Phytoplankton biomass from continuous plankton recorder data: an assessment of the phytoplankton colour index

A 15 month time-series of simultaneous fluorometric measurements of chlorophyll a, and phytoplankton cell abundance and a phytoplankton colour index (PCI) from continuous plankton recorder (CPR) samples are compared for the Iberian margin, western Europe. The relationships between fluorometrically determined chlorophyll, cell abundance and PCI are all significant, with PCI showing a better relationship to chlorophyll than cell abundance. This supports previous research which suggests that the PCI is a useful index of phytoplankton standing stock and, furthermore, may be a measure of cells that are not preserved or counted in the routine microscopic processing of CPR samples. The relationship between PCI and chlorophyll appears to vary seasonally, although more extensive data are needed to quantify this variability. The study also compares the PCI and fluorometer data with chlorophyll values obtained from satellite imagery (SeaWiFS) for the Iberian margin. Significant correlations exist during the 15 month time-series between these three estimates of phytoplankton concentration. This result demonstrates that the similar to50 year time-series of PCI for the North Atlantic can provide valuable information on changes in phytoplankton standing stock.