Abstract

Pico- (0.2–2 μm) and small (2–10 μm) nanophytoplanktonic assemblages were investigated along the Strait of Magellan at the beginning of the austral autumn. Three main areas were identified on the basis of hydrological data: an area characterised by high runoff from fjords, a divergence zone and an inland sea in the shallow eastern part of the channel (Paso Ancho). In the study area picophytoplankton abundance ranged from 8.10 × 10⁵ to 3.28 × 10⁷ cells L⁻¹. Cyanobacteria outnumbered picoeukaryotes on average by one order of magnitude; however prokaryotes to picoeukaryotes ratios varied along the Strait. Nanophytoplankton (2.20 × 10⁵ to 5.0⁹ × 106 cells L⁻¹) were mainly represented by cells 2–3 μm (60%, mean value). Picophytoplankton and nanophytoplankton accounted respectively on average for 46% and 27% of the total chlorophyll a which ranged from 0.02 μg L⁻¹ to 1.0 μg L⁻¹ with a mean value of 0.32 μg L⁻¹. Along the Strait phytoplankton displayed quite uniform size structure. Higher chlorophyll a concentrations were observed in the northernmost part of Paso Ancho where microphytoplankton was dominant, while lower chlorophyll a concentrations and lower prokaryotes: picoeukaryotes ratios (2 to 6) were found in the divergence zone.