IMIS

The Continuous Plankton Recorder (CPR) survey has been operating in the North Atlantic and North Sea since 1946 and measures the abundance of approximately 450 phytoplankton and zooplankton taxa (Warner and Hays, 1994). The CPR is a high-speed plankton recorder that is towed behind ‘ships of opportunity’ through the surface layer of the ocean (~10 m depth). Water passes through the recorder, and plankton are filtered by a slow moving silk (mesh size 270 μm). A second layer of silk covers the first and both are reeled into a tank containing 4% formaldehyde. Upon returning to the laboratory, the silk on is unwound and cut into sections corresponding to 10 nautical miles and approximately 3 m3 of filtered sea water. The colour of each section of CPR silk is then evaluated and categorized according to four levels of ‘greenness’ (green, pale green, very pale green and no colour) using a standard colour chart; these numbers are given a numerical value as a measure of the ‘Phytoplankton Colour Index’. This is a semiquantitative measure of phytoplankton biomass; the silk gets its green colour from the chloroplasts of the filtered phytoplankton. Phytoplankton cells are then identified and recorded as either present or absent across 20 microscopic fields spanning each section of silk; CPR phytoplankton abundance is therefore a semiquantitative estimate (i.e. the species is recorded once per field independent of the number of cells in a field). However, the proportion of cells captured by the silk reflects the major changes in abundance, distribution, and community composition of the phytoplankton (Robinson, 1970), and is consistent and comparable over time. Zooplankton analysis then carried out in two stages with small (<2 mm) zooplankton identified and counted on-silk and larger (>2 mm) zooplankton enumerated off-silk. The collection and analysis of CPR samples have been carried out using a consistent methodological approach since 1958, making the CPR survey the longest continuous dataset of its kind in the world (Edwards and Richardson, 2004).