant’s ear sponges, and trialled the zooplankton sampler, which unfortunately did not yield much zooplankton. We were working on a rocky bottom throughout the entire dive which meant we did not see as many of the species typical in sediment environments such as holothurians, brittle stars, and sea urchins.
After the dive we collected a CTD at the site. Bea Vinha took water samples to filter for particulate organic carbon (POC). As more biodiversity was seen at this site compared to the first dive at Cadamosto Seamount, Bea will compare the POC values at each location. As POC is often used as a measurement for food quantity, increased POC at the latest dive site could be the driver behind the species distribution. If the POC values are comparable between sites then the hypothesis is that the currents likely make this site suitable for high biodiversity.
In addition to amazing videos from the ROV, the baited camera lander provided some exciting deep-sea pictures this week. On this fourth deployment we used squid as bait, and the team got their first full-length picture of a Synaphobranchid eel. There were also potentially different species of the rat tail fish and cusk eels present. After the fish and eels wiped the lander clean in just 5 hours, a large shrimp (possibly Cerataspis monstrosus) appeared as well. The benthic lander team had an exciting milestone this week as the respirometer recovery on 13 August marked Prof. Andrew Sweetman’s 300th lander deployment of his seagoing career since he began his count in 2015.
On Monday, some of the science team got a tour of the engine room. It was exciting to see a new part of the ship and journey down to the bottom of the staircase – we usually limit our activity to the main three floors. Tomas from the ship’s crew explained the forward and aft thruster, ballast water system, compressor systems for all the fridges and freezers on board, main propulsions, and generators. It was fascinating to hear about how the ship sterilises its ballast water with UV light and a filtration system to help prevent the spread of invasive species as the ship travels around the ocean. We were also shown a map of all the tanks on board which can hold diesel oil, ballast water, dirty oil, or fresh water for domestic use. Most of the tanks on board are diesel, which once allowed the ship to go all the way from the Spanish Antarctic station to Las Palmas without refuelling! The crew who work in the engine room are also largely responsible for emergency procedures and can initiate firefighting methods using CO2 and steer the ship should the Bridge lose control. After using the on-board electricity and water supply for nearly a month without giving it much thought, it was a great to see all the work that goes into providing these services and keeping the ship running.
Our Monday ended with an Autosub6000 surface test set against the incredible backdrop of Brava island. Just before launching we watched as a school of cetaceans swam and played near the vessel. We then enjoyed cake with dinner to celebrate the second on-board birthday: Antonio! He told me having a birthday at sea makes the day special by default. He also said that the day was different and fun with a highlight being the WhatsApp video call home courtesy of Roger extending some Wi-Fi privileges to the birthday boy!
