Characterization of spring phytoplankton communities in the R�o de La Plata maritime front using pigment signatures and cell microscopy

Abstract

The surface abundance and species composi- tion of phytoplankton communities were studied in a section across the continental shelf between the Riod e La Plata and the oceanic waters of the Subtropical Convergence, during late spring (November 1999). Al- gal communities were examined using light microscopy and HPLC-derived (high-performance liquid chroma- tography) pigment concentrations. The CHEMTAX program was used to estimate the chlorophyll a (chl a) biomass of different algal classes. The inclusion of the most abundant members of the chl c pigment family (chl c1, chl c2, chl c3 and chl c2 monogalactosyldiacyl- glyceride esters) in the pigment matrix improved the CHEMTAX interpretation of field data. Using this novel approach four haptophyte populations were distinguished across the studied section, even though they had qualitatively similar pigment signatures, although one subtype lacked 19¢-hexanoyloxyfucoxan- thin (Hex-Fuco). Five different phytoplankton assem- blages, spatially segregated by the prevailing environmental conditions, were distinguished during the studied period. All of them showed a complex community structure, formed by a background of small-sized cells such as cyanobacteria, cryptophytes, haptophytes and prasinophyceans, on which diatom, cryptophyte or some haptophyte blooms were over- lapped. In the estuarine assemblage, where maximum chl a concentrations where found, diatoms were always the dominant group (30-60% of total chl a), but cryptophytes (10-40%), prasinophyceans (2-20%) and dinoflagellates (2-12%) were also relevant. In the coastal assemblage diatoms were also the dominant group (35-45%), but haptophytes lacking Hex-Fuco were subdominant (20-35%). The continental shelf assemblage showed an almost exclusive dominance (90%) of haptophytes resembling the coccolithophorid E. huxleyi. Another haptophyte (Phaeocystis sp.) was dominant (75-85%) in the Malvinas Current assem- blage. The Brazil Current assemblage was characterized by the codominance of cyanobacteria (45%) and ha- ptophytes (35%). These results are discussed in rela- tionship to the complex hydrographic features of the area.