The phytoplankton colour index (PCI) of the Continuous Plankton Recorder
(CPR) survey is an in situ measure of ocean colour, which is considered a proxy of
the phytoplankton biomass. PCI has been extensively used to describe the major
spatiotemporal patterns of phytoplankton in the North Atlantic Ocean and North
Sea since 1931. Regardless of its wide application, the lack of an adequate evalu-
ation to test the PCI’s quantitative nature is an important limitation. To address
this concern, a field trial over the main production season has been undertaken to
assess the numerical values assigned by previous investigations for each category of
the greenness of the PCI. CPRs were towed across the English Channel from
Roscoff to Plymouth consecutively for each of 8 months producing 76 standard
CPR samples, each representing 10 nautical miles of tow. The results of this ex-
periment test and update the PCI methodology, and confirm the validity of this
long-term in situ ocean colour data set. In addition, using a 60-year time series of
the PCI of the western English Channel, a comparison is made between the previ-
ous and the current revised experimental calculations of PCI.