A comparison of HPLC pigment signatures and electron microscopic observations for oligotrophic waters of the North Atlantic and Pacific Oceans

The use of HPLC pigment analysis has become a primary tool for investigating the taxonomic composition of natural phytoplankton populations. In this study, we compare, for the first time, the taxonomic composition based upon HPLC pigment signatures with direct electron microscopic taxonomic identifications from two sets of open ocean oligotrophic field samples. Electron microscopic observations at sites in the Atlantic and Pacific Oceans (Hydrostation S and Station ALOHA, respectively) agree with taxonomic partitioning based upon HPLC algorithms in the, upper water-column samples, but there is increasing disagreement between the two methods in deeper water samples. This disparity probably results from depth-dependent changes in cellular pigment content and accessory pigment-to-chlorophyll ratios. At both locations, the eukaryotic ultraplankton was similar in taxonomic composition, at least at the class level, and the Prymnesiophyceae and the newly described Pelagophyceae were the two most abundant groups of eukaryotes.