In the autumn, the waters surrounding the Antarctic Peninsula were still home to large quantities of krill and salps, ready to spawn. Thanks to warmer water temperatures, the ice formation began later in the year, as a result of which single-celled algae, the chief food source for these organisms, were available in higher concentrations. How life in the Southern Ocean will adapt to such changes was a central topic during this year’s Antarctic season on board the research vessel Polarstern, which will end when, on Monday 11 June 2018, the ship returns to its homeport, Bremerhaven, after nearly six months at sea.
Using echo sounders, dragnets and dip nets, and with research dives, the participants in Polarstern’s latest Antarctic expedition investigated the behaviour of krill and salps at 100 different points around the Antarctic Peninsula. These two groups of animals represent the first predatory link in the food chain, and feed on free-floating single-celled algae: phytoplankton. Whereas the krill are also a preferred food for many types of fish, seals and whales, very few species feed on the gelatinous salps. Accordingly, one of the researchers’ goals was to determine which of the two tends to be dominant under specific conditions.
“Even though it was late in the year, we found a great deal of mature krill and salps,” reports Prof. Bettina Meyer. The biologist from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg (HIFMB) led the Antarctic expedition from mid-March to mid-May, and offers a possible explanation: “The phytoplankton concentration – chiefly diatoms – was incredibly high for the time of year: the Antarctic autumn.” These single-celled algae still had sufficient light for photosynthesis, since the ice formation hadn’t yet begun, despite the late time of year.
“We were surprised to find krill and salps in the same area, though in different water layers. We had previously assumed that, given their differing biologies and feeding behaviours, the two species wouldn’t ever be found in the same waters,” explains Meyer.
In Polarstern’s onboard laboratories, the scientists then took a closer look at the krill and salps: they surveyed more than 13,000 individual organisms and determined their sex and maturity, e.g. to determine whether they still could have successfully reproduced in the autumn. In the course of the expedition, they confirmed that yes, they could have; and that, in a steadily warming Antarctic, we’re likely to see larger and larger salp concentrations. “It remains to be seen how this will affect the krill, but our initial findings indicate that more salps won’t mean more competition for food. On the contrary: the salps appear to profit from the swarming krill, as they use the krill’s droppings as a food source,” says Bettina Meyer.
In further experiments conducted on board, the team, consisting of researchers from seven countries, investigated the ability of salps and krill to adapt to various conditions regarding the temperature and food supply. They also explored how massive salp concentrations affect the plankton assemblages and carbon flow in the Southern Ocean. These experiments are essential to making predictions on how the local ecosystem will change when the progressive warming of the Antarctic Peninsula causes changes in the ratio of salps to krill.
Following these tests, the staff worked to refine marine technologies during a transit journey. The goal: to achieve more accurate measurements of carbon flows, both in the ocean, and between the ocean and atmosphere. The team of 28 researchers, led by AWI oceanographer Dr Volker Strass, used Polarstern’s return trip from Punta Arenas (Chile) to Bremerhaven to try out a new tow-behind measuring system. The new system will make it possible to measure several physical, chemical and biological variables, together with how they overlap – and to do so rapidly, simultaneously and precisely. “This will allow us to identify those processes that influence phytoplankton photosynthesis and, through primary production, the ocean’s level of carbon dioxide uptake,” says Strass, who also led the expedition. Following the successful test of the topAWI (towed ocean profiler of the AWI) system, the oceanographer is confident that, armed with a better grasp of these processes, he will soon be able to more precisely gauge the effects that physical changes produced by climate change have on marine ecosystems and their corresponding biogeochemical flows.
Over the course of the next four weeks, RV Polarstern will undergo routine maintenance and repairs at the Lloyd Werft in Bremerhaven, and is scheduled to leave port again on 10 July 2018.
