MEPS 142:55-73 (1996)  -  doi:10.3354/meps142055

The seasonal variation in the composition of algal communities in the lower St. Lawrence Estuary was examined using HPLC pigments and cell taxonomy, and the efficiency of these 2 techniques was compared. A major centric diatom bloom was observed in July 1992, preceded by an increase in pennate diatoms in June, likely caused by bottom resuspension due to spring runoff. Grazing in June and July was indicated by the presence of pyropheophorbide a, a copepod grazing product tracer, and chlorophyll degradation pigments, likely associated with sloppy feeding and with the presence of cells (diatoms) with high chlorophyllase activity and acidic cell sap. Various pheopigments and degradation products of chl a were found in these 2 months. This is consistent with observations of maximum abundances of major copepod species and of herbivorous ciliates in June preceding the summer diatom bloom. May was characterized by nanoflagellates from Chrysophyceae, Cryptophyceae and Chlorophyceae, lower values of algal biomass and production and higher light harvesting efficiency. Mixing prevented the establishment of vertical fluorescence patterns in May and September and probably lowered the effective daily light exposure of algae, which translated into lower light acclimation than in summer and higher ratios of photosynthetic pigments to chl a. Low-light acclimation was also observed in the deep (>20 m) June and July populations, affecting marker pigment coefficients used to calculate relative algal contributions. Increases in relative amounts of chlorophyllide a and in the allomer of chl a in September were interpreted as signs of algal senescence. The September populations were composed of a number of chlorophyte and chromophyte (fucoxanthin-containing) algae. Low pigment concentrations and low numbers of observations complicated the identification task for that month. Pigment and microscopic approaches were compared on the basis of (1) clustering, using each separately, (2) correlations between pigments and cell groups, and (3) transformation of pigment data into algal group contributions to chl a. The 2 approaches generally gave similar results even though they showed different characteristics: the presence of small cells was often a problem for microscopic identifications, while the lack of specificity of some markers (e.g. fucoxanthin) reduced taxonomic precision from the pigment approach. Combining both was certainly advantageous, in that cell-pigment correlations helped in the assignment of a number of pigment markers. Pigments also helped in ascribing taxonomic identities for unidentified flagellates, which were numerous in June and September. Thus, the choice between using the methods singly or together will depend partly on the degree of taxonomic detail needed.

HPLC · Pigments · Cell microscopy · Phytoplankton communities · Cluster analysis · St. Lawrence Estuary