MEPS 323:59-73 (2006)  -  doi:10.3354/meps323059

ABSTRACT: Size-fractionated phytoplankton was studied using pigments and carbon (C) biomass derived from light microscopy at a fixed location on the NW Iberian shelf between May 2001 and June 2002. The annual contributions of pico-, nano- and microphytoplankton to the total autotrophic C biomass were 9.7, 63 and 27.3%, respectively, with unidentified nanoflagellates (63%) and diatoms (22%) being the most important components. The CHEMTAX program was used to determine the chlorophyll a (chl a) contribution of 9 pigment groups in nano-microplankton and 8 in picoplankton. ‘Diatoms II’ (species containing chl c₃), ‘haptophytes II’ (species of the genus Chrysochromulina containing chl c₂-MGDG [monogalactosyl diacylglyceride] [14:0/14:0] ) and ‘chlorophytes’, represented 34, 17 and 16%, respectively, of the total chl a in the nano-microplankton fraction. The major pigment groups in the picoplankton were ‘chlorophytes’, ‘Synechococcus’ and ‘prasinophytes II’ (Order Mamiellales), which accounted for 34, 22 and 10% of the total chl a in this size class. ‘Prochlorococcus’ was traced using divinyl (DV) chl a and was present seasonally, especially during downwelling and stratified poleward conditions (October to November). The lowest chl a and autotrophic C values throughout the study (152 ± 45 ng chl a l^–1 and 31 ± 13 µg C l^–1) corresponded to the homogenous poleward flow or Iberian Poleward Current (IPC) from December to February. During this period the phytoplankton composition changed abruptly to a diverse pico-nanoplanktonic community that represented ~95% of the total autotrophic C biomass. ‘Chlorophytes’, ‘cryptophytes’ and ‘haptophytes II’ in the nanoplankton, and ‘chlorophytes’ and ‘Synechococcus’ in the picoplankton were the main contributers to chl a in IPC waters, as calculated by CHEMTAX.

KEY WORDS: Phytoplankton · Pigments · Carbon biomass · Poleward current · Iberian upwelling · CHEMTAX