Characterization of phytoplankton communities in the lower St. LawrenceEstuary using HPLC-detected pigments and cell microscopy

Abstract

The seasonal variation in the composition of algal communities in thelower St. Lawrence Estuary was examined rising HPLC pigments and celltaxonomy, and the efficiency of these 2 techniques was compared. A majorcentric diatom bloom was observed in July 1992, preceded by an increase inpennate diatoms in June, likely caused by bottom resuspension due to springrunoff. Grazing in June and July was indicated by the presence ofpyropheophorhide a, a copepod grazing product tracer, and chlorophylldegradation pigments, likely associated with sloppy feeding and with thepresence of cells (diatoms) with high chlorophyllase activity and acidic cellsap. Various pheopigments and degradation products of chl a were found inthese 2 months. This is consistent with observations of maximum abundances ofmajor copepod species and of herbivorous ciliates in June preceding thesummer diatom blood. May was characterized by nanoflagellates fromChrysophyceae, Cryptophyceae and Chlorophyceae, lower values of algal biomassand production and higher light harvesting efficiency. Mixing prevented theestablishment el vertical fluorescence patterns in May and September andprobably lowered the effective daily light exposure of algae, whichtranslated into lower light acclimation than in summer and higher ratios ofphotosynthetic pigments to chl a. Low-light acclimation was also observed inthe deep (> 20 m) June and July populations, affecting marker pigmentcoefficients used to calculate relative algal contributions. Increases inrelative amounts of chlorophyllide a and in the allomer of chl a in Septemberwere interpreted as signs of algal senescence. The September populations werecomposed of a number of chlorophyte and chromophyte (fucoxanthin-containing)algae. Low pigment concentrations and low numbers of observations complicatedthe identification task for that month. Pigment and microscopic approacheswere compared on the basis of (1) clustering, using each separately, (2)correlations between pigments and cell groups, and (3) transformation ofpigment data rate algal group contributions to chl a. The 2 approachesgenerally gave similar results even though they showed differentcharacteristics: the presence of small cells was often a problem formicroscopic 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 correlationshelped in the assignment of a number of pigment markers. Pigments also helpedin ascribing taxonomic identities for unidentified flagellates, which werenumerous in June and September. Thus, the choice between using the methodssingly or together will depend partly on the degree of taxonomic detailneeded.