| Summary: | The western Antarctic Peninsula (WAP) is one of the most productive regions in the Southern Ocean. However, little is known about the phytoplankton composition in nearshore waters, in fjords and channels between 63º and 67°S, where Antarctic krill and baleen whales are conspicuous. This study represents the first attempt to describe spatial and temporal composition of the phytoplankton community (species, cell concentration, phytoplankton biomass) in twelve relatively unexplored nearshore sites of the WAP. Sampling was carried out in the frame of a Citizen Science project during late summer of 2016 and during the spring–summer 2016–2017. Species identification and enumeration were performed by light and scanning electron microscopy and phytoplankton carbon biomass was estimated by using cell-volume conversion. The highest phytoplankton abundance and biomass values were found in December-January, and were mainly represented by nanophytoflagellates (2–20 µm). Cryptophytes were more abundant in early summer and prasinophyceans in late summer. The abundance of large bloom-forming diatoms was unexpectedly low. Three blooming flagellated taxa were found during the sampling season, chronologically: Pyramimonas sp. in Neko Harbor (March 3, 2016, 1.4 × 106 cells L−1, and 327 µgC L−1), cryptophytes in Wilhelmina Bay (December 14, 2016, 6.4 × 106 cells L−1, and 97.5 µgC L−1) and unidentified unarmored dinoflagellates near Danco Island (December 18, 2016, 9.5 × 106 cells L−1, and 1597 µgC L−1). The last one represents, as far as we know, the first record of a dinoflagellate bloom in the WAP. It is to note that blooming organisms, analyzed morphologically, do not coincide with previously described Antarctic species. Fil: Mascioni, Martina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Ficología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina Fil: Almandoz, Gaston Osvaldo. Universidad Nacional de La ...
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