By Kelsey Archer Barnhill
After high winds at Cadamosto Seamount caused us to head for the shelter of Fogo Island, we changed the plan of our third dive and revisited Fogo’s continental shelf. Our first dive there consisted of mixed sediment and rocky views, but this second dive brought us down to an area dominated with large geological structures.
Almost immediately upon starting the dive transect, we saw that the octocoral Acanella sp. was common in the area. We took three samples for Bea Vinha’s trophic ecology work. On previous dives we’d been surprised at the apparent lack of sponges in the region and hadn’t yet taken a sponge sample. However, during this dive we began to see white glass sponges of various sizes and took advantage of their abundance to take samples. One of the sponges we collected was the Venus flower basket sponge. This sponge associates with a mated pair of shrimp who spend their entire life inside the sponge once they grow too large to escape the small holes.
Organism associations or symbionts ended up being the theme of the dive as we also collected two other symbionts, one we expected and another which took us by surprise. We sampled an octocoral from the family Pleuxaridae for taxonomic work to identify the species. This coral had 4 symbiotic brittle stars living on it, which is a common occurrence in the cold-water coral world. However, each of the three Acanella sp. samples also had one polychaete, all apparently of the same species. Despite not intending to sample the polychaetes, once we discovered them, we did some opportunistic sampling and preserved them for stable isotope and fatty acid analyses along with all the intentional samples.
This was a special dive for me as it was the first dive I was able to spend in the ROV cabin. Like most members of the science team, I watched the previous dives on this cruise from the scientific laboratory, either observing the imagery and helping spot animals or annotating on the ocean floor observation protocol (OFOP) software. For this dive I took on a new role of taking pictures on the ROV’s scientific camera for the majority of the dive. In addition to the 4K camera we use during ROV dives and the other cameras used by the pilots to monitor Luso during the dive, there is also a scientific camera attached. This camera has a wider frame of view than the 4K and occasionally during the dive I was asked to aim my camera towards one of the ROV manipulator arms to check everything looked alright. The scientific camera is operated with a joystick remote to move the field of view in combination with a computer to focus, zoom, and take the pictures. If a scientist is not able to actively take pictures then the camera will automatically take one picture during the dive every minute. I enjoyed moving the camera around and getting a sneak peek of what Luso might see next. I was also able to turn the angle from side to side to view organisms and geological features just outside of Luso’s view. Being in the cabin was a great experience as I got to see how the ROV team worked. Before this experience I did not know that the ROV co-pilot speaks to the bridge to confirm ship movements and location. It was also exciting to see how the co-pilot uses the mini manipulator arm in the control cabin to make Luso’s arm move underwater. The ROV pilots are truly a great team and it is really impressive to watch how they work and communicate together.
While most of my scientific remit on board relates to the ROV operations, I also make sure to catch up with the other teams on board. Autosub6000 had its first successful mission on Thursday and was deployed again on Sunday. On the first dive, Susan Evans was able to obtain her first eDNA samples using RoCSI and collected 200mL samples in all 24 filters. After the AUV is recovered, these samples are stored in the -80˚C freezer to be analysed back on shore. Between each mission the tubes in RoCSI must be cleaned to prepare for the next dive. Susan was excited that a CTD was taken at the same location as the first dive – this means she can compare the eDNA samples collected via both methods. On the latest Autosub6000 mission Susan was able to get 1.5L samples with RoCSI at 3000m water depth. The next Autosub6000 missions aim to combine RoCSI sampling with high resolution multibeam bathymetry and photography.
The team from the Lyell Centre, Heriot-Watt University finished their week-long on board sediment incubation experiments on Friday with exciting initial results. This experiment, run by Daniëlle De Jonge, Alycia Smith, and Prof. Andrew Sweetman had two single-stressor treatments (reduced particulate organic carbon (POC) influx quality, and increased temperature) as well as one combined multiple stressor treatment. From respiration measurements they found reduced POC flux quality slightly lowered respiration rates while the 2˚C temperature rise increased respiration rates. In the dual stressor treatment it appeared the two stressors interacted as the respiration rate increase was not as high as in the temperature increase alone. Increased temperature has a significant impact on respiration rates, nearly doubling the measurements found in the control treatment. These findings support the hypothesis that as our ocean warms the sediments will become a less successful carbon sink. As POC sinks to the seafloor, some is eaten and respired and the rest is buried. Increased respiration rates on the seafloor means decreased POC burial, suggesting decreased carbon storage rates in our future ocean. These results might also impact what we know about sediment community oxygen consumption (SCOC). When seafloor areas are compared, higher SCOC generally means the area is more biologically active. However, in this experiment they found increased temperature also increased SCOC. The hypothesis behind why this change occurred is that respiration increased just to keep regular maintenance going. With more energy going to supporting normal functions, this could mean less energy for other functions such as reproduction. In this case increased respiration rates does not mean there is more secondary production, but that more energy is needed just to sustain the same level of biomass. This hypothesis will be explored in further detail through the stable isotope samples taken which will be analysed on shore. You can listen to Alycia Smith explain this experiment in the video below.
In the run-up to the weekend, we had a couple of days with high winds which prevented us from deploying the ROV and Autosub6000. To fill this time we took CTD samples and did some multibeam bathymetry mapping. Increased CTD operations means that there is a lot of water filtering happening on board, led by Susan Evans, Bea Vinha, and Andrea Gori. At one point water filtration for POC, fatty acid, and stable isotope analysis had been going continuously for over 24 hours! Susan filters water for eDNA and Bea and Andrea filter water for trophic ecology and bentho-pelagic coupling studies.
The multibeam mapping can happen at any time of the day, which meant that I, along with some others, moved to a late shift. I spent two nights on a multibeam bathymetry watch in the late night/early hours of the morning. We have been collecting bathymetry in the area to fill in some gaps in existing bathymetric maps to better understand the area and more accurately choose locations for AUV and ROV missions. The addition of these late-night multibeam shifts means I sleep until lunchtime, and whilst I have adjusted to the new schedule just fine (with a little help from the on board coffee machine) I can’t say that bean soup when I wake up at lunchtime hits the spot in the same way cereal does for the first meal of the day.
While all the exciting science has kept us busy, there is still some time for relaxing. One activity we do on board is decorate styrofoam cups and models. Murray Roberts and I brought some styrofoam and pens on board for everyone to use. People draw on the cups and models as they please and then hand them off to one of the ROV or CTD technicians. They are then sent down on the equipment and crushed under pressure. When they return to the ship they are miniatures! Drawing on these cups are a fun evening activity.
When we are lucky we also get treated to live music! A few nights ago the UTM technicians Roger Mocholi Segura, Iván Casal and Mario Sanchez treated the ship to an impromptu concert on drums and guitar. It was great to listen to their music as we mapped off of Fogo and was absolutely a highlight of the week.
We are looking forward more scheduled AutoSub6000 and ROV dives coming up and can’t wait to see more images of the seafloor!
Shipboard entertainment, courtesy of Roger, Ivan and Mario. Video by Murray Roberts / UEDIN / iMirabilis2