Science update 4 from Richard O’Driscoll

We are now four weeks into the Ross Sea Life in a Changing Climate (ReLiCC) 2021 voyage on RV Tangaroa and our time in the Ross Sea is rapidly coming to an end. Over the past 7 days we've been privileged to see some amazing sights both above and below the water.

 

We completed the coastal work on Monday, visiting Daniell Peninsula and Wood Bay in addition to the Cape Adare, Possession Island, and Cape Hallett sites I mentioned last week. In planning the voyage track I'd crossed my fingers that the weather would be nice in at least some of the coastal areas. But we were incredibly lucky to get calm, and often sunny, days at all five locations! As somebody remarked "if you haven't got any good photos by now, you shouldn't use a camera". And there have certainly been hundreds of amazing pictures taken by those onboard. A few images have already made it onto the voyage website and onto NIWA social channels, but there are many more to show once we are no longer limited by the constraints of satellite bandwidth.

Icebergs, bergy bits, and sea-ice along the coast added to the vista, but continued to provide challenges to the science objective of exploring life on the seabed at depths from 30 to 200 metres along the north Victoria Land coast.

We had to take a very circuitous route around the ice to get into the Daniell Peninsula site, and once we were there we could only get close enough to deploy NIWA's deep-towed imaging system (DTIS) at depths of 300 m. The final coastal site of Wood Bay, on the north side of Cape Washington was relatively ice-free and this allowed us to do a full suite of sampling including: five DTIS runs at depths of 40-130 m; three deployments of the Boxfish remote operated vehicle (ROV); and two grab samples to collect samples for genetic and chemical analyses. The images from the underwater cameras more than matched the scenery above, with extensive beds of Antarctic scallops, colourful sponges, swimming feather stars (crinoids), and icefish guarding circular nests.

Icefish are one of the unique fish of the Antarctic region. They don't have haemoglobin in their blood, so the gills are white instead of red. Icefish construct nests on the seabed to lay their eggs. The nests are then guarded by the adults. We saw this nesting behaviour on DTIS on our previous voyaged in 2019, but the video from the ROV in Wood Bay is even better!

Another unique Antarctic fish is the Antarctic silverfish. These small midwater fish are one of the 'keystone' species of the Ross Sea ecosystem, providing food for penguins, seals, and whales, as well as for larger fish, including toothfish. Like toothfish, silverfish are a type of Antarctic cod (notothenid). Notothenids have developed anti-freeze in their blood which allows them to live in water with temperatures below -1°C (seawater freezes at -1.9°C). At these temperatures the blood of normal fish would freeze!

The life history of silverfish is closely coupled to their environment. They spawn in winter or early spring and their eggs float up under the sea-ice. There they hatch in the platelet ice (thin overlapping discs of ice that form underneath the lower surface of the solid 2-4 m thick sea-ice). When the eggs hatch the platelets provide protection from predators, but also a source of food in the form of tiny algae that grows on the underside of the sea-ice. As the sea-ice melts the developing larvae continue to feast on the plant (phyto-) plankton that blooms in the summer surface waters. As they develop, they move further down into the water column into the colder sub-zero water where vast swarms of the 5 cm long whitebait-like juveniles are found. The adult (10-20 cm) silverfish are widespread in depths of 200-500 m across the Ross Sea shelf.

On this voyage we have a specific objective to learn more about the life history and distribution of silverfish. Italian colleagues working in Terra Nova Bay (TNB) have found dense concentrations of eggs under the sea-ice in Oct-Nov. It is still not known whether adult silverfish migrate into the bays to spawn, or whether the eggs drift inshore with the currents. On Tuesday we deployed an upward-looking moored echosounder at 500 m in the deep trench that goes into TNB. This instrument will record from May to December over the next two winters to hopefully document the inward and outward spawning migration of adult silverfish. As this research is a collaboration between NIWA and Italian scientists it was appropriate that we also saw the Italian research vessel Laura Bassi while in TNB. We passed her at an acceptable social distance as they departed and we went into the bay.

Throughout the voyage we have also been studying the offshore distribution of silverfish using the suite of fisheries echosounders on Tangaroa. Silverfish are hard to see on echosounders as they don't have an air-filled swimbladder so they are a very weak acoustic target. However by using sensitive equipment across a range of frequencies from 18 to 200 kHz (including a new broadband system from 55-85 kHz) we have learnt to recognise the signal associated with silverfish. To check ourselves, we are carrying out regular midwater trawls to 'ground-truth' the marks we see on our echosounders. These trawls have caught silverfish in a range of sizes from larvae to adults. Most of the fish are measured and weighed, then kept as samples for further work by NIWA, and collaborators at University of Otago, and in China and Italy.

Apologies for the overload of information on silverfish, but they are a personal favourite! To me they epitomise the Ross Sea ecosystem just as much as penguins. Silverfish are a uniquely Antarctic species which are integrally linked with their environment, and may therefore be particularly susceptible to change. And if you want a penguin angle, it is probably no coincidence that there is an emperor penguin colony at Cape Washington near the entrance to TNB! We saw a few emperor penguins on ice floes while we worked, and also a distant pod of 5-6 orca.

Today (Friday) we are at the most southerly point of our voyage at 75 degrees south. For the last three days we have been zig-zagging our way east along this line of latitude. This transect has taken us across a massive bloom of phytoplankton – visible from satellites that monitor ocean colour from space. The water in this bloom is pea-green – more like the Firth of Thames than the Southern Ocean! The water sampling and biogeochemistry teams have been working overtime sampling, filtering and experimenting. The usual sample volumes have been reduced as filters clog so quickly with the abundant life! Today we will deploy a special tow-fish to collect about 400 litres of surface water to carry out further onboard experiments for the next 8 days as we steam north. NIWA scientist Cliff Law and his team are particularly interested in the effects of trace-metals on phytoplankton, so we have to take special care to avoid contamination with any of the metal surfaces on the ship. The crew are wearing surgical gloves when handling the instruments!

Tonight we will steam north to the Iselin Bank to pick up and re-deploy the last of our passive acoustic moorings (whale listening posts). We will then start the long (8-day) journey home. We are due into Wellington on 15 February, but I will send another update next Friday with a summary of the voyage. As always, if you have any questions, you can e-mail me direct.

Best wishes,
Richard