By Kelsey Archer Barnhill
Monday morning started off with recovering the first deployment of the baited trap along with the second baited camera lander. There was quite an exciting moment as a shark was attracted to the fish in the trap as we waited to bring the lander on board. Alycia Smith and Daniëlle De Jonge took pictures of the fish and amphiopods which fell prey to the trap before measuring and dissecting the samples. Five rat tail fish, 4 cusk eels and hundreds of amphiopods were caught in total and three of the samples were heavily damaged as amphiopods or the shark had taken chunks out of them before we were able to recover the lander. Stable isotope and ecotoxicological samples were taken, with a focus on determining the baseline levels of metal concentrations in the organisms. Science team and crew members alike came down to visit the lab and ogle at the deep-sea fish.
With camera lander images now available from both bait types, the team was able to take a first look at comparing the pictures. Andrew Sweetman explained that it seems there are different species compositions which are attracted to the squid and the mackerel, which could have implications on scavenger composition in the region under climate change as the dominant food source is predicted to shift towards squid. The four images below are taken by the lander, all courtesy Prof. Andrew Sweetman / Lyell Centre – Heriot-Watt University / iMirabilis2
On Monday evening we had our second ROV dive of Leg 1 at Cadamosto Seamount. After completing the bathymetric mapping of the area, the expedition leaders created different transects up the slope of the seamount, all meeting in the middle at the summit for ROV dive planning. We started the dive at the 2000 m contour and the ROV pilots skilfully navigated up the transect, following the path to the top of the seamount.
During an ROV dive there is a trade-off between sampling and completing the transect. If you stop to sample and zoom-in on every interesting rock and organism then the transect will not be completed. Alternatively, you can speed up the transect and miss sampling opportunities if zonation in the types of animals present in each area occurs. We were fortunate to have an incredibly successful dive on both fronts as sampling was made in key areas and we stayed on schedule to follow the transect to the peak. We sampled brittle stars and sea urchins at the start of the dive, which were representative of the areas. When we first touched bottom we saw large volcanic basaltic rocks with thousands of brittle stars dominating the area. As the area briefly changed on our ascent, a sandy area opened up where a field with hundreds of white sea urchins was visible. Bea Vinha will use these samples in her PhD thesis for stable isotope and fatty acid analyses to understand the trophic ecology of the area. The ROV science team on board consists of biologists and ecologists as the mission of these dives has a biological focus. However, we were all impressed with the geological features in the area. The Cadamosto seamount is one of the most seismically active areas in Cabo Verde. There have been recent volcanotectonic activity at the seamount, which could one day form a new island in Cabo Verde. The large cooled lava flows were impressive to behold at the seamount as we saw different structures across depths.
As we moved on from the brittle star and sea urchin dominated areas, we began seeing more corals, specifically the Scleractinian coral Enallopsammia. After observing that the coral was prevalent in the area, Andrea Gori made the call to sample it as a potential species for the on board aquaria experiments. The ROV team tried different strategies in sampling the coral, both using the manipulator arm to place it into the bio box as well as using the suction sampler to break off smaller fragments and store them in the suction containers. One benefit of using the suction chambers is that they are better sealed than the bioboxes. This is advantageous during cold-water coral sampling as the temperature at the seafloor is around 5 degrees, while the surface water is over 20 degrees. Keeping the corals in the suction containers ensures the corals are not exposed to extreme heat shock upon retrieval. Both sampling methods were used on this dive to decide which is the best sampling technique for future dives. We also sampled some dead coral for geochemical analyses for onshore partners to reconstruct multidecadal coral growth changes. Bea took some small pieces off of the sampled corals for stable isotope and fatty acid analyses.
Soon after corals became more common at the seamount we reached the summit. The geology was distinct at the top, with red rocks indicating previous hydrothermal activity instead of the volcanic black rocks describing the area. There were also thousands of little white stick-looking organisms present. As the ROV team is currently troubleshooting the zoom on the camera, we did not get a close look at these organisms but are hypothesizing they could be a type of coral or carnivorous sponge. Luckily we plan to return to the site during a future dive, as we aim to target another transect to categorize the habitat on another side of the seamount. Hopefully then we can get a better look or even a sample to understand what these creatures are!
Once the ROV was recovered, Andrea and Bea retrieved the samples and then processed them in the downstairs laboratory with my help. The samples for stable isotope and fatty acid analyses were stored in the -80 degree freezer. The corals were cut into fragments for the experiment and glued onto mounts before being placed in the on board aquaria tank for recovery. We are monitoring their health and hope to see their polyps come out before starting any work on them. As corals use their polyps to feed, we may try to bribe them to come out with some artemia brine shrimp.
After the ROV was back on deck we took a CTD on top of the seamount to have water samples of the area. Susan Evans filtered water for eDNA while Andrea Gori, Bea Vinha, and myself filtered water for trophic ecology and evidence of bentho-pelagic coupling. The CTD took water samples at four different depths (the bottom, 100 m off the bottom, the oxygen minimum zone, and 50 m) and we sampled three replicates of 1 litre at each depth for Bea’s PhD work.
Yesterday, the benthic lander team from the Lyell Centre, Heriot-Watt University finished monitoring temperature effects within the on board incubation experiment. After injecting isotopically labelled algae into 8 different sediment cores, inorganic carbon and oxygen consumption rates were recorded. In this experiment 4 cores were injected with fresh, high quality algae and 4 with dialysed algae, which means it was grown in an environment to make the algae lower quality. One climate change prediction is that lower surface water productivity will create lower quality food to sink to the seafloor. As this experiment looks at CO2 production and oxygen consumption to quantify seafloor respiration under climate change, the work was done both at the current seafloor temperature of 6.5 degrees as well as the climate change predicted 8.5 degrees. This is a multiple stressor experiment where both the effects of temperature, food quality, and the combined effect of the two drivers is explored.
At dinner time we celebrated ROV technician Miguel Souto’s Birthday with a card signed by the science team, some decorations in the dining room, and a cake courtesy of Juan! Miguel normally spends his birthday with his family but shared that he felt quite happy to be on board with us this year.
Overnight we completed a multicorer sample at 4394 m. We have to congratulate the technicians, Iván Casal and Mario Sanchez, as this was the deepest multicore they’ve ever done! It was a successful sample as well with a layer of foraminifera on top with mud underneath. This sample was taken for partners back on shore to look at the foraminifera.
This morning the third camera trap deployment was retrieved. This lander was sent down with mackerel as the bait. On the first deployment the mackerel returned picked clean, but on this retrieval amphipods were still on the bait eating the remainders of the bait. The one deployment with squid returned completely empty as everything had been eaten.
We are currently on route to our next dive location off of Fogo Island. We planned to complete another dive today at Cadamosto Seamount but high wind in the area has delayed those plans. We are looking forward to the next dive and hope to be able to return to Cadamosto to keep exploring the deep!