The CPR Survey
The CPR is a plankton sampling instrument designed to be towed from merchant ships on their normal sailings. Alister Hardy used the first prototype to sample krill in the Antarctic on the "Discovery" cruises of 1925-27. He modified the design for use in the North Sea and started collecting plankton in the 1930s. The CPR was deployed in the North Sea regularly from 1946, on a number of routes.
The CPR is about one metre in length. The body is made of gunmetal, (phosphor bronze), or stainless steel in later versions from 1997. The nose cone is filled with lead. The tail section is made of mild steel which is rust proofed. The CPR is towed on a 10mm diameter wire rope at a depth of about 10 metres. The wire is connected to the CPR nose via a shock absorber. The CPR has been operated successfully at speeds of up to 25 knots, and its robust design allows deployment in rough seas without fear of excessive damage. Successful tows have been continued through sea states of up to wind force 11. Deployments and recoveries have been made in wind force 8 conditions from larger merchant ships. Each CPR is now fitted with a fender to reduce the risk of damage on deployment and recovery
The CPR works by filtering plankton from the water over long distances (up to 500 nautical miles) on a moving filter band of silk (270 micron mesh size). The filter silk band is wound through the CPR on rollers turned by gears, which are powered by an impeller. The cut away diagram shows the layout of the plankton filtering mechanism (sometimes described as the 'internal mechanism' or 'cassette') and the impeller.
The internal mechanism (also shown) is a self-contained cartridge that is loaded with the filtering silk at the laboratory and placed inside the CPR prior to deployment. On some tows, the ships are supplied with several internal mechanisms, which they load into the CPR to increase the sampling range. On return to the laboratory, the silk is removed from the mechanism and divided into samples (known as blocks) representing 10 nautical miles of towing. The plankton on these samples are then analysed according to standard procedures.
Before cutting, the colour of the silk is compared to a colour chart and given a 'green-ness' value of 0 (no greenness), 1 (very pale green), 2 (pale green) or 6.5 (green). Other colours are not recorded. This is a subjective analysis, with arbitrary values returned, but it can be the first indication of phytoplankton blooms on our samples.
After cutting into blocks, microscopic analysis of the plankton contained on the sample is undertaken. A subsample of the block is examined under high power magnification to identify and count phytoplankton species present. (the sub-sample is about 0.001 of the whole sample). This analysis is known as 'Phytoplankton field analysis'.
Another sub-sample analysis for small zooplankton is then carried out under a lower magnification, where all individuals seen in a traverse of the silk are identified and counted. (This sub sample is about 0.02 of the whole sample). This analysis is known as 'zooplankton traverse analysis'.
The last part of the analysis process is that all zooplankton larger than about 2mm are identified and counted from the whole sample. They are spotted by eye, but identified under the microscope. This is known as 'zooplankton eye count analysis'.
After analysis, the counts are checked and added to the CPR database, which contains details of the plankton found on over 170000 samples taken since 1946 in the North Sea and North Atlantic Ocean.