Methodology
In the field
- The CPR (Diagram:AAD)
Water enters the Continuous Plankton Recorder (CPR) through a square aperture (1.27 cm each side), about the size of a thumbnail. It then flows down an expanding tunnel, which effectively reduces the water pressure to minimise damage to the captured plankton, and exits through the rear of the device. The movement of water past the CPR turns an external propeller at the rear of the device that operates a drive shaft and gear system, which advances the silk filtering mesh. Plankton in the water are filtered onto this constantly moving band of silk. This filtering silk meets a second band of covering silk, effectively sandwiching the plankton, and is then wound onto a spool in a storage tank containing formalin. Both phytoplankton and zooplankton are retained.
- The CPR being deployed on a SOOP (Image:SAHFOS)
Ships of Opportunity (SOOPs) have been used extensively to tow CPRs in the North Atlantic for the past 70 years, and more recently in the North Pacific, South Atlantic, and Southern Ocean. If a towing point (davit) and winch is not already available at the stern of the ship to tow a CPR, then they can be fitted. As no research staff accompany the CPR, the ship's crew deploy the device once the ship reaches deep enough water outside the harbour. The CPR is towed at around 7-10 m depth at the normal cruising speed of the vessel (usually 15-20 knots). The deployment and retrieval of the CPR does not interfere with normal ship operations. For long routes, the CPR is brought onboard every 450 nautical miles whilst underway to change the internal cassette and then redeployed. On a SOOP, GPS coordinates are recorded at the start and end point of each tow as well as at waypoints for changes of bearing.
- The CPR is lowered to approximately 10 m depth, 100 m behind the vessel (Image: J.Kitchener/AAD)
The CPR is fitted with a Star-Oddi recorder measuring temperature, depth, and pitch and roll of the instrument.
Detailed descriptions of the CPR device and its sampling characteristics are summarised in Richardson et al. (2006) and Hosie et al. (2003). Please click link below for full pdf download.
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- The silk is cut into section representing 5 nm of towing distance (Image:Anita Slotwinski/CSIRO)
Cutting of the silk and the Phytoplankton Colour Index
The 6 m length of the plankton silk 'sandwich' is carefully unrolled and laid out on a bench. Prior to being cut, the colour of the silk is compared with the colour of a standard set of colour charts by laying the silk against a white background. This Phytoplankton Colour Index is estimated for each numbered division (sample) on the graduated silk. This index has 4 levels depending on the greenness of the silk (1: very pale green; 2: pale green; 3: green; 4: very green). Phytoplankton Colour has proven to be a good index of phytoplankton biomass (chlorophyll) estimate fluoremetrically and from satellite (see Richardson et al. (2006) for more information). Although the phytoplankton colour assessment is done 'by eye', there is very close agreement between CPR analysts on the estimated colour of silks.
The silk is then cut into segments representing 5 nautical miles of towing distance using the ships location information to relate the plankton data from the CPR silk to a real position. The size of the segments varies slightly and is calculated depending on the speed the silk was wound through the CPR unit; 5 nautical miles is approximately equal to 5 cm.
- Phytoplankton analysis showing 20 fields (each 295 µm across) (Diagram:Anthony Richardson/CSIRO).
Phytoplankton analysis (on-silk)
The covering and filtering silks are opened out with the plankton facing upwards. The filtering silk is laid on a glass plate sprayed with a little water to keep the silk moist. Under a compound microscope with a large 150 mm x 100 mm stage, twenty fields of view (295 µm) situated in a cross pattern are then counted for phytoplankton cells by moving the stage (right). Within each field, presence-absence is recorded for each phytoplankton species. A taxa seen in 20 out of 20 fields of view is considered to be very common, while a taxon only recorded in 1 out of 20 is rare.
- Zooplankton analysis (Image:David McLeod/CSIRO)
Zooplankton analysis (off-silk)
The silk is then rinsed off into a 2 litre beaker using tap water, ensuring that all animals are removed from the silk. The zooplankton washed off the silk are then poured through a 100 µm sieve with the filtrate containing the majority of the phytoplankton being reserved in a beaker for archiving. The zooplankton sample is then transferred from the sieve to a Bogorov tray for identification to the lowest possible taxa. Copepods are generally identified to species, and other zooplankton groups to higher taxonomic groups (e.g. chaetognaths, decapod larvae, hyperiid amphipods).
Exact zooplankton counts for zooplankton are recorded and not the categorical system of the North Atlantic survey. All information is recorded on data sheets and later transferred to the AusCPR database. The zooplankton sample and the filtrate (containing the phytoplankton) are then reduced in volume by running through 100 µm and 10 µm sieves respectively. The zooplankton sample, filtrate and silk segment are then transferred to a 30 ml vile containing 5% formalin for archiving. Archived samples are available for later analysis (please contact the AusCPR survey if you would like to use the samples for your own analysis).