central southern North Atlantic
The Continuous Plankton Recorder (CPR) survey is a long-term, subsurface, marine plankton monitoring programme consisting of a network of CPR transects towed monthly across the major geographical regions of the North Atlantic. It has been operating in the North Sea since 1931 with some standard routes existing with virtually unbroken monthly coverage back to 1946. After each tow, the CPR samples are returned to the laboratory for routine analysis, including the estimation of phytoplankton biomass (Phytoplankton Colour Index, PCI) and the identification of up to 500 different phytoplankton and zooplankton taxa (Warner and Hays, 1994). Direct comparisons between the Phytoplankton Colour Index and other Chlorophyll a estimates, including SeaWiFS satellite estimates, indicate strong positive correlations (Batten et al., 2003; Raitsos et al., 2005).
The second step of the phytoplankton analysis involves counting phytoplankton cells under high magnification (x450) to identify and count taxa. Each CPR sample represents ~3 m3 of filtered seawater. Using this analysis method, 200 phytoplankton taxa have been routinely identified and counted by the CPR survey since 1958. Because of the mesh size of CPR silks, many phytoplankton species are only semi-quantitatively sampled owing to the small size of the organisms. There is, thus, a bias towards recording larger armoured flagellates and chain-forming Diatoms, and smaller-species abundance estimates from cell counts will probably be underestimated in relation to other water sampling methods. However, the proportion of the population that is retained by the CPR silk reflects the major changes in abundance, distribution, and specific composition (i.e. the percentage retention is roughly constant within each species even with very small-celled species; Edwards et al., 2006).
The CPR now has a water sampler housed on board certain units to provide additional data and sample the whole size spectrum of plankton using molecular techniques from bacteria and viruses to flagellates and other taxa not normally identified using standard CPR analysis.