Continuous Plankton Recorder - Sampling and Analysis

Sampling and Analysis

The CPR is a plankton sampling instrument designed to be towed from merchant ships, or ships of opportunity, on their normal sailings. As of December 31, 2007, CPRs have been towed a total of 5,460,042 nmi (10,111,998 km) by 278 ships since the survey’s inception. Recorders have been towed in all oceans of the world, the Mediterranean, Baltic and North Seas and in freshwater lakes. However, SAHFOS CPR sampling primarily focuses on the northwest European shelf and the Northeast and Northwest Atlantic, with these regions undergoing monthly sampling; regular sampling is also now carried out in the North Pacific. Additionally, sister CPR surveys, not conducted by SAHFOS but using similar methodology, are operated from the USA and Australia.

The CPR is towed at a depth of approximately 10 metres. Water passes through the CPR and plankton are filtered onto a slow-moving band of silk (270 micrometre mesh size) and covered by a second silk. The silks and plankton are then spooled into a storage tank containing formalin. On return to the laboratory, the silk is removed from the mechanism and divided into samples representing 10 nautical miles (19 km) of tow.

CPR samples are analyzed in two ways. Firstly, the Phytoplankton Colour Index (PCI) is determined for each sample. The colour of the silk is evaluated against a standard colour chart and given a 'green-ness' value based on the visual discoloration of the CPR silk produced by green chlorophyll pigments; the PCI is a semiquantitative estimate of phytoplankton biomass. In this way the PCI takes into account the chloroplasts of broken cells and small phytoplankton which cannot be counted during the microscopic analysis stage. After determination of the PCI, microscopic analysis is undertaken for each sample, and individual phytoplankton and zooplankton taxa are identified and counted. Nearly 500 phyto- and zooplankton taxa have been identified on CPR samples since 1948.