CHARACTERIZATION OF PHYTOPLANKTON COMMUNITIES IN THE LOWER ST-LAWRENCE ESTUARY USING HPLC-DETECTED PIGMENTS AND CELL MICROSCOPY

The seasonal variation in the composition of algal communities in the lower St. Lawrence Estuary was examined using HPLC pigments and cell t axonomy, and the efficiency of these 2 techniques was compared. A majo r centric diatom bloom was observed in July 1992, preceded by an incre ase in pennate diatoms in June, likely caused by bottom resuspension d ue to spring runoff. Grazing in June and July was indicated by the pre sence of pyropheophorbide a, a copepod grazing product tracer, and chl orophyll degradation pigments, likely associated with sloppy feeding a nd with the presence of cells (diatoms) with high chlorophyllase activ ity and acidic cell sap. Various pheopigments and degradation products of chl a were found in these 2 months. This is consistent with observ ations of maximum abundances of major copepod species and of herbivoro us ciliates in June preceding the summer diatom bloom. May was charact erized by nanoflagellates from Chrysophyceae, Cryptophyceae and Chloro phyceae, 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 e ffective daily light exposure of algae, which translated into lower li ght acclimation than in summer and higher ratios of photosynthetic pig ments 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 populatio ns were composed of a number of chlorophyte and chromophyte (fucoxanth in-containing) algae. Low pigment concentrations and low numbers of ob servations complicated the identification task for that month. Pigment and microscopic approaches were compared on the basis of (1) clusteri ng, using each separately, (2) correlations between pigments and cell groups, and (3) transformation of pigment data into algal group contri butions to chl a. The 2 approaches generally gave similar results even though they showed different characteristics: the presence of small c ells was often a problem for microscopic identifications, while the la ck of specificity of some markers (e.g. fucoxanthin) reduced taxonomic precision from the pigment approach. Combining both was certainly adv antageous, in that cell-pigment correlations helped in the assignment of a number of pigment markers. Pigments also helped in ascribing taxo nomic 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 neede d.