Annual variations of phytoplankton biomass in the Eastern English Channel: comparison by pigment signatures and microscopic counts

From November 1995 to July 1997, phytoplankton composition was followed fortnightly, microscopically as well as by pigment analysis, in the coastal and offshore waters of the Eastern English Channel. By relating chlorophyll (Chl) c(3), peridinin and alloxanthin to biomass estimates of Phaeocystis sp., Dinophyceae and Cryptophyceae, respectively, it has been shown that these pigments are good quantitative markers of these classes. Fucoxanthin was quantitatively related to brown algae (sum of diatoms, Prymnesiophyceae, Raphidophyceae and Chrysophyceae) if the largest cells of diatoms (size >200 mu m) were nut taken into account. On the other hand, no relationship was found between green algae and Chl b, perhaps due to uncertainties in the microscopic counts for cells of small size (<5 mu m) and/or with membranes of low visibility. While Prasinophyceae and Chrysophyceae were sometimes observed, prasinoxanthin and 19'-butanoyloxyfucoxanthin were never detected. Finally, Chl a did not always give a good estimation of the total phytoplankton biomass. Thus, during the spring bloom of Phaeocystis sp. (Prymnesiophyceae), total biomass was overestimated by pigment analysis with respect to microscopic counts, while during summer biomass was underestimated by pigment analysis.