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cienceAbstract Phytoplankton composition and abundance were studied at a fixed station (P2, Rı́a of Pontevedra, NW Spain) weekly during a 2-year period (1999–2000). In addition to microscopic cell counts, a chemotaxonomic approach based on HPLC pigment analysis and CHEMTAX data processing was studied on two size classes. The contribution of the picoplankton fraction to the total chlorophyll (chl) a averaged 13 10%. Pigment suites of the picoplankton fraction were mainly provided by picoeukaryotes. Chl b dominated in the picoplankton whereas chls c (c2, c1 and c3) were the major accessory chlorophylls in the micro-nanoplankton. Despite this, fucoxanthin was by far the most abundant carotenoid in both size classes (often>70 % of total carotenoids). Major pigment groups in the picoplankton were prasinophytes (with prasinoxanthin and carotenoids of the uriolide series) and chlorophytes, which contributed up to 60 % total chl a during winter. Diatoms and haptophytes were other relevant picoplanktonic groups along the seasonal cycle. Micro-nanoplankton was dominated by diatoms I (chl c1 and chl c2) and diatoms II (chl c3 and chl c2), which contributed up to 70 % of total chl a in spring. Chl c composition during diatom blooms exhibited higher chl c1: chl c2 ratios in winter–spring and higher chl c3: chl c2 ratios in summer–autumn