AME 49:255-265 (2007)  -  DOI: https://doi.org/10.3354/ame01144

ABSTRACT: The aim of our study was to describe the diversity and dynamics of size-fractionated phytoplankton at a fixed station (P2, Ría de Pontevedra, NW Spain) from late spring to early winter. Phytoplankton successions were assessed weekly from May to November 2004 using pigment analyses, light microscopy and fluorescence in situ hybridization coupled with tyramide signal amplification (FISH-TSA). Periodic upwelling events triggered the development of diatom populations followed by periods of relaxation/downwelling conditions where remineralization processes (maxima of NH₄⁺), raised temperatures and/or low salinity waters from rainfall favored the increase of picophytoplankton. Marked differences were observed between diversity and dynamics of the micro-nano (>3 µm) and pico (<3 µm) size fractions. Micro-nanoplankton organisms contributed 85% of chl a on average (range, 55 to 99%); diatoms and cryptophytes were the dominant pigment groups (~70% chl a in this size category). In the picoplankton, chlorophytes and non-chlorophytes contributed to an identical proportion (50%) of total picoeukaryotes determined by FISH-TSA. A large fraction of these (~65%) could be accounted for by group-specific probes. Molecular data suggested that chl b-containing picoplankton were mostly accounted for by prasinophyceans belonging to the order Mamiellales (55% of total chlorophyte cells), with Micromonas pusilla as its major component (~70% of Mamiellales, ~20% of all picoeukaryotes), similar to that found in other temperate coastal locations. In contrast, HPLC analyses severely underestimated the contribution of prasinophyceans and indicated <10% Mamiellales among chlorophytes. Among chromophytes, the haptophyte probe targeted ~25% of non-chlorophyte cells, and pelagophyceans were sporadically abundant (13% non-chlorophyte cells, May to September). After pigment analyses the main chromophytes were diatom II (chl c₃-containing), pelagophyceans and haptophytes with 20, 13 and 8% of picoplanktonic chl a, respectively.

KEY WORDS: Phytoplankton diversity · Pigments · FISH-TSA · Microscopy · Picoplankton · Ría de Pontevedra