In 2009, the composition of phytoplankton was studied in the open NW Mediterranean (MEDOC area) during three key ecological situations: bloom (March-April), post-bloom (May) and late stratification (September). Pigments were quantified by HPLC and cells by flow cytometry and optical microscopy. Despite the great variability between stations within each cruise, statistical PCA analysis revealed that stations were more different intercruises than intra-cruises. The bloom period was very dynamic. We found an initial prebloom situation with strong vertical mixing, and slightly more stratified waters towards the end of the cruise, when a diatom bloom started. A chlorophyll maximum was observed at surface rather than at depth. Prymnesiophytes were the next phytoplankton group in terms of abundance, while prasinophytes, pelagophytes and Synechococcus contributed more to the bulk phytoplankton during the first part of the cruise (the pre-bloom). Dinoflagellates were residual. During the post-bloom cruise the stations were also very heterogeneous. Two stations located south in the area presented a marked deep chlorophyll maximum (DCM) and were dominated by prymnesiophytes and pelagophytes, with a significant presence of diatoms in surface and low concentrations of Synechococcus, dinoflagellates and prasinophytes in general. On all other stations there was not a clear DCM. The highlight of this cruise was the massive dominance of Synechococcus on some stations and cryptophytes on others. In September the stations were more homogeneous. Here, prymnesiophytes dominated, with a significant presence of Synechococcus and Prochlorococcus along the water column, and pelagophytes at the DCM and below. Prasinophytes, cryptophytes and dinoflagellates were scarce. In terms of phytoplankton groups, diatoms dominated during the bloom situation and their importance decreased while stratification increased. Prymnesiophytes contributed always significantly ( ̴30%) to the bulk phytoplankton with a slight preference for surface layers. Synechococcus presented bloom concentrations in few stations of the post-bloom cruise (to our knowledge a first report of this kind in the NW Mediterranean), and significant concentrations during all cruises. Cryptophytes were in general very scarce except in some very shallow samples of few stations during the post-bloom cruise. Pelagophytes contributed around 20% at depth during the post-bloom and late stratification cruises. Prochlorococcus appeared only in the late stratification cruise and especially in the top layers of the DCM. Prasinophytes were present in all situations. Photosynthetic dinoflagellates were always residual

XXXII Trobades Científiques de la Mediterrània, Planeta Oceà - Planet Ocean, celebradas del 5 al 7 de octubre de 2016 en Maó, Menorca.-- Homenatge als Drs. Marta Estrada, Jordi Font i Jordi Salat, pioners de l'oceanografia mediterrània moderna. A tribute to Drs. Marta Estrada, Jordi Font and Jordi Salat, pioneers of modern Mediterranean oceanography.-- 1 page

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