Snails and 'snot' tell acid story
The team is researching the effects of ocean acidification on tiny marine snails, known as pteropods, and planktonic, single-celled, shell-forming organisms called foraminifera. Pteropods are an important food source for marine predators in the Antarctic food web and sometimes replace krill as the dominant zooplankton group in parts of the Southern Ocean. Foraminifera are prey for many small marine invertebrates and fish. Both organisms are indicators of changes in the ecosystem that could have profound implications for commercial fish species, seals and whales.
About 40% of man-made carbon dioxide is absorbed by the Southern Ocean and forms a weak acid (carbonic acid) when it mixes with water. This acid readily releases hydrogen ions, and as acidity is determined by the concentration of hydrogen ions (measured on the pH scale), the more acidic a solution, the more hydrogen ions are present and the lower the pH. Increasing hydrogen ions affect the ability of pteropods and foraminifera to form shells, resulting in thinner, lighter, and pitted or etched shells. As colder water absorbs more carbon dioxide than warmer water, the effects of ocean acidification will be seen first in the Southern Ocean. According to Dr John Baxter, a scientific advisor to government from the Scottish Natural Heritage who has joined 'Team Acid' on the ship, ocean acidity has increased by 30% (a pH change of 0.1) since the beginning of the Industrial Revolution and is already affecting shell-forming marine organisms. Observed effects include thinner shells, fewer pteropods in areas where they were previously common, and an increase in gelatinous organisms such as jellyfish and salps.
To do this they are deploying a 'rectangular midwater trawl' (RMT) – a pair of rectangular mesh nets – at different latitudes, from 47– 54°S, along a line from Hobart to Casey. They hope to catch larger pteropods with a 4 mm mesh net, but the main species they're looking for is the tiny (0.5–1 mm) Limacina helicina antarctica, which will be caught in a 150 micron mesh net. The microscopic foraminifera will also be sieved from the water brought up in the trawl and preserved for later shell integrity analysis.
Team Acid will conduct eight trawls; four in subantarctic waters (45–49°S), three in polar waters (54–56°S) and one in the narrow channel of water where the subantarctic and polar waters meet (51°S). They expect to see a change in the shell weight, size and species of pteropods as we move further south into the colder and more acidified water, and hope to collect a good sample of the common Limacina helicina antarctica.
On the trawl deck the ship's crew winch the two RMTs into the heaving seas. Each net has a 'cod end' attached to it – cylindrical canisters to contain the sample. The nets remain closed until they reach the required depth, between 20 and 200 m below the surface, at which time the team can remotely open the net to collect the sample.
The team has chosen to sample between 20 and 200 m as this is the region where scientists think the pteropods construct their shells. This hypothesis is based on an analysis of pteropod shells collected in ocean sediment traps. These shells contained isotopes (different forms of molecules such as carbon and oxygen) typical of the water column at these depths.
Fifteen minutes after deployment, the RMTs are retrieved. Team Acid and their accompanying paparazzi crowd into the ship's 'wet lab' and begin bucketing and sieving through the samples. Both cod ends contain a glutinous mass of salps – 'another bucket of snot' as one crew member describes it – but this subantarctic sample also yields some surprises – about 12 large pteropods (Clio recurva) and six of the smaller Limacina helicinaantarctica. A tiny squid, a large selection of amphipods (small crustaceans) and some translucent predatory worms called chaetognaths, also appear. The huge abundance of salps and other gelatinous creatures is typical of these waters. Some theories suggest an increase in salps is occurring, creating a 'jellyfish ocean'.
At the seventh RMT site at 54°S, Team Acid hit the jackpot. One large Clio recurva shell and a whopping six small Clio pyramidata antarctica shells are captured. Dr Roberts wears a huge grin as she preserves the impressive specimens in ethanol.
The final RMT goes in at 58°S – inside waters managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) – and pulls up an amazing array of species – a magnificent pelagic polychaete (worm), lots of amphipods, juvenile krill, ctenophores (small jellyfish-like creatures), two naked (shell-less) pteropods, small salps and some mysterious, gelatinous, eyeball-like spheres, which someone suggests could be fish eggs. The naked pteropods are particularly interesting. Scientific theory suggests that these may become the dominant pteropods in the ocean as ocean acidification increases.
When they return to Australia, team member Alex Pentony Vran, an engineer from the Australian National University, will examine the mechanical properties of the captured pteropod shells to provide definitive evidence that they are becoming more fragile. Previous work has focussed on changes in shell weight and the use of optical microscopy to examine shell thickness. In contrast, Mr Pentony Vran will take the shells captured on this trip, apply force to them with an extremely fine diamond-tipped probe, and measure their response to this force. This will allow him to put a figure on how strong or weak the shells are.
The team has plenty of work ahead of them, but after five days of frenetic activity, they can now enjoy the voyage at a snail's pace.
Corporate Communications, Australian Antarctic Division