Size distribution of algal pigments and phytoplankton assemblages in a coastal--estuarine environment: contribution of small eukaryotic algae

The size structure and the taxonomic composition of phytoplankton assemblages were assessed in the lower Urdaibai estuary from February 1998 to January 1999 by means of high-pressure liquid chromatography of algal pigments, along with microscopic observations of live and fixed cells. Four size classes were delineated: microplankton (>20 μm), large nanoplankton (8–20 μm), small nanoplankton (1–8 μm) and picoplankton (<1 μm). The dominant marker pigments in the samples were fucoxanthin, 19′-butanoyloxyfucoxanthin (19′but), chlorophyll (Chl) b, alloxanthin, peridinin and lutein–zeaxanthin. During the winter–spring transition, most of the Chl a was found in the microplankton size fraction. However, during the rest of the observation period, dominated by nanoplankton, a great proportion of the total Chl a (60–80%) appeared in the <8 μm fraction. The relative importance of the picophytoplankton was highest from April to November; its contribution during this period ranged from 10% to ∼40% of the total Chl a. Observations using microscopy confirmed the dominance of diatoms during most of the year and the presence of prymnesiophytes, prasinophytes, chrysophytes, cryptophytes, dinoflagellates, euglenophytes and cyanobacteria. Based on signature pigments and microscopic observations, the following groups of algae were established: fucoxanthin-containing, 19′but-containing, chlorophytes, euglenophytes, dinoflagellates, cryptophytes and cyanobacteria, whose contribution to total Chl a is discussed.