Continuous Plankton Recorder

View from the Aurora Australis trawl deck (Photo: Wendy Pyper)
View from the Aurora Australis trawl deck (Photo: Wendy Pyper)
A single pink krill is visible on the silk (Photo: Wendy Pyper)The internal cassette of the CPR containing the silk mesh wound around a spool (Photo: Wendy Pyper)The silk mesh is removed from the CPR and preserved in formalin for later microscopy (Photo: Wendy Pyper)

21 September

We’re currently at 56° South, past Macquarie Island and well into the sub-Antarctic. It’s 1.3°C with the wind chill bringing the temperature down to about −6°C.

I have been feeling somewhat queasy these past three days so blog posts and general computer work has been a little intermittent. The rolling and pitching of the ship doesn’t prevent the work of the continuous plankton recorder (CPR) team, however. Marine biologist, Rob King, has been responsible for deploying this nifty little device off the trawl deck of the ship since we left Hobart.

The CPR is a self-contained automatic sampler towed behind the ship at normal ship speed and can operate in nearly all sea conditions. As the CPR is towed along, water and zooplankton enter a small 1.25 × 1.25 cm aperture in the nose cone, which then expands into a wider collecting tunnel, slowing down the water flow. The plankton are then trapped between two bands of 270 µm mesh silk (6 m long × 15 cm wide) loaded in a removable cassette. The silk and plankton 'sandwich' is wound onto a take-up spool inside a formalin preserving chamber, all driven by passing water turning an external propeller. Regardless of the speed of the vessel, the sheets of silk are advanced at a fixed rate of 1 cm per nautical mile travelled. Each tow represents a 450 nautical mile track of continuous sampling. Back in the laboratory, each set of silks is unrolled and cut into sections representing five nautical mile samples. Plankton are then identified by microscope and counted.

We caught the first krill of the trip in the CPR silk yesterday, looking a little worse for wear as you can see in the photo. This little specimen will contribute to a 20 year record of data collected in the Southern Ocean. Recent analysis of this data by CPR project leader, the Australian Antarctic Division’s Dr Graham Hosie, has shown significant changes in the composition, distribution and diversity of plankton and krill. This includes a decline in the numbers of krill caught, an increase in smaller zooplankton and sudden massive blooms of other plankton species spread over large areas. Scientists are not sure what is driving this change yet, but documented changes in ocean temperature and sea ice could be involved.

According to Dr Hosie the CPR project has covered 70% of the Southern Ocean, taken more than 30 000 samples, identified and mapped 230 species and towed the recorder for more than 278 000 kilometres. We’ll add a few thousand more kilometres to that total by the end of this trip.