Phytoplankton and pigment distributions in an anticyclonic slope water oceanic eddy (SWODDY) in the southern Bay of Biscay

An anticyclonic slope water oceanic eddy (SWODDY), named AE6, was sampled in the southern Bay of Biscay from 12 to 31 August 1998 to assess changes in the abundance and composition of phytoplankton assemblages related to the mesoscale feature. SWODDY AE6 showed characteristic biological signatures. A twofold increase in chlorophyll a concentration was found at the eddy centre relative to surrounding waters. Picoplankton cells accounted for a lower fraction of total chlorophyll a values at the eddy centre (44-50%) than outside the eddy (54-61%). Microscopic cell counts and HPLC pigment analysis showed that diatoms were almost entirely confined to the eddy centre, but both techniques yielded different results when studying other phytoplankton groups. Microscopic cell counts indicated that the spatial distribution of diatoms, dinoflagellates and unidentified flagellates was significantly influenced by SWODDY AE6, showing maximum abundance inside the mesoscale feature. HPLC pigment analysis provided more detailed information about the composition of pico-nanoplanktonic organisms. Pigment data processed by means of the CHEMTAX program showed 'chlorophytes', 'haptophytes' and 'dinoflagellates II' (having haptophyte-like pigments and gyroxanthin-diester) as the more abundant 'pigment classes' at the eddy centre, whereas dominance of 'chlorophytes' and higher contribution of 'cyanobacteria' (type Synechococcus) were estimated in the surrounding waters.